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SEX-LINKED INHERITANCE IN
DROSOPHILA

BY

T. H. MORGAN AND C. B. BRIDGES



[Illustration]



WASHINGTON
PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON
1916



CARNEGIE INSTITUTION OF WASHINGTON
PUBLICATION NO. 237.



PRESS OF GIBSON BROTHERS, INC.
WASHINGTON, D. C.

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{3}

CONTENTS.

                                                                   PAGE.

  PART I. INTRODUCTORY                                                5

      Mendel's law of segregation                                     5
      Linkage and chromosomes                                         5

      Crossing-over                                                   7

      The Y chromosome and non-disjunction                            8

      Mutation in _Drosophila ampelophila_                           10
      Multiple allelomorphs                                          11
          Sex-linked lethals and the sex ratio                       14
      Influence of the environment on the realization of two
          sex-linked characters                                      16
      Sexual polymorphism                                            17
      Fertility and sterility in the mutants                         18
      Balanced inviability                                           19
      How the factors are located in the chromosomes                 20
      The sex-linked factors of _Drosophila_                         21
      Map of chromosome X                                            22
      Nomenclature                                                   24

  PART II. NEW DATA                                                  25
      White                                                          25
      Rudimentary                                                    25
      Miniature                                                      26
      Vermilion                                                      27
      Yellow                                                         27
      Abnormal abdomen                                               27
      Eosin                                                          28
      Bifid                                                          28
          Linkage of bifid with yellow, with white, and with
              vermilion                                              29
          Linkage of cherry, bifid, and vermilion                    30
      Reduplicated legs                                              31
      Lethal 1                                                       31
      Lethal 1a                                                      32
      Spot                                                           33
      Sable                                                          34
          Linkage of yellow and sable                                35
          Linkage of cherry and sable                                37
          Linkage of eosin, vermilion, and sable                     37
          Linkage of miniature and sable                             40
          Linkage of vermilion, sable, and bar                       40
      Dot                                                            44
          Linkage of vermilion and dot                               44
      Bow                                                            46
          Bow by arc                                                 47
      Lemon body-color                                               48
          Linkage of cherry, lemon, and vermilion                    48
      Lethal 2                                                       49
      Cherry                                                         51
          A system of quadruple allelomorphs                         51
          Linkage of cherry and vermilion                            51
          Compounds of cherry                                        52
      Fused                                                          53
           Linkage of eosin and fused                                54
           Linkage of vermilion, bar, and fused                      56
  {4}
      Forked                                                         58
          Linkage of vermilion and forked                            59
          Linkage of cherry and forked                               59
          Linkage of forked, bar, and fused                          60
          Linkage of sable, rudimentary, and forked                  61
          Linkage of rudimentary, forked, and bar                    62
      Shifted                                                        63
          Linkage of shifted and vermilion                           63
          Linkage of shifted, vermilion, and bar                     64
      Lethals _sa_ and _sb_                                          64
      Bar                                                            66
      Notch                                                          66
      Depressed                                                      67
          Linkage of depressed and bar                               67
          Linkage of cherry, depressed, and vermilion                68
      Club                                                           69
          Genotypic club                                             70
          Linkage of club and vermilion                              70
          Linkage of yellow, club, and vermilion                     70
          Linkage of cherry, club, and vermilion                     72
      Green                                                          73
      Chrome                                                         74
      Lethal 3                                                       74
      Lethal 3_a_                                                    75
      Lethal 1_b_                                                    76
      Facet                                                          76
          Linkage of facet, vermilion, and sable                     77
          Linkage of eosin, facet, and vermilion                     78
      Lethal _sc_                                                    79
      Lethal _sd_                                                    79
      Furrowed                                                       80
      Additional data for yellow, white, vermilion, and miniature    80
          New data contributed by A. H. Sturtevant and H. J. Muller  82
      Summary of the previously determined cross-over values         83
      Summary of all data upon linkage of gens in chromosome I.      84
      BIBLIOGRAPHY.                                                  86

       *       *       *       *       *


{5}

PART I. INTRODUCTORY.

MENDEL'S LAW OF SEGREGATION.

Although the ratio of 3 to 1 in which contrasted characters reappear in the
second or F_2 generation is sometimes referred to as Mendel's Law of
Heredity, the really significant discovery of Mendel was not the 3 to 1
ratio, but the segregation of the characters (or rather, of the germinal
representatives of the characters) which is the underlying cause of the
appearance of the ratio. Mendel saw that the characters with which he
worked must be represented in the germ-cells by specific producers (which
we may call factors), and that in the fertilization of an individual
showing one member of a pair of contrasting characters by an individual
showing the other member, the factors for the two characters meet in the
hybrid, and that _when the hybrid forms germ-cells the factors segregate
from each other without having been contaminated one by the other._ In
consequence, half the germ-cells contain one member of the pair and the
other half the other member. When two such hybrid individuals are bred
together the combinations of the pure germ-cells give three classes of
offspring, namely, two hybrids to one of each of the pure forms. Since the
hybrids usually can not be distinguished from one of the pure forms, the
observed ratio is 3 of one kind (the dominant) to 1 of the other kind (the
recessive).

There is another discovery that is generally included as a part of Mendel's
Law. We may refer to this as the _assortment_ in the germ-cells of the
products of the segregation of two or more pairs of factors. If assortment
takes place according to chance, then definite F_2 ratios result, such as
9:3:3:1 (for two pairs) and 27:9:9:9:3:3:3:1 (for three pairs), etc. Mendel
obtained such ratios in peas, and until quite recently it has been
generally supposed that free assortment is the rule when several pairs of
characters are involved. But, as we shall try to show, the emphasis that
has been laid on these ratios has obscured the really important part of
Mendel's discovery, namely, _segregation_; for with the discovery in 1906
of the fact of linkage the ratios based on free assortment were seen to
hold only for combinations of certain pairs of characters, not for other
combinations. But the principle of segregation still holds for each pair of
characters. Hence segregation remains the cardinal point of Mendelism.
Segregation is to-day Mendel's Law.

LINKAGE AND CHROMOSOMES.

It has been found that when _certain_ characters enter a cross together
(_i. e._, from the same parent) their factors tend to pass into the same
gamete of the hybrid, with the result that other ratios than the chance
ratios described by Mendel are found in the F_2 generation. {6} Such cases
of linkage have been described in several forms, but nowhere on so
extensive a scale as in the pomace fly, _Drosophila ampelophila_. Here,
over a hundred characters that have been investigated as to their linkage
relations are found to fall into four groups, the members of each group
being linked, in the sense that they tend to be transmitted to the gametes
in the same combinations in which they entered from the parents. The
members of each group give free assortment with the members of any of the
other three groups. A most significant fact in regard to the linkage shown
by the _Drosophila_ mutants is that _the number of linked groups
corresponds to the number of pairs of the chromosomes._ If the gens for the
Mendelian characters are carried by the chromosomes we should expect to
find demonstrated in _Drosophila_ that there are as many groups of
characters that are inherited together as there are pairs of chromosomes,
provided the chromosomes retain their individuality. The evidence that the
chromosomes are structural elements of the cell that perpetuate themselves
at every division has continually grown stronger. That factors have the
same distribution as the chromosomes is clearly seen in the case of
sex-linked characters, where it can be shown that any character of this
type appears in those individuals which from the known distribution of the
X chromosomes must also contain the chromosome in question. For example, in
_Drosophila_, as in many other insects, there are two X chromosomes in the
cells of the female and one X chromosome in the cells of the male. There is
in the male, in addition to the X, also a Y chromosome, which acts as its
mate in synapsis and reduction. After reduction each egg carries an X
chromosome. In the male there are two classes of sperm, one carrying the X
chromosome and the other carrying the Y chromosome. Any egg fertilized by
an X sperm produces a female; any egg fertilized by a Y sperm produces a
male. The scheme of inheritance is as follows.

            +------------------------------+
            |                              |
            | Eggs                    X--X |
            | Sperm                   X--Y |
            +------------------------------+
            | Daughter                 XX  |
            | Son                      XY  |
            |                              |
            +------------------------------+

The sons get their single X chromosome from their mother, and should
therefore show any character whose gen is carried by such a chromosome. In
sex-linked inheritance all sons show the characters of their mother. A male
transmits his sex-linked character to his daughters, who show it if
dominant and conceal it if recessive. But any daughter will transmit such a
character, whether dominant or recessive, to half of her sons. The path of
transmission of the gen is the same as the path followed by the X
chromosome, received here {7} from the male. Many other combinations show
the same relations. In the case of non-disjunction, to be given later,
there is direct experimental evidence of such a nature that there can no
longer be any doubt that the X chromosomes are the carriers of certain gens
that we speak of as sex-linked. This term (sex-linked) is intended to mean
that such characters are carried by the X chromosome. It has been objected
that this use of the term implies a knowledge of a factor for sex in the X
chromosome to which the other factors in that chromosome are linked; but in
fact we have as much knowledge in regard to the occurrence of a sex factor
or sex factors in the X chromosome as we have for other factors. It is true
we do not know whether there is more than one sex-factor, because there is
no crossing-over in the male (the heterozygous sex), and crossing-over in
the female does not influence the distribution of sex, since like parts are
simply interchanged. It follows from this that we are unable as yet to
locate the sex factor or factors in the X chromosome. The fact that we can
not detect crossing-over under this condition is not an argument against
the occurrence of linkage. We are justified, therefore, in speaking of the
factors carried by the X chromosome as sex-linked.

CROSSING-OVER.

When two or more sex-linked factors are present in a male they are always
transmitted together to his daughters, as must necessarily be the case if
they are carried by the unpaired X chromosome. If such a male carrying, let
us say, two sex-linked factors, is mated to a wild female, his daughters
will have one X chromosome containing the factors for both characters,
derived from the father, and another X chromosome that contains the factors
that are normal for these two factors (the normal allelomorphs). The sons
of such a female will get one or the other of these two kinds of
chromosomes, and should be expected to be like the one or the other
grandparent. In fact, most of the sons are of these two kinds. But, in
addition, there are sons that show one only of the two original mutant
characters. Clearly an interchange has taken place between the two X
chromosomes in the female in such a way that a piece of one chromosome has
been exchanged for the homologous piece of the other. The same conclusion
is reached if the cross is made in such a way that the same two sex-linked
characters enter, but, one from the mother and the other from the father.
The daughter gets one of her sex chromosomes from her mother and the other
from her father. She should produce, then, two kinds of sons, one like her
mother and one like her father. In fact, the majority of her sons are of
these two kinds, but, in addition, there are two other kinds of sons, one
kind showing both mutant characters, the other kind showing normal
characters. Here again the results must be due to interchange between the
two X's in the hybrid female. _The number of_ {8} _the sons due to exchange
in the two foregoing crosses is always the same, although they are of
contrary classes._ Clearly, then, the interchange takes place irrespective
of the way in which the factors enter the cross. We call those classes that
arise through interchange between the chromosomes "cross-over classes" or
merely "cross-overs." The phenomenon of holding together we speak of as
linkage.

By taking a number of factors into consideration at the same time it has
been shown that _crossing-over involves large pieces of the chromosomes_.
The X chromosomes undergo crossing-over in about 60 per cent of the cases,
and the crossing-over may occur at any point along the chromosome. When it
occurs once, whole ends (or halves even) go over together and the exchange
is always equivalent. If crossing-over occurs twice at the same time a
middle piece of one chromosome is intercalated between the ends of the
other chromosome. This process is called double crossing-over. It occurs
not oftener than in about 10 per cent of cases for the total length of the
X chromosome. Triple crossing-over in the X chromosome is extremely rare
and has been observed only about a half dozen times.

While the genetic evidence forces one to accept crossing-over between the
sex chromosomes in the female, that evidence gives no clue as to how such a
process is brought about. There are, however, certain facts familiar to the
cytologist that furnish a clue as to how such an interchange might take
place. When the homologous chromosomes come together at synapsis it has
been demonstrated, in some forms at least, that they twist about each other
so that one chromosome comes to lie now on the one side now on the other of
its partner. If at some points the chromosomes break and the pieces on the
same side unite and pass to the same pole of the karyokinetic spindle, the
necessary condition for crossing-over will have been fulfilled.

THE Y CHROMOSOME AND NON-DISJUNCTION.

Following Wilson's nomenclature, we speak of both X and Y as sex
chromosomes. Both the cytological and the genetic evidence shows that when
two X chromosomes are present a female is produced, when one, a male. This
conclusion leaves the Y chromosome without any observed relation to
sex-determination, despite the fact that the Y is normally present in every
male and is confined to the male line. The question may be asked, and in
fact has been asked, why may not the presence of the Y chromosome determine
that a male develop and its absence that a female appear? The only answer
that has yet been given, outside of the work on _Drosophila_, is that since
in some insects there is no Y chromosome, there is no need to make such an
assumption. But in _Drosophila_ direct proof that Y has no such function is
furnished by the evidence discovered by Bridges in the case of
non-disjunction. (Bridges, 1913, 1914, 1916, and unpublished results.) {9}

Ordinarily all the sons and none of the daughters show the recessive
sex-linked characters of the mother when the father carries the dominant
allelomorph. The peculiarity of non-disjunction is that sometimes a female
produces a daughter like herself or a son like the father, although the
rest of the offspring are perfectly regular. For example, a vermilion
female mated to a wild male produces vermilion sons and wild-type
daughters, but rarely also a vermilion daughter or a wild-type son. The
production of these exceptions (primary exceptions) by a normal XX female
must be due to an aberrant reduction division at which the two X
chromosomes fail to disjoin from each other. In consequence both remain in
the egg or both pass into the polar body. In the latter case an egg without
an X chromosome is produced. Such an egg fertilized by an X sperm produces
a male with the constitution XO. These males received their single X from
their father and therefore show the father's characters. While these XO
males are exceptions to sex-linked inheritance, the characters that they do
show are perfectly normal, that is, the miniature or the bar or other
sex-linked characters that the XO male has are like those of an XY male,
showing that the Y normally has no effect upon the development of these
characters. But that the Y does play some positive rôle is proved by the
fact that all the XO males have been found to be absolutely sterile.

While the presence of the Y is necessary for the fertility of the male, it
has no effect upon sex itself. This is shown even more strikingly by the
phenomenon known as secondary non-disjunction. If the two X chromosomes
that fail to disjoin remain in the egg, and this egg is fertilized by a Y
sperm, an XXY individual results. This is a female which is like her mother
in all sex-linked characters (a matroclinous exception), since she received
both her X chromosomes from her mother and none from her father. As far as
sex is concerned this is a perfectly normal female. The extra Y has no
effect upon the appearance of the characters, even in the case of eosin,
where the female is much darker than the male. The only effect which the
extra Y has is as an extra wheel in the machinery of synapsis and
reduction; for, on account of the presence of the Y, both X's of the XXY
female are sometimes left within the ripe egg, a process called secondary
non-disjunction. In consequence, an XXY female regularly produces
exceptions (to the extent of about 4 per cent). A small percentage of
reductions are of this XX-Y type; the majority are X-XY. The XY eggs,
produced by the X-XY reductions, when fertilized by Y sperm, give XYY
males, which show no influence of the extra Y except at synapsis and
reduction. By mating an XXY female to an XYY male, XXYY females have been
produced and these are perfectly normal in appearance. We may conclude from
the fact that visibly indistinguishable males have been produced with the
formulas XO, XY, and XYY, and {10} likewise females with the formulas XX,
XXY, and XXYY, that the Y is without effect either on the sex or on the
visible characters (other than fertility) of the individual.

The evidence is equally positive that sex is quantitatively determined by
the X chromosome--that two X's determine a female and one a male. For in
the case of non-disjunction, a zero or a Y egg fertilized by an X sperm
produces a male, while conversely an XX egg fertilized by a Y sperm
produces a female. It is thus impossible to assume that the X sperms are
normally female-producing because of something else than the X or that the
Y sperm produce males for any other reason than that they normally
fertilize X eggs. Both the X and the Y sperm have been shown to produce the
sex opposite to that which they normally produce when they fertilize eggs
that are normal in every respect, except that of their X chromosome
content. These facts establish experimentally that sex is determined by the
combinations of the X chromosomes, and that the male and female
combinations are the causes of sex differentiation and are not simply the
results of maleness and femaleness already determined by some other agent.

Cytological examination has demonstrated the existence of one XXYY female,
and has checked up the occurrence in the proper classes and proportions of
the XXY females. Numerous and extensive breeding-tests have been made upon
the other points discussed. The evidence leaves no escape from the
conclusion that the genetic exceptions are produced as a consequence of the
exceptional distribution of the X chromosomes and that the gens for the
sex-linked characters are carried by those chromosomes.

MUTATION IN DROSOPHILA AMPELOPHILA.

The first mutants were found in the spring of 1910. Since then an
ever-increasing series of new types has been appearing. An immense number
of flies have come under the scrutiny of those who are working in the
Zoological Laboratory of Columbia University, and the discovery of so many
mutant types is undoubtedly due to this fact. But that mutation is more
frequent in _Drosophila ampelophila_ than in some of the other species of
_Drosophila_ seems not improbable from an extensive examination of other
types. It is true a few mutants have been found in other _Drosophilas_, but
relatively few as compared with the number in _D. ampelophila_. Whether
_ampelophila_ is more prone to mutate, or whether the conditions under
which it is kept are such as to favor this process, we have no knowledge.
Several attempts that we have made to produce mutations have led to no
conclusive results.

The mutants of _Drosophila_ have been referred to by Baur as "mutations
through loss," but inasmuch as they differ in no respect that we can
discover from other mutants in domesticated animals and plants, there is no
particular reason for putting them into this category unless {11} to imply
that new characters have not appeared, or that those that have appeared
must be due to loss in the sense of absence of something from the
germ-plasm.

In regard to the first point, several of the mutants are characterized by
what seem to be additions. For example, the eye-color sepia is darker than
the ordinary red. At least three new markings have been added to the
thorax. A speck has appeared at the base of the wing, etc. These are
recessive characters, it is true, but the character "streak," which
consists of a dark band added to the thorax, is a dominant. If dominance is
supposed to be a criterion as to "presence," then it should be pointed out
that among the mutants of _Drosophila_ a number of dominant types occur.
But clearly we are not justified by these criteria in inferring anything
whatever in regard to the nature of the change that takes place in the
germ-plasm. Probably the only data which give a basis for attempting to
decide the nature of the change in the germ-plasm are from cases where
multiple allelomorphs are found. Several such cases are known to us, and
two of these are found in the X chromosome group, namely, a quadruple
system (white, eosin, cherry, red), and a triple system (yellow, spot,
gray). In such cases each member acts as the allelomorph of any other
member, and only two can occur in any one female, and only one in any male.
If the normal allelomorph is thought of as the positive character, which
one of the mutants is due to its loss or to its absence? If each is
produced by a loss it must be a different loss that acts as an allelomorph
to the other loss. This is obviously absurd unless a different idea from
the one usually promulgated in regard to "absence" is held.

MULTIPLE ALLELOMORPHS.

It appears that Cuénot was the first to find a case (in mice) in which the
results could be explained on the basis that more than two factors may
stand in the relation of allelomorphs to each other. In other words, a
given factor may become the partner of more than one other factor,
although, in any one individual, no more than two factors stand in this
relation. While it appears that his evidence as published was not
demonstrative, and that, at the time he wrote, the possibility of such
results being due to very close linkage could not have been appreciated as
an alternative explanation, nevertheless it remains that Cuénot was right
in his interpretation of his results and that the factors for yellow, gray,
gray white-belly, and black in mice form a system of quadruple
allelomorphs.

There are at least two such systems among the factors in the first
chromosome in _Drosophila_. The first of these includes the factor for
white eyes, that for eosin eyes, and that for cherry eyes, and of course
that allelomorph of these factors present in the wild fly and which when
present gives the red color. In this instance the normal {12} allelomorph
dominates all the other three, but in mice the mutant factor for yellow
dominates the wild or "normal" allelomorph.

The other system of multiple allelomorphs in the first chromosome is a
triple system made up of yellow (body-color), spot (on abdomen), and their
normal allelomorph--the factor in the normal fly that stands for "gray."

In general it may be said that there are two principal ways in which it is
possible to show that certain factors (more than two) are the allelomorphs
of each other. First, if they are allelomorphs only two can exist in the
same individual; and, in the case of sex-linked characters, while two may
exist in the same female, only one can exist in the male, for he contains
but one X chromosome. Second, all the allelomorphs should give the same
percentages of crossing-over with each other factor in the same chromosome.

It is a question of considerable theoretical importance whether these cases
of multiple allelomorphs are only extreme cases of linkage or whether they
form a system quite apart from linkage and in relation to normal
allelomorphism. It may be worth while, therefore, to discuss this question
more at length, especially because _Drosophila_ is one of the best cases
known for such a discussion.

The factors in the first chromosome are linked to each other in various
degrees. When they are as closely linked as yellow body-color and white
eyes crossing-over takes place only once in a hundred times. If two factors
were still nearer together it is thinkable that crossing-over might be such
a rare occurrence that it would require an enormous number of individuals
to demonstrate its occurrence. In such a case the factors might be said to
be completely linked, yet each would be supposed to have its normal
allelomorph in the homologous chromosome of the wild type. Imagine, then, a
situation in which one of these two mutant factors (a) enters from one
parent and the other mutant factor (b) from the other parent. The normal
allelomorph of a may be called A. It enters the combination with b, while
the normal allelomorph B of b enters the combination with a. Since b is
completely linked to A and a to B, the result will be the same as though a
and b were the allelomorphs of each other, for in the germ-cells of the
hybrid aBAb the assortment will be into aB and Ab, which is the same as
though a and b acted as segregating allelomorphs.

There is no way from Mendelian data by which this difference between a true
case of multiple allelomorphs and one of complete linkage (as just
illustrated) can be determined. There is, however, a different line of
attack which, in a case like that of _Drosophila_, will give an answer to
this question. The answer is found in the way in which the mutant factors
arise. This argument has been fully developed in the book entitled "The
Mechanism of Mendelian Inheritance," and will therefore not be repeated
here. It must suffice to say that if two mutant {13} types that behave as
allelomorphs of each other arise separately from the wild form, one of them
must have arisen as a double mutation of two factors so close to each other
as to be completely linked--a highly improbable occurrence when the
infrequency of mutations is taken into consideration.[1] The evidence
opposed to such an interpretation is now so strong that there can be little
doubt that multiple allelomorphs have actually appeared.

On _a priori_ grounds there is no reason why several mutative changes might
not take place in the same locus of a chromosome. If we think of a
chromosome as made up of a chain of chemical particles, there may be a
number of possible recombinations or rearrangements within each particle.
Any change might make a difference in the end-product of the activity of
the cell, and give rise to a new mutant type. It is only when one
arbitrarily supposes that the only possible change in a factor is its loss
that any serious difficulty arises in the interpretation of multiple
allelomorphs.

One of the most striking facts connected with the subject of multiple
allelomorphs is that the same kind of change is effected in the same organ.
Thus, in the quadruple system mentioned above, the color of the eye is
affected. In the yellow-spot system the color of the body is involved. In
mice it is the coat-color that is different in each member of the series.
While this is undoubtedly a striking relation and one which seems to fit
well with the idea that such effects are due to mutative changes in the
same fundamental element that affects the character in question, yet on the
other hand it would be dangerous to lay too much emphasis on this point,
because any given organ may be affected by other factors in a similar
manner, and also because a factor frequently produces more than a single
effect. For instance, the factor that when present gives a white eye
affects also the general yellowish pigment of the body. If red-eyed and
white-eyed flies are put for several hours into alcohol, the yellowish
body-color of the white-eyed flies is freely extracted, but not that of the
red-eyed flies. In the living condition the difference between the
body-colors of the red- and of the white-eyed flies is too slight to be
visible, but after extraction in alcohol the difference is striking. There
are other effects also that follow in the wake of the white factor. Now, it
is quite conceivable that in some specific case one of the effects might be
more striking than the one produced in that organ more markedly affected by
the other factor of the allelomorphic series. In such a case the relation
mentioned above might seemingly disappear. For this reason it is well not
to insist too strongly on the idea that multiple allelomorphs affect the
same part in the same way, even although at present that appears to be the
rule for all known cases.

{14}

SEX-LINKED LETHALS AND THE SEX RATIO.

Most of the mutant types of _Drosophila_ show characteristics that may be
regarded as superficial in so far as they do not prevent the animal from
living in the protected life that our cultures afford. Were they thrown
into open competition with wild forms, or, better said, were they left to
shift for themselves under natural conditions, many or most of the types
would no doubt soon die out. So far as we can see, there is no reason to
suppose that the mutations which can be described as superficial are
disproportionally more likely to occur than others. Of course, superficial
mutations are more likely to survive and hence to be seen; while if
mutations took place in important organs some of them would be expected to
affect injuriously parts essential to the life of the individual and in
consequence such an individual perishes. The "lethal factors" of
_Drosophila_ may be supposed to be mutations of some such nature; but as
yet we have not studied this side of the question sufficiently, and this
supposed method of action of the lethals is purely speculative. Whatever
the nature of the lethals' action, it can be shown that from among the
offspring obtained from certain stocks expected classes are missing, and
the absence of these classes can be accounted for on the assumption that
there are present mutant factors that follow the Mendelian rule of
segregation and which show normal linkage to other factors, but whose only
recognizable difference from the normal is the death of those individuals
which receive them. The numerical results can be handled in precisely the
same way as are other linkage results.

There are some general relations that concern the lethals that may be
mentioned here, while the details are left for the special part or are
found in the special papers dealing with these lethals. A factor of this
kind carried by the X chromosome would be transmitted in the female line
because the female, having two X chromosomes, would have one of them with
the normal allelomorph (dominant) of the lethal factor carried by the other
X chromosome. Half of her sons would get one of her X's, the other half the
other. Those sons that get the lethal X will die, since the male having
only one X lacks the power of containing both the lethal and its normal
allelomorph. The other half of the sons will survive, but will not transmit
the lethal factor. In all lethal stocks there are only half as many sons as
daughters. The heterozygous lethal-bearing female, fertilized by a normal
male, will give rise to two kinds of daughters; one normal in both X's, the
other with a normal X and a lethal-bearing X chromosome. The former are
always normal in behavior, and the latter repeat in their descendants the
2:1 sex-ratio.

Whether a female bearing the same lethal twice (_i.e._, one homozygous for
a given lethal) would die, can not be stated, for no such females are
obtainable, because the lethal males, which alone could bring about {15}
such a condition, do not exist. The presumption is that a female of this
kind would also die if the lethal acts injuriously on some vital function
or structure.

Since only half of the daughters of the lethal-bearing females carry the
lethal, the stock can be maintained by breeding daughters separately in
each generation to insure obtaining one which repeats the 2:1 ratio. There
is, however, a much more advantageous way of carrying on the stock--one
that also confirms the sufficiency of the theory.

In carrying on a stock of a lethal, advantage can be taken of linkage. A
lethal factor has a definite locus in the chromosome; if, then, a
lethal-bearing female is crossed to a male of another stock with a
recessive character whose factor lies in the X chromosome very close to the
lethal factor, half the daughters will have lethal in one X and the
recessive in the other. The lethal-bearing females can be picked out from
their sisters by the fact that they give a 2:1 sex-ratio, and by the fact
that nearly all the sons that do survive show the recessive character. If
such females are tested by breeding to the recessive males, then the
daughters which do not show the recessive carry the lethal, except in the
few cases of crossing-over. Thus in each generation the normal females are
crossed to the recessive males with the assurance that the lethal will not
be lost. If instead of the single recessive used in this fashion, a double
recessive of such a sort that one recessive lies on each side of the lethal
is used, then in each generation the females which show neither recessive
will almost invariably contain the lethal, since a double cross-over is
required to remove the lethal.

It is true that females carrying two _different_ lethals might arise and
not die, because the injurious effect of each lethal would be dominated by
its allelomorph in the other X chromosome. Such females can not be obtained
by combining two existing lethals, since lethal males do not survive. They
can occur only through a new lethal arising through mutation in the
homologous chromosome of a female that already carries one lethal. Rare as
such an event must be, it has occurred in our cultures thrice. The presence
of a female of this kind will be at once noticed by the fact that she
produces no sons, or very rarely one, giving in consequence extraordinary
sex-ratios. The rare appearance of a son from such a female can be
accounted for in the following way: If crossing-over occurs between her X
chromosomes the result will be that one X will sometimes contain two
lethals, the other none. The latter, if it passes into a male, will lead to
the development of a normal individual. The number of such males depends on
the distance apart of the two lethals in the chromosome. There is a crucial
test of this hypothesis of two lethals in females giving extraordinary
ratios. This test has been applied to the cases in which such females were
found, by Rawls (1913), by Morgan (1914_c_), and again by Stark (1915), and
it has been found to confirm the explanation. The daughters of {16} such a
female should all (excepting a rare one due to crossing-over) give 2:1
ratios, because each daughter must get one or the other X chromosome of her
mother, that is, one or the other lethal. Although the mother was
fertilized by a normal male, every daughter is heterozygous for one or the
other of the lethal factors. The daughters of the two-lethal females differ
from the daughters of the one-lethal female in that the former mother, as
just stated, gives all lethal-bearing daughters; the latter transmits her
lethal to only half of her daughters.

INFLUENCE OF THE ENVIRONMENT ON THE REALIZATION OF TWO SEX-LINKED
CHARACTERS.

The need of a special environment in order that certain mutant characters
may express themselves has been shown for abnormal abdomen (Morgan,
1912_d_, 1915_b_) and for reduplication of the legs (Hoge, 1915). In a
third type, club, described here (page 69), the failure of the unfolding of
the wing which occurs in about 20 per cent of the flies is also without
much doubt an environmental effect, but as yet the particular influence
that causes the change is unknown.

A very extensive series of observations has been made on the character
called abnormal abdomen. In pure cultures kept moist with abundance of
fresh food all the flies that hatch for the first few days have the black
bands of the abdomen obliterated or made faint and irregular. As the
bottles get dry and the food becomes scarce the flies become more and more
normal, until at last they are indistinguishable from the normal flies.
Nevertheless these normal-looking flies will give rise in a suitable
environment to the same kind of flies as the very abnormal flies first
hatched. By breeding from the last flies of each culture, and in dry
cultures, flies can be bred from normal ancestors for several generations,
and then by making the conditions favorable for the appearance of the
abnormal condition, the flies will be as abnormal as though their ancestors
had always been abnormal. Here, then, is a character that is susceptible to
the variations in the environment, yet whatever the realized condition of
the soma may be, that condition has no effect whatever on the nature of the
germ-plasm. A more striking disproof of the theory of the inheritance of
acquired characters would be hard to find.

A demonstration is given in this instance of the interaction between a
given genotypic constitution and a special environment. The character
abnormal is a sex-linked dominant. Therefore, if an abnormal male is mated
to a wild female the daughters are heterozygous for abnormal, while the
sons, getting their X chromosome from their mother, are entirely normal. In
a wet environment all the daughters are abnormal and the sons normal. As
the culture dries out the daughters' color becomes normal in appearance.
But while the sons {17} will never transmit abnormality to any of their
descendants in any environment, the daughters will transmit (if bred to
normal males) in a suitable environment their peculiarity to half of their
daughters and to half of their sons. The experiment shows convincingly that
the abnormal abdomen appears in a special environment only in those flies
that have a given genotypic constitution.

As the cultures dry out the abnormal males are the first to change over to
normal, then the heterozygous females, and lastly the homozygous females.
It is doubtful if any far-reaching conclusion can be drawn from this
series, because the first and second classes differ from each other not
only in the presence of one or of two factors for abnormal, but also by the
absence in the first case (male) of an entire X chromosome with its
contained factors. The second and third classes differ from each other only
by the abnormal factor.

Similar results were found in the mutant type called reduplicated legs,
which is a sex-linked recessive character that appears best when the
cultures are kept at about 10° C. As Miss M. A. Hoge has shown, this
character then becomes realized in nearly all of the flies that have the
proper constitution, but not in flies of normal constitution placed in the
same environment. Here the effect is produced by cold.

SEXUAL POLYMORPHISM.

Outside the primary and secondary sexual differences between the male and
the female, there is a considerable number of species of animals with more
than one kind of female or male. Darwin and his followers have tried to
explain such cases on the grounds that more than one kind of female (or
male) might arise through natural selection, in consequence of some
individuals mimicking a protected species. It is needless to point out here
how involved and intricate such a process would be, because the mutation
theory has cut the Gordian knot and given a simpler solution of the origin
of such diandromorphic and digynomorphic conditions.

In _Drosophila_ a mutant, eosin eye-color, appeared in which the female has
darker eyes than the male. If such stock is crossed with cherry (another
sex-linked recessive mutant, allelomorphic to eosin) the females in the F_2
generation are alike (for the pure eosin and the eosin-cherry compound are
not separable), but the cherry males and the eosin males are quite
different in appearance. Here we have a simulation, at least, of a
diandromorphic species. Such a group perpetuates itself, giving one type of
female (inasmuch as eosin and cherry females are very closely similar) and
two types of males, only one of which is like the females. A population of
this kind is very directly comparable to certain polymorphic types that
occur in nature. In _Colias philodice_ there is one type of male, yellow,
and two types of females, yellow and {18} white. In _Colias eurydice_ the
male is orange and the females are orange or white. In _Papilio turnus_ the
male is yellow and the females either yellow or black. Those cases are
directly comparable to an eosin-cherry population, except that in
Lepidoptera the female is heterozygous for the sex differential, in Diptera
the male.

Since in _Drosophila_ the results are explicable on a sex-linked basis, a
similar explanation may apply to polymorphism in butterflies. By suitable
combinations of eosin and cherry most of the cases of polymorphism in
butterflies may be simulated. To simulate the more complex cases, such as
that of _Papilio polytes_ and _memnon_, another allelomorph like eosin
would have to be introduced. A population of mixed cherry and white would
give three somatic types of females (cherry, cherry-white, and white) and
two of males (cherry and white).

FERTILITY AND STERILITY IN THE MUTANTS.

Aside from the decrease in fertility that occurs in certain stocks (a
question that need not be treated here), there are among the types
described in the text two cases that call for special comment. When the
mutant type called "rudimentary" was first discovered, it was found that
the females were sterile but the males were fully fertile. Later work has
revealed the nature of the sterility of the female. The ovaries are present
and in the young flies appear normal, but while in the normal flies the
eggs in the posterior portion enlarge rapidly during the first few days
after hatching, in the rudimentary females only a very few (about 15) eggs
enlarge. The other eggs in the ovary remain at a lower stage of their
development. Rarely the female lays a few eggs; when she does so some of
the eggs hatch, and if she has been mated to a rudimentary male, the
offspring are rudimentary females and males. The rudimentary females mate
in the normal time with rudimentary or with normal males, and their sexual
behavior is normal. Their sterility is therefore due to the failure of the
eggs to develop properly. Whether in addition to this there is some
incompatibility between the sperm and the eggs of this type (as supposed to
be the case at one time) is not conclusively disproved, but is not probable
from the evidence now available.

In the mutant called "fused" the females are sterile both with wild males
and with males from their own stock. An examination of the ovaries of these
females, made by Mr. C. McEwen, shows clearly that there are fewer than the
normal number of mature eggs, recalling the case of rudimentary.

It should be noticed that there is no apparent relation between the
sterility of these two types and the occurrence of the mutation in the X
chromosome, because other mutations in the X do not cause sterility, and
there is sterility in other mutant types that are due to factors in other
chromosomes. {19}

BALANCED INVIABILITY.

The determination of the cross-over values of the factors was at first
hindered because of the poor viability of some of the mutants. If the
viability of each mutant type could be determined in relation to the
viability of the normal, "coefficients of viability" could serve as
corrections in working with the various mutant characters. But it was found
(Bridges and Sturtevant, 1914) that viability was so erratic that
coefficients might mislead. At the same time it was becoming more apparent
that poor viability is no necessary attribute of a character, but depends
very largely on the condition of culture. Competition among larvæ was found
to be the chief factor in viability. Mass cultures almost invariably have
extremely poor viability, even though an attempt is made to supply an
abundance of food. Special tests (Morgan and Tice, 1914) showed that even
those mutants which were considered the very poorest in viability were
produced in proportions fairly close to the theoretical when only one
female was used for each large culture bottle and the amount and quality of
food was carefully adjusted.

For the majority of mutants which did well even under heavy competition in
mass cultures the pair-breeding method reduced the disturbances due to
viability to a point where they were negligible.

Later a method was devised (Bridges, 1915) whereby mutations of poor
viability could be worked with in linkage experiments fairly accurately and
whereby the residual inviability of the ordinary characters could be
largely canceled. This method consists in balancing the data of a certain
class with poor viability by means of an equivalent amount of data in which
the same class occurs as the other member of the ratio. Thus in obtaining
data upon any linkage case it is best to have the total number of
individuals made up of approximately equal numbers derived from each of the
possible ways in which the experiment may be conducted. In the simplest
case, in which the results are of the form AB:Ab:aB:ab, let us suppose that
the class ab has a disproportionately low viability. If, then, ab occurs in
an experiment as a cross-over class, that class will be too small and a
false linkage value will be calculated. The remedy is to balance the
preceding data by an equal amount of data in which ab occurs as a
non-cross-over. In these latter the error will be the opposite of the
previous one, and by combining the two experiments the errors should be
balanced to give a better approximation to the true value. When equal
amounts of data, secured in these two ways, are combined, all four classes
will be balanced in the required manner by occurring both as
non-cross-overs and as cross-overs. The error, therefore, should be very
small. For three pairs of gens there are eight classes, and in order that
each of them may appear as a non-cross-over, as each single cross-over, and
as the double cross-over, four experiments must be made. {20}

HOW THE FACTORS ARE LOCATED IN THE CHROMOSOMES.

A character is in the first chromosome if it is transmitted by the
grandfather to half of his grandsons, while, in the reciprocal cross, the
mother transmits her character to all her sons (criss-cross inheritance)
and to half of her granddaughters and to half of her grandsons; in other
words, if the factor that differentiates the character has the same
distribution as the X chromosome. If, however, a new mutant type does not
show this sex-linked inheritance, its chromosome is determined by taking
advantage of the fact that in _Drosophila_ there is no crossing-over in the
male between factors in the same chromosome. For instance, if a new mutant
type is found not to be sex-linked, its group is determined by the
following tests: It is crossed to black, whose factor is known to be in the
second chromosome, and to pink, whose factor lies in the third chromosome.
If the factor of the new form should happen to be in the second chromosome,
then, in the cross with black, no double recessive can appear, so that the
F_{2} proportion is 2:1:1:0; but with pink, the mutant type should give the
proportion 9:3:3:1, typical of free assortment.

If, however, the factor of the new form is in the third chromosome, then,
when crossed to black, the double recessive and the 9:3:3:1 proportion
appear in F_{2}. But when crossed to pink no double recessive appears in
F_{2}, and the proportion 2:1:1:0 occurs.

If these tests show that the new mutant does not belong to either the
second or third chromosome, that is, if both with black and with pink the
9:3:3:1 ratio is obtained, then by exclusion the factor lies in the fourth
chromosome, in which as yet only two factors have been found.

We propose to give in a series of papers an account of the mutant races of
_Drosophila_ and the linkage shown in their inheritance. In this paper we
shall consider only the members of the first chromosome, describing a large
number of new mutants with their linkage relations and summarizing to date
all the linkage data relating to the first chromosome. In later papers we
propose to consider the members of the second, third, and fourth
chromosomes.

The list at the top of page 21 gives the names of the factors dealt with in
this paper. They stand in the order of their discovery, the mutant forms
reported here for the first time being starred.

In each experiment the percentage of crossing-over is found by dividing the
number of the cross-overs by the sum of the non-cross-overs and the
cross-overs, and multiplying this quotient by 100. The resulting
percentages, or cross-over values, are used as measures of the distances
between loci. Thus if the experiments give a cross-over value of 5 per cent
for white and bifid, we say that white and bifid lie 5 units apart in the X
chromosome. Other experiments show that yellow and white are about 1 unit
apart, and that yellow and bifid are about 6 units apart. We can therefore
construct a diagram with yellow as {21} the zero, with white at 1, and with
bifid at 6. If we know the cross-over values given by a new mutant with any
two mutants of the same chromosome whose positions are already determined,
then we can locate the new factor with accuracy, and be able to predict the
cross-over value which the new factor will give with any other factor whose
position is plotted.

_The sex-linked factors of Drosophila._

  +------------+----------+-------+-------+--------+------------+---------+
  | Gen.       |  Part    |Figure.|Symbol.| Locus. | Date found.|Found by.|
  |            |affected. |       |       |        |            |         |
  +------------+----------+-------+-------+--------+------------+---------+
  |White       |Eye-color | 11    | w     | 1.1    | May 1910   |Morgan.  |
  |Rudimentary |Wings     | A     | r     | 55.1   | June 1910  |Morgan.  |
  |Miniature   |Wings     | 7-8   | m     | 36.1   | Aug. 1910  |Morgan.  |
  |Vermilion   |Eye-color | 10    | v     | 33.0   | Nov. 1910  |Morgan.  |
  |Yellow      |Body-color| 5     | y     | 0.0    | Jan. 1911  |Wallace. |
  |Abnormal    |Abdomen   | 4     | A'    | 2.4    | July 1911  |Morgan.  |
  |Eosin       |Eye-color | 7-8   | w^e   | 1.1    | Aug. 1911  |Morgan.  |
  |Bifid       |Wings     | B     | b_i   | 6.3    | Nov. 1911  |Morgan.  |
  |Reduplicated|Legs      |       |       | 34.7   | Nov. 1911  |Hoge.    |
  |Lethal 1    |Life      |       | l_1   | 0.7    | Feb. 1912  |Rawls.   |
  |Lethal 1_a_*|Life      |       | l_1a  | 3.3    | Mar. 1912  |Rawls.   |
  |Spot*       |Body-color| 14-17 | y^s   | 0.0    | April 1912 |Cattell. |
  |Sable*      |Body-color| 2     | s     | 43.0   | July 1912  |Bridges. |
  |Dot*        |Thorax    |       |       | 33 ±   | July 1912  |Bridges. |
  |Bow*        |Wings     | C     |       |        | Aug. 1912  |Bridges. |
  |Lemon*      |Body-color| 3     | l_m   | 17.5   | Aug. 1912  |Wallace. |
  |Lethal 2    |Life      |       | l_2   | 12.5±  | Sept. 1912 |Morgan.  |
  |Cherry      |Eye-color | 9     | w^c   | 1.1    | Oct. 1912  |Safir.   |
  |Fused*      |Venation  | D     | f_u   | 59.5   | Nov. 1912  |Bridges. |
  |Forked*     |Bristles  | E     | f     | 56.5   | Nov. 1912  |Bridges. |
  |Shifted*    |Venation  | F     | s_h   | 17.8   | Jan. 1913  |Bridges. |
  |Lethal sa   |Life      |       | l_sa  | 23.7   | Jan. 1913  |Stark.   |
  |Bar         |Eye-shape | 12-13 | B'    | 57.0   | Feb. 1913  |Tice.    |
  |Notch       |Wing      |       | N'    | 2.6    | Mar. 1913  |Dexter.  |
  |Depressed*  |Wing      | G     | d_p   | 18.0   | April 1913 |Bridges. |
  |Lethal sb   |Life      |       | l_sb  | 16.7   | April 1913 |Stark.   |
  |Club*       |Wings     | H     | c_l   | 14.6   | May 1913   |Morgan.  |
  |Green*      |Body-color|       |       |        | May 1913   |Bridges. |
  |Chrome*     |Body-color|       |       |        | Sept. 1913 |Bridges. |
  |Lethal 3    |Life      |       | l_3   | 26.5   | Dec. 1913  |Morgan.  |
  |Lethal 3_a_ |Life      |       | l_3a  | 19.5   | Jan.  1914 |Morgan.  |
  |Lethal 1_b_*|Life      |       | l_1b  | 1.1-   | Feb. 1914  |Morgan.  |
  |Facet*      |Eye       |       | f_a   | 2.2    | Feb.  1914 |Bridges. |
  |Lethal _sc_ |Life      |       | l_sc  | 66.2   | April 1914 |Stark.   |
  |Lethal _sd_ |Life      |       | l_sd  |        | May 1914   |Stark.   |
  |Furrowed    |Eye       |       | f_w   | 38.0   | Nov. 1914  |Duncan.  |
  +------------+----------+-------+-------+--------+------------+---------+

The factors are located preferably by short distances (_i.e._, by those
cases in which the amount of crossing-over is small), because when the
amount of crossing-over is large a correction must be made for double
crossing-over, and the correction can be best found through breaking up the
long distances into short ones, by using intermediate points.

Conversely, when a long distance is indicated on the chromosome diagram,
the actual cross-over value found by experiment (_i.e._, the percentage of
cross-overs) will be less than the diagram indicates, because the diagram
has been corrected for double crossing-over. {22}

               0.0 | Yellow, spot
               0.7 | Lethal I
               1.1-| Lethal Ib
               1.1 | White, eosin, cherry
               2.2 | Facet
               2.4 | Abnormal
               2.6 | Notch
               3.3 | Lethal Ia
                   |
               6.3 | Bifid
                   |
                   |
                   |
                   |
                   |
                   |
              12.5 | Lethal II
              14.6 | Lethal sb
              16.7 | Club
              17.5 | Lemon
              17.8 | Shifted
              18.0 | Depressed
              19.5 | Lethal IIIa
                   |
                   |
              23.7 | Lethal sa
                   |
              26.5 | Lethal III
                   |
                   |
                   |
              33.0 | Vermilion
              33.± | Dot
              34.7 | Reduplicated
              36.1 | Miniature
              38.0 | Furrowed
                   |
                   |
              43.0 | Sable
                   |
                   |
                   |
                   |
                   |
                   |
              55.1 | Rudimentary
              56.5 | Forked
              57.0 | Bar
              59.5 | Fused
                   |
                   |
                   |
              66.2 | Lethal sc

              DIAGRAM I.

{23}

Diagram I has been constructed upon the basis of all the data summarized in
table 65 (p. 84) for the first or X chromosome. It shows the relative
positions of the gens of the sex-linked characters of _Drosophila_. One
unit of distance corresponds to 1 per cent of crossing-over. Since all
distances are corrected for double crossing-over and for coincidence, the
values represent the _total_ of crossing-over between the loci. The
uncorrected value obtained in any experiment with two loci widely separated
will be smaller than the value given in the map.

It may be asked what will happen when two factors whose loci are more than
50 units apart in the same chromosome are used in the same experiment? One
might expect to get more than 50 per cent of cross-overs with such an
experiment, but double crossing-over becomes disproportionately greater the
longer the distance involved, so that in experiments the observed
percentage of crossing-over does not rise above 50 per cent. For example,
if eosin is tested against bar, somewhat under 50 per cent of cross-overs
are obtained, but if the distance of bar from eosin is found by summation
of the component distances the interval for eosin bar is 56 units.

In calculating the loci of the first chromosome, a system of weighting was
used which allowed each case to influence the positions of the loci in
proportion to the amount of the data. In this way advantage was taken of
the entire mass of data.

The factors (lethal 1, white, facet, abnormal, notch, and bifid) which lie
close to yellow were the first to be calculated and plotted. The next step
was to determine very accurately the position of vermilion with respect to
yellow. There are many separate experiments which influence this
calculation and all were proportionately weighted. Then, using vermilion as
the fixed point the factors (dot, reduplicated, miniature, and sable) which
lie close to vermilion were plotted. The same process was repeated in
locating bar with respect to vermilion and the factors about bar with
reference to bar. The last step was to interpolate the factors (club,
lethal 2, lemon, depressed, and shifted), which form a group about midway
between yellow and vermilion. Of these, club is the only one whose location
is accurate. The apparent closeness of the grouping of these loci is not to
be taken as significant, for they have been placed only with reference to
the distant points yellow and vermilion and not with respect to each other;
furthermore, the data available in the cases of lemon and depressed are
very meager.

The factors which are most important and are most accurately located are
yellow, white (eosin), bifid, club, vermilion, miniature, sable, forked,
and bar. Of these again, white (eosin), vermilion, and bar are of prime
importance and will probably continue to claim first rank. Of the three
allelomorphs, white, eosin, and cherry, eosin is the most useful. {24}

NOMENCLATURE.

The system of symbols used in the diagrams and table headings is as
follows: The factor or gen for a recessive mutant character is represented
by a lower-case letter, as v for vermilion and m for miniature. The symbols
for the dominant mutant characters bar, abnormal, and notch are B', A', and
N'. There are now so many characters that it is impossible to represent all
of them by a single letter. We therefore add a subletter in such cases, as
bifid (b_i), fused (f_u), and lethal 2 (l_2). In the case of multiple
allelomorphs we usually use as the base of the symbol the symbol of that
member of the system which was first found and add a letter as an exponent
to indicate the particular member, as y^s for spot, w^e for eosin, and w^c
for cherry. The normal allelomorphs of the mutant gens are indicated by the
converse letter, as V for not-vermilion, B_i for not-bifid, and b' for
not-bar. In the table headings the normal allelomorphs are indicated by
position alone without the use of a symbol.

Thus the symbol [draw] indicates that the female in question carried eosin,
not-vermilion, and bar in one chromosome and not-eosin, vermilion, and
not-bar in the other. The symbol [draw] when used in the heading of a
column in a table indicates that the flies classified under this heading
are the result of single crossing-over between eosin and vermilion in a
mother which was the composition [draw]; the symbol tells at the same time
that the flies that result from a single cross-over between eosin and
vermilion in the mother are of the two contrary classes, eosin bar and
vermilion. When a fly shows two or more non-allelomorphic characters the
names are written from left to right in the order of their positions from
the zero end of the map.

       *       *       *       *       *


{25}

PART II. NEW DATA.

WHITE.

(Plate II, figure 11.)

The recessive character white eye-color, which appeared in May 1910, was
the first sex-linked mutation in _Drosophila_ (Morgan, 1910_a_, 1910_b_).
Soon afterwards (June 1910) rudimentary appeared, and the two types were
crossed (Morgan, 1910_c_). Under the conditions of culture the viability of
rudimentary was extremely poor, but the data demonstrated the occurrence of
recombination of the factors in the ovogenesis so that white and
rudimentary, though both sex-linked, were brought together into the same
individual. The results were not fully recognized as linkage, because white
and rudimentary are so far apart in the chromosome that they seemed to
assort freely from each other.

Owing to the excellent viability and the perfect sharpness of separation,
white was extensively used in linkage experiments, especially with
miniature and yellow (Morgan, 1911_a_; Morgan and Cattell, 1912 and 1913).
White has been more extensively used than any other character in
_Drosophila_, though it is now being used very little because of the fact
that the double recessives of white with other sex-linked eye-colors, such
as vermilion, are white, and consequently a separation into the true
genetic classes is impossible. The place of white has been taken by eosin,
which is an allelomorph of white and which can be readily used with any
other eye-color.

The locus of white and its allelomorphs is only 1.1 units from that of
yellow, which is the zero of the chromosome. Yellow and white are very
closely linked, therefore giving only about one cross-over per 100 flies.

All the published data upon the linkage of white with other sex-linked
characters have been collected into table 65.

RUDIMENTARY.

Rudimentary, which appeared in June 1910, was the second sex-linked
character in _Drosophila_ (Morgan, 1910_c_). Its viability has always been
very poor; in this respect it is one of the very poorest of the sex-linked
characters. The early linkage data (Morgan, 1911_a_) derived from mass
cultures have all been discarded. By breeding from a single F_1 female in
each large culture bottle it has been possible to obtain results which are
fairly trustworthy (Morgan, 1912_g_; Morgan and Tice, 1914). These data
appear in table 65, which summarizes all the published data. {26}

The locus of rudimentary is at 55.1, for a long time the extreme right end
of the known chromosome, though recently several mutants have been found to
lie somewhat beyond it.

[Illustration: Fig. A. _a._ rudimentary wing; _b._ the wild fly for
comparison.]

The rudimentary males are perfectly fertile, but the rudimentary females
rarely produce any offspring at all, and then only a very few. The reason
for this is that most of the germ-cells cease their development in the
early growth stage of the eggs (Morgan, 1915_a_).

MINIATURE.

(Plate II. figures 7 and 8.)

The recessive sex-linked mutant miniature wings appeared in August 1910
(Morgan, 1911b and 1912a). The viability of miniature is fair, and this
stock has been used in linkage experiments more than any {27} other, with
the single exception of white. While the wings of miniature usually extend
backwards, they are sometimes held out at right angles to the body, and
especially in acid bottles the miniature flies easily become stuck to the
food or the wings become stringy, so that other wing characters are not
easy to distinguish in those flies which are also miniature. At present
vermilion, whose locus is at 33, in being used more frequently in linkage
work. The locus of miniature at 36.1 is slightly beyond the middle of the
chromosome.

VERMILION.

(Plate II. figure 10.)

The recessive sex-linked mutant vermilion eye-color (Morgan, 1911_c_ and
1912_a_) appeared in November 1910, and has appeared at least twice since
then (Morgan and Plough, 1915). This is one of the best of the sex-linked
characters, on account of its excellent viability, its sharp distinction
from normal with very little variability, its value as a double recessive
in combination with other sex-linked eye-colors, and because of its
location at 33.0, very near to the middle of the known chromosome.

YELLOW.

(Plate I. figure 5.)

The recessive sex-linked mutant yellow body and wing-color appeared in
January 1911 (Morgan, 1911_c_ and 1912_a_). Its first appearance was in
black stock; hence the fly was a double recessive, then called brown. Later
the same mutation has appeared independently from gray stock. Yellow was
found to be at the end of the X chromosome, and this end was arbitrarily
chosen as the zero or the "left end," while the other gens are spoken of as
lying at various distances to the right of yellow. Recently a lethal gen
has been located less than one-tenth of a unit (-0.04) to the left of
yellow, but yellow is still retained as the zero-point.

The viability of yellow is fairly good and the character can be separated
from gray with great facility, and in consequence yellow has been used
extensively, although at present it is being used less than formerly, since
eosin lies only 1.1 units distant from yellow and is generally preferred.

ABNORMAL ABDOMEN.

(Plate I. figure 4.)

The dominant sex-linked character abnormal abdomen appeared in July 1911
(Morgan, 1911_d_). It was soon found that the realization of the abnormal
condition depended greatly upon the nature of the environment (Morgan,
1912). Recently a very extensive study of this character has been published
(Morgan, 1915). As this case has been reviewed in the introduction, there
is little further to be said here. {28} Because of the change that takes
place as the culture grows older (the abnormal changing to normal), this
character is not of much value in linkage work. The location of the factor
in the X chromosome at 2.4 has been made out from the data given by Morgan
(1915_b_). These data, which in general include only the abnormal classes,
are summarized in table 1.

TABLE 1.--_Linkage data, from Morgan, 1915b._

  +------------------+-----------+-----------+------------+
  |     Gens.        |  Total.   |   Cross-  | Cross-over |
  |                  |           |   overs.  |  values.   |
  +------------------+-----------+-----------+------------+
  | Yellow white     |  28,018   |    334    |    1.2     |
  | Yellow abnormal  |  15,314   |    299    |    2.0     |
  | White abnormal   |  16,300   |    277    |    1.7     |
  +------------------+-----------+-----------+------------+

EOSIN.

(Plate II, figures 7 and 8.)

The recessive sex-linked mutation eosin eye-color appeared in August 1911
in a culture of white-eyed flies (Morgan 1912_a_). The eye-color is
different in the male and female, the male being a light pinkish yellow,
while the female is a rather dark yellowish pink. Eosin is allelomorphic to
white and the white-eosin compound or heterozygote has the color of the
eosin male. There is probably no special significance in this coincidence
of color, since similar dilutions to various degrees have been demonstrated
for all the other eye-colors tested (Morgan and Bridges, 1913). Since eosin
is allelomorphic to white, its locus is also at 1.1. Eosin is the most
useful character among all those in the left end of the chromosome.

BIFID.

The sex-linked wing mutant bifid, which appeared in November 1911, is
characterized by the fusion of all the longitudinal veins into a heavy
stalk at the base of the wing. The wing stands out from the body at a wide
angle, so that the fusion is easily seen. At the tip of the wing the third
longitudinal vein spreads out into a delta which reaches to the marginal
vein. The fourth longitudinal vein reaches the margin only rarely. There is
very often opposite this vein a great bay in the margin, or the whole wing
is irregularly truncated.

The stock of bifid was at first extremely varied in the amount of this
truncation. By selection a stock was secured which showed only very greatly
reduced wings like those shown in figures _a_, b. Another stock (figs. _c_,
_d_) was secured by outcrossing and selection which showed wings of nearly
normal size and shape, which always had the bifid stalk, generally the
spread positions (not as extreme), and often the delta and the shortened
fourth longitudinal vein. We believe that the extreme reduction in size
seen in the one stock was due to an added modifier of {29} the nature of
beaded, since this could be eliminated by outcrossing and selection.

[Illustration: FIG. B.--Bifid wing. _c_ and _d_ show the typical condition
of bifid wings. All the longitudinal veins are fused into a heavy stalk at
the base of the wing. _a_ shows the typical position in which the bifid
wings are held. The small size of the wings in _a_ and _b_ is due to the
action of a modifier of the nature of "beaded" which has been eliminated in
_c_, d.]

LINKAGE OF BIFID WITH YELLOW, WITH WHITE, AND WITH VERMILION.

The stock of the normal (not-beaded) bifid was used by Dr. R. Chambers,
Jr., for determining the chromosome locus of bifid by means of its linkage
relations to vermilion, white, and yellow (Chambers, 1913). We have
attempted to bring together in table 2 the complete data and to calculate
the locus of bifid.

TABLE 2.--_Linkage data, from Chambers, 1913._

  +-----------------+------------+-------------+--------------+
  |      Gens.      |   Total.   |    Cross-   |  Cross-over  |
  |                 |            |    overs.   |   values.    |
  +-----------------+------------+-------------+--------------+
  | Yellow bifid    |    3,175   |      182    |     5.8      |
  | White bifid     |   20,800   |    1,127    |     5.3      |
  | Bifid vermilion |    2,509   |      806    |    32.1      |
  +-----------------+------------+-------------+--------------+

{30}

In the crosses between white and bifid there were 1,127 cross-overs in a
total of 20,800 available individuals, which gives a cross-over value of
5.3. In the crosses between yellow and bifid there were 182 cross-overs in
a total of 3,175 available individuals, which gives a cross-over value of
5.8. In crosses between bifid and vermilion there were 806 cross-overs in a
total of 2,509, which gives a cross-over value of 32.1. On the basis of all
the data summarized in table 65, bifid is located at 6.3 to the right of
yellow.

LINKAGE OF CHERRY, BIFID, AND VERMILION.

In a small experiment of our own, three factors were involved--cherry,
bifid, and vermilion. A cherry vermilion female was crossed to a bifid
male. Two daughters were back-crossed singly to white bifid males. The
female offspring will then give data for the linkage of cherry white with
bifid, while the sons will show the linkage of the three gens, cherry,
bifid, and vermilion. The results are shown in table 3.

TABLE 3.--_P_1 cherry vermilion [female] [female] × bifid [male] [male]. B.
C.[2] F_1 wild-type [female] × white bifid [male] [male]._

  |-----------------------------------
  |      |       F_2 females.        |
  |      |---------------------------+
  |      |                           |
  |Refer-| Non-cross-  |Cross-overs. |
  |ence. |   overs.    |             |
  |      |-------------+-------------+
  |      |White-|Bifid.|White-|Wild- ~
  |      |cherry|      |cherry|type. ~
  |      |      |      |bifid.|      |
  |------+------+------+------+------+
  |  262 |   40 |   46 |   1  |   2  |
  |  263 |   47 |   45 |   3  |   3  |
  |      |------+------+------+------+
  |Total.|   87 |   91 |   4  |   5  |
  |-----------------------------------

  |----------------------------------------------------------------------|
  |      |                         F_2 males.                            |
  |      |-----------------------------+---------------------------------|
  |      | w^c        v | w^c b        | w^c           |  w^c  b_i   v   |
  |Refer-| ------------ | ---+-------- | ---------+--- |  ---+-----+---  |
  |ence. |     b_i      |           v  |     b_i    v  |                 |
  |      |--------------+--------------+---------------+-----------------|
  ~      |Cherry |Bifid.|Cherry| Ver-  |Cherry.|Bifid  | Cherry   |Wild- |
  ~      | ver-  |      |bifid.|milion.|       | ver-  |  bifid   |type. |
  |      |milion.|      |      |       |       |milion.|vermilion.|      |
  |------+-------+------+------+-------+-------+-------+----------+------|
  |  262 |  45   |  38  |   3  |   2   |   11  |   13  |     ..   |  ..  |
  |  263 |  30   |  50  |   1  |   3   |    8  |   10  |      1   |  ..  |
  |      |-------+------+------+-------+-------+-------+----------+------|
  |Total.|  75   |  88  |   4  |   5   |   19  |   23  |      1   |   0  |
  |----------------------------------------------------------------------|

Both males and females give a cross-over value of 5 units for cherry bifid,
which is the value determined by Chambers. The order of the factors, viz,
cherry, bifid, vermilion, is established by taking advantage of the double
cross-over classes in the males. The male classes give a cross-over value
of 20 for bifid vermilion and 24 for cherry vermilion, which are low
compared with values given by other experiments. The locus of bifid at 6.3
is convenient for many linkage problems, but this advantage is largely
offset by the liability of the bifid flies to become stuck in the food and
against the sides of the bottle. Bifid flies can be separated from the
normal with certainty and with great ease. {31}

REDUPLICATED LEGS.

In November 1912 Miss Mildred Hoge found that a certain stock was giving
some males whose legs were reduplicated, either completely or only with
respect to the terminal segments (described and figured, Hoge, 1915).
Subsequent work by Miss Hoge showed that the condition was due to a
sex-linked gen, but that at room temperature not all the flies that were
genetically reduplicated showed reduplication. However, if the flies were
raised through the pupa stage in the ice-box at a temperature of about 10°
to 12° a majority of the flies which were expected to show reduplication
did so. The most extremely reduplicated individual showed parts of 14 legs.

In studying the cross-over values of reduplicated, only those flies that
have abnormal legs are to be used in calculation, as in the case of
abnormal abdomen where the phenotypically normal individuals are partly
genetically abnormal. Table 4 gives a summary of the data secured by Miss
Hoge.

TABLE 4.--_Summary of linkage data upon reduplicated legs, from Hoge,
1915._

  +---------------------------+---------+---------+------------+
  |         Gens.             |  Total. |  Cross- | Cross-over |
  |                           |         |  overs. |  values.   |
  |---------------------------+---------+---------|------------|
  |                           |         |         |            |
  |White reduplicated         |    418  |   121   |    29.0    |
  |Reduplicated vermilion     |    667  |    11   |     1.7    |
  |Reduplicated bar           |    583  |   120   |    20.6    |
  |                           |         |         |            |
  +---------------------------+---------+---------+------------+

The most accurate data, those upon the value for reduplicated and
vermilion, give for reduplicated a distance of 1.7 from vermilion, either
to the right or to the left. The distance from white is 29, which would
place the locus for reduplication to the left of vermilion, which is at 33.
The data for bar give a distance of 21, but since bar is itself 24 units
from vermilion, this distance of 21 would seem to place the locus to the
right of vermilion. The evidence is slightly in favor of this position to
the right of vermilion at 34.7, where reduplicated may be located
provisionally. In any case the locus is so near to that of vermilion that
final decision must come from data involving double crossing-over, _i. e._,
from a three-locus experiment.

LETHAL 1.

In February 1912 Miss E. Rawls found that certain females from a wild stock
were giving only about half as many sons as daughters. Tests continuing
through five generations showed that the sons that appeared were entirely
normal, but that half of the daughters gave again 2 : 1 sex-ratios, while
the other half gave normal 1 : 1 sex-ratios. {32}

The explanation of this mode of transmission became clear when it was found
that the cause of the death of half of the males was a particular factor
that had as definite a locus in the X chromosome as have other sex-linked
factors (Morgan, 1912_e_). Morgan mated females (from the stock sent to him
by Miss Rawls) to white-eyed males. Half of the females, as expected, gave
2 : 1 sex-ratios, and daughters from these were again mated to white males.
Here once more half of the daughters gave 2 : 1 sex-ratios, but in such
cases the sons were nearly all white-eyed and only rarely a red-eyed son
appeared, when under ordinary circumstances there should be just as many
red sons as white sons. The total output for 11 such females was as follows
(Morgan, 1914_b_): white [female], 457; red [female], 433; white [male],
370; red [male], 2. It is evident from these data that there must be
present in the sex-chromosome a gen that causes the death of every male
that receives this chromosome, and that this lethal factor lies very close
to the factor for white eyes. The linkage of this lethal (now called lethal
1) to various other sex-linked gens was determined (Morgan 1914_b_), and is
summarized in table 5. On the basis of these data it is found that the gen
lethal 1 lies 0.4 unit to the left of white, or at 0.7.

TABLE 5.--_Summary of linkage data upon lethal 1, from Morgan, 1914b, pp.
81-92._

  +------------------------+---------+--------+-------------+
  |          Gens.         |  Total. | Cross- |  Cross-over |
  |                        |         | overs. |   values.   |
  +------------------------+---------+--------+-------------+
  |                        |         |        |             |
  | Yellow lethal 1        |    131  |    1   |     0.8     |
  | Yellow miniature       |    131  |   45   |    34.4     |
  | Lethal 1 white         |  1,763  |    7   |     0.4     |
  | Lethal 1 miniature     |    814  |  323   |    39.7     |
  | White miniature        |    994  |  397   |    39.9     |
  |                        |         |        |             |
  +------------------------+---------+--------+-------------+

LETHAL 1a.

In the second generation of the flies bred by Miss Rawls, one female gave
(March 1912) only 3 sons, although she gave 312 daughters. It was not known
for some time (see lethals 3 and 3_a_) what was the cause of this extreme
rarity of sons. It is now apparent, however, that this mother carried
lethal 1 in one X and in the other X a new lethal which had arisen by
mutation. The new lethal was very close to lethal 1, as shown by the rarity
of the surviving sons, which are cross-overs between lethal 1 and the new
lethal that we may call lethal 1a. There is another class of cross-overs,
namely, those which have lethal 1 and get lethal 1_a_ by crossing-over.
These doubly lethal males must also die, but since they are theoretically
as numerous as the males (3) free from both lethals, we must double this
number (3 × 2) to get the total number of cross-overs. There were 312
daughters, but as the sons are normally about 96 per cent of the number of
the females, {33} we may take 300 as the number of the males which died.
There must have been, then, about 2 per cent of crossing-over, which makes
lethal 1_a_ lie about 2 units from lethal 1. This location of lethal 1_a_
is confirmed by a test that Miss Rawls made of the daughters of the
high-ratio female. Out of 98 of these daughters none repeated the high
sex-ratio and only 2 gave 1 [female] : 1 [male] ratios. The two daughters
which gave 1 : 1 ratios are cross-overs. There should be an equal number of
cross-overs which contain both lethals. These latter would not be
distinguishable from the non-cross-over females, each of which carries one
or the other lethal. In calculation, allowance can be made for them by
doubling the number of observed cross-overs (2 × 2) and taking 98 - 2 as
the number of non-cross-overs. The cross-over fraction {6 + 4}/{300 + 96}
gives 2.6 as the distance between the two lethals. Lethal 1_a_ is probably
to the right of lethal 1 at 0.7 + 2.6 = 3.3.

SPOT.

(Plate II, figures 14 to 17.)

In April 1912 there was found in the stock of yellow flies a male that
differed from yellow in that it had a conspicuous light spot on the upper
surface of the abdomen (Morgan, 1914_a_). In yellow flies this region is
dark brown in color. In crosses with wild flies the spot remained with the
yellow, and although some 30,000 flies were raised, none of the gray
offspring showed the spot, which should have occurred had crossing-over
taken place. The most probable interpretation of spot is that it was due to
another mutation in the yellow factor, the first mutation being from gray
to yellow and the second from yellow to spot.

Spot behaves as an allelomorph to yellow in all crosses where the two are
involved and is completely recessive to yellow, _i. e._, the yellow-spot
hybrid is exactly like yellow. A yellow-spot female, back-crossed to a spot
male, produces yellows and spots in equal numbers.

In a cross of spot to black it was found that the double recessive, spot
black, flies that appear in F_2 have, in addition to the spot on the
abdomen, another spot on the scutellum and a light streak on the thorax.
These two latter characters ("dot and dash") are very sharply marked and
conspicuous when the flies are young, but they are only juvenile characters
and disappear as the flies become older. The spot flies never show the "dot
and dash" clearly, and it only comes out when black acts as a developer.
These characters furnish a good illustration of the fact that mutant gens
ordinarily affect many parts of the body, though these secondary effects
often pass unnoticed.

In the F_2 of the cross of spot by black one yellow black fly appeared,
although none are expected, on the assumption that spot and yellow {34} are
allelomorphic. Unless due to crossing-over it must have been a mutation
from spot back to yellow. Improbable as this may seem to those who look
upon mutations as due to losses from the germ-plasm, yet we have records of
several other cases where similar mutations "backwards" have taken place,
notably in the case of eosin to white, under conditions where the
alternative interpretation of crossing-over is excluded.

SABLE.

(Plate I, figure 2.)

In an experiment involving black body-color[3] a fly appeared (July 19,
1912) whose body-color differed slightly from ordinary black in that the
trident mark on the thorax was sharper and the color itself was brighter
and clearer. This fly, a male, was mated to black females and gave some
black males and females, but also some gray (wild body-color) males and
females, showing not only that he was heterozygous for ordinary recessive
black, but at the same time that his dark color must be due to another kind
of black. The gray F_1 flies when mated together gave a series of gray and
dark flies in F_2 about as follows: In the females 3 grays to 1 dark; in
the males 3 grays to 5 dark in color. The result indicated that the new
black color, which we call sable, was due to a sex-linked factor. It was
difficult to discover which of the heterogeneous F_2 males were the new
blacks. Suspected males were bred (singly) to wild females, and the F_2
dark males, from those cultures that gave the closest approach to a 2 gray
[female] : 1 gray [male] : 1 dark [male], were bred to their sisters in
pairs in order to obtain sable females and males. Thus stock homozygous for
sable but still containing black as an impurity was obtained. It became
necessary to free it from black by successive individual out-crossings to
wild flies and extractions.

This account of how sable was purified shows how difficult it is to
separate two recessive factors that give closely similar somatic effects.
If a character like sable should be present in any other black stock, or if
a character like black should be present in sable, very erratic results
would be obtained if such stocks were used in experiments, before such a
population had been separated into its component races.

Sable males of the purified stock were mated to wild females and gave
wild-type (gray) males and females. These inbred gave the results shown in
table 6.

No sable females appeared in F_2, as seen in table 6. The reciprocal cross
gave the results shown in table 7.

{35}

The F_1 males were sable like their mother. The evidence thus shows that
sable is a sex-linked recessive character. Our next step was to determine
the linkage relations of sable to certain other sex-linked gens, namely,
yellow, eosin, cherry, vermilion, miniature, and bar.

TABLE 6.--_P_1 wild [female] [female] × sable [male]. F_1 wild-type
[female] [female] × F_1 wild-type [male] [male]._

  +---------------+-------------------+-------------------+---------------+
  |               |                   |                   |               |
  | Reference.[4] |Wild-type [female].| Wild-type [male]. | Sable [male]. |
  |               |                   |                   |               |
  +---------------+-------------------+-------------------+---------------+
  |               |                   |                   |               |
  |     88 C      |        218        |       100         |       70      |
  |    143 C      |        245        |       108         |       72      |
  |    146 C      |        200        |       115         |       82      |
  |               +-------------------+-------------------+---------------+
  |        Total  |        663        |       323         |      224      |
  |               |                   |                   |               |
  +---------------+-------------------+-------------------+---------------+

TABLE 7.--_P_1 sable [female] × wild [male] [male]. F_1 wild-type [female]
× F_1 sable [male]._

  +--------------+-------------+-------------+-------------+-------------+
  |              |             |             |             |             |
  | Reference.   | Wild-type   | Wild-type   |  Sable      |   Sable     |
  |              |  [female].  |  [male].    | [female].   |   [male].   |
  +--------------+-------------+-------------+-------------+-------------+
  |              |             |             |             |             |
  |       4 I    |      10     |      10     |     6       |     10      |
  |              |             |             |             |             |
  +--------------+-------------+-------------+-------------+-------------+

LINKAGE OF YELLOW AND SABLE.

The factor for yellow body-color lies at one end of the known series of
sex-linked gens. As already stated, we speak of this end as the left end of
the diagram, and yellow as the zero in locating factors.

When yellow (not-sable) females were mated to (not-yellow) sable males they
gave wild-type (gray) daughters and yellow sons. These inbred gave in F_2
two classes of females, namely, yellow and gray, and four classes of males,
namely, yellow and sable (non-cross-overs), wild type and the double
recessive yellow sable (cross-overs). From off-spring (F_3) of the F_2
yellow sable males by F_2 yellow females, pure stock of the double
recessive yellow sable was made up and used in the crosses to test linkage.

In color the yellow sable is quite similar to yellow black, that is, a rich
brown with a very dark brown trident pattern on the thorax. Yellow sable is
easier to distinguish from yellow than is yellow black, even when the flies
have not yet acquired their adult body-color.

Yellow sable males were bred to wild females and F_1 consisted of wild-type
males and females. These inbred gave the results shown in table 8. {36}

TABLE 8.--_P_1 wild [female] [female] × yellow sable [male] [male]. F_1
wild-type [female] [female] × F_1 wild-type [male] [male]._

  +-----------+---------+--------------+--------------+-------+-----------+
  |           |         |Non-cross-over|  Cross-over  |       |           |
  |           |  Wild-  |    [male].   |    [male].   |       |           |
  | Reference.|  type   +-------+------+-------+------+ Total |Cross-over |
  |           |[female].| Yellow| Wild-|       |      | males.|  value.   |
  |           |         | sable.| type.|Yellow.|Sable.|       |           |
  +-----------+---------+-------+------+-------+------+-------+-----------+
  |           |         |       |      |       |      |       |           |
  |    44 I   |    292  |  110  |   43 |   75  |  36  |  264  |     42    |
  |    45 I   |    384  |  104  |   58 |   71  |  60  |  293  |     45    |
  |           +---------+--------------+-------+------+-------+-----------+
  |    Total  |    676  |  214  |  101 |  146  |  96  |  557  |     43    |
  |           |         |       |      |       |      |       |           |
  +-----------+---------+-------+------+-------+------+-------+-----------+

Some of the F_1 females were back-crossed to yellow sable males and gave
the data for table 9.

TABLE 9.--_P_1 wild-type [female] [female] × yellow sable [male] [male]. B.
C. F_1 wild-type [female] × yellow sable [male] [male]._

  +----------+-------------------------+---------------+-------+----------+
  |          |                         |               |       |          |
  |          |     Non-cross-overs.    |  Cross-overs. |       |          |
  |          |                         |               |       |          |
  |Reference.+-----------+-------------+-------+-------+ Total.|Cross-over|
  |          |           |             |       |       |       |  value.  |
  |          | Wild-type.|Yellow sable.|Yellow.| Sable.|       |          |
  |          |           |             |       |       |       |          |
  +----------+-----------+-------------+-------+-------+-------+----------+
  |          |           |             |       |       |       |          |
  |  31 I    |    108    |      51     |   58  |   56  |   273 |    42    |
  |  49 I    |    265    |     175     |  161  |  169  |   770 |    43    |
  |          +-----------+-------------+-------+-------+-------+----------+
  |    Total |    373    |     226     |  219  |  225  | 1,043 |    43    |
  |          |           |             |       |       |       |          |
  +----------+-----------+-------------+-------+-------+-------+----------+

In these tables the last column (to the right) shows for each culture the
amount of crossing-over between yellow and sable. These values are found by
dividing the number of cross-overs by the total number of individuals which
might show crossing-over, that is, males only or both males and females, as
the case may be. Free assortment would give 50 per cent of cross-overs and
absolute linkage 0 per cent of cross-overs. Except where the percentage of
crossing-over is very small these values are expressed to the nearest unit,
since the experimental error might make a closer calculation misleading.

The combined data of tables 8 and 9 give 686 cross-overs in a total of
1,600 individuals in which crossing-over might occur. The females of table
8 are all of one class (wild type) and are useless for this calculation
except as a check upon viability. The cross-over value of 43 per cent shows
that crossing-over is very free. We interpret this to mean that sable is
far from yellow in the chromosome. Since yellow is at one end of the known
series, sable would then occupy a locus somewhere near the opposite end.
This can be checked up by finding its linkage relations to the other
sex-linked factors. {37}

LINKAGE OF CHERRY AND SABLE.

The origin of cherry eye-color (Plate II, fig. 9) has been given by Safir
(Biol. Bull., 1913). From considerations which will be discussed later in
this paper we regard cherry as allelomorphic to white in a quadruple
allelomorph system composed of white, eosin, cherry, and their normal red
allelomorph. Cherry will then occupy the same locus as white, which is one
unit to the right of yellow, and will show the same linkage relations to
other factors as does white. A slightly lower cross-over value should be
given by cherry and sable than was given by yellow and sable.

When cherry (gray) females were crossed to (red) sable males the daughters
were wild type and the sons cherry. Inbred these gave the results shown in
table 10.

TABLE 10.--_P_1 cherry [female][female] × sable [male][male]. F_1 wild-type
[female] × F_1 cherry [male] [male]._

  +---------+---------+---------+--------------+------------+------+------+
  |         |         |         |  Non-cross-  | Cross-over |      |      |
  |         |  Wild-  | Cherry  | over [male]. |   [male].  |      |Cross-|
  |  Refer- |  type   |[female].+-------+------+------+-----+Total | over |
  |  ence.  |[female].|         |Cherry.|Sable.|Cherry|Wild-|males.|value.|
  |         |         |         |       |      |sable.|type.|      |      |
  +---------+---------+---------+-------+------+------+-----+------+------+
  |         |         |         |       |      |      |     |      |      |
  | 24  I   |    94   |   105   |   51  |   42 |  20  |  43 |  156 |  40  |
  | 55  I   |   101   |   131   |   63  |   52 |  38  |  48 |  201 |  43  |
  | 55' I   |    96   |    94   |   52  |   31 |  29  |  30 |  142 |  42  |
  |         +---------+---------+-------+------+------+-----+------+------+
  | Total   |   291   |   330   |  166  |  125 |  87  | 121 |  499 |  42  |
  |         |         |         |       |      |      |     |      |      |
  +---------+---------+---------+-------+------+------+-----+------+------+

The percentage of crossing-over between cherry and sable is 42. Since
cherry is one point from yellow, this result agrees extremely well with the
value 43 for yellow and sable. Since yellow and eosin lie at the left end
of the first chromosome, the high values, namely, 43 and 42, agree in
making it very probable that sable lies near the other end (_i. e._, to the
right). Sable will lie farther to the right than vermilion, for vermilion
has been shown elsewhere to give 33 per cent of crossing-over with eosin.
The location of sable to the right of vermilion has in fact been
substantiated by all later work.

LINKAGE OF EOSIN, VERMILION, AND SABLE.

Three loci are involved in the next experiment. Since eosin is an
allelomorph of cherry, it should be expected to give with sable the same
cross-over value as did cherry. When eosin (red) sable females were crossed
to (red) vermilion (gray) males, the daughters were wild type and the males
were eosin sable. Inbred these gave the classes shown in table 11. {38}

TABLE 11.--_P_1 eosin sable [female] × vermilion [male][male]. F_1
wild-type [female][female] × F_1 eosin sable [male][male]._

  +------+--------------------------+
  |      |                          |
  |      |       F_2 females.       |
  |      |                          |
  |      +------------+-------------+
  |      | w^e      s | w^e         |
  |Refer-| ---------- | -----+----- |
  |      |            |           s |
  |ence. +------+-----+------+------+
  |      |      |     |      |      |
  |      |      |     |      |      |
  |      |Eosin |Wild-|Eosin.|Sable.~
  |      |sable.|type.|      |      ~
  |      |      |     |      |      |
  +------+------+-----+------+------+
  |      |      |     |      |      |
  | 26 I | 132  | 171 |  113 |  109 |
  | 26'I |  96  | 146 |   86 |   78 |
  |      +------+-----+------+------+
  |Total.| 228  | 317 |  199 |  187 |
  +------+------+-----+------+------+

  +------+---------------------------------------------------------+
  |      |                                                         |
  |      |                         F_2 males.                      |
  |      |                                                         |
  |      +---------------+--------------+-------------+------------+
  |      | w^e       s   | w^e    v     | w^e         | w^e  v   s |
  |Refer-| -----------   | -----+-----  | --------+-- | ---+---+-- |
  |      |        v      |           s  |       v   s |            |
  |ence. +-------+-------+-------+------+------+------+------+-----+
  |      |       |       |       |      |      |      |      |     |
  |      |       |       |Eosin  |      |      |Ver-  |Eosin |     |
  ~      | Eosin |Ver-   |ver-   |Sable.|Eosin.|milion|ver-  |Wild-|
  ~      | sable.|milion.|milion.|      |      |sable.|milion|type.|
  |      |       |       |       |      |      |      |sable.|     |
  +------+-------+-------+-------+------+------+------+------+-----+
  |      |       |       |       |      |      |      |      |     |
  | 26 I |  127  |  163  |  75   |  76  |   37 |  14  |   2  |  5  |
  | 26'I |   74  |  128  |  76   |  59  |   18 |  21  |   4  |  3  |
  |      +-------+-------+-------+------+------+------+------+-----+
  |Total.|  201  |  291  | 151   | 135  |   55 |  35  |   6  |  8  |
  +------+-------+-------+-------+------+------+------+------+-----+

If we consider the male classes of table 11, we find that the smallest
classes are eosin vermilion sable and wild type, which are the expected
double cross-over classes if sable lies to the right of vermilion, as
indicated by the crosses with eosin and with yellow. The classes which
represent single crossing-over between eosin and vermilion are eosin
vermilion, and sable, and those which represent single crossing-over
between vermilion and sable are eosin and vermilion sable. These relations
are seen in diagram II.

    w^e                                                V              s
  --+--------------------------------------------------+--------------+
                                     X                         X
  --+--------------------------------------------------+--------------+
    W                                                  v              S

DIAGRAM II.--The upper line represents an X chromosome, the lower line its
mate. The cross connecting lines indicate crossing-over between pairs of
factors.

                      w^e                      s   {Eosin sable.
  Non-cross-overs     --------------------------   {
                                         v         {Vermilion.

                      w^e                v         {Eosin vermilion.
  Single cross-overs  ------------+-------------   {
                                               s   {Sable.

                      w                            {Eosin.
                      -----------------------+--   {
                                         v     s   {Vermilion sable.

                      w^e                v     s   {Eosin vermilion sable.
  Double cross-overs  ------------+----------+--   {
                                                   {Wild-type.

If we consider the female classes of table 11, we get information as to the
cross-over value of eosin and sable, namely, 42 units. The male classes
will be considered in connection with the cross that follows.

The next experiment involves the same three gens which now enter in
different relations. A double recessive, eosin vermilion (gray) female {39}
was mated to (red red) sable males and gave 202 wild-type[5] females and
184 eosin vermilion males. Two F_1 pairs gave the results shown in table 12
(the four classes of females not being separated).

TABLE 12.--_P_1 eosin vermilion F_1 wild-type [female] × F_1 eosin
vermilion [male] [male]._

  +------+--------+-------------------------------------------------------+
  |      |        |                        F_2 males.                     |
  |      |        +-------------+-------------+-------------+-------------+
  |      |        | w^e   v     | w^e       s | w^e   v   s | w^e         |
  |      |        | ----------- | -----+----- | --------+-- | ----+---+-- |
  |Refer-| F_2    |           s |         v   |             |       v   s |
  | ence.|females.+------+------+------+------+------+------+------+------+
  |      |        |      |      |      |      |Eosin |      |      |      |
  |      |        |Eosin |      |      |      |verm- |Wild- |      |Verm- |
  |      |        |verm- |Sable.|Eosin |Verm- |ilion |type. |Eosin.|ilion |
  |      |        |ilion.|[male]|sable.|ilion.|sable.|[male]|[male]|sable.|
  |      |        |[male]|      |[male]|[male]|[male]|      |      |[male]|
  +------+--------+------+------+------+------+------+------+------+------+
  | 59 C |   133  |  40  |  33  |   7  |  16  |   5  |   5  |   2  |   1  |
  | 61 C |   101  |  34  |  26  |   8  |  11  |   3  |   7  |   1  |   0  |
  |      +--------+------+------+------+------+------+------+------+------+
  |Total |   234  |  74  |  59  |  15  |  27  |   8  |  12  |   3  |   1  |
  +------+--------+------+------+------+------+------+------+------+------+

If we combine the data for males given in table 12 with those of table 11,
we get the following cross-over values. Eosin vermilion, 32; vermilion
sable, 12; eosin sable, 41.

{40}

LINKAGE OF MINIATURE AND SABLE.

The miniature wing has been described (Morgan, Science, 1911) and the wing
figured (Morgan, Jour. Exp. Zool., 1911). The gen for miniature lies about
3 units to the right of vermilion, so that it is still closer to sable than
is vermilion. The double recessive, miniature sable, was made up, and males
of this stock were bred to wild females (long gray). The wild-type
daughters were back-crossed to double recessive males and gave the results
(mass cultures) shown in table 13.

TABLE 13.--_P_1 wild [female] [female] × miniature sable [male] [male]. B.
C. F_1 wild-type [female] [female] × miniature sable [male] [male]._

  +-----------+---------------------+-----------------+-------+-------+
  |           |                     |                 |       |       |
  |           |   Non-cross-overs.  |  Cross-overs.   |       |       |
  |           |                     |                 |       | Cross-|
  | Reference.+----------+----------+----------+------+ Total.|  over |
  |           |          |          |          |      |       | value.|
  |           |Miniature |Wild-type.|Miniature.|Sable.|       |       |
  |           |  sable.  |          |          |      |       |       |
  +-----------+----------+----------+----------+------+-------+-------+
  |           |          |          |          |      |       |       |
  |  38 I     |    245   |    283   |    15    |  17  |   560 |   6   |
  |  43 I     |    191   |    236   |    13    |  18  |   458 |   7   |
  |  46 I     |    232   |    274   |    24    |  21  |   551 |   8   |
  |           +----------+----------+----------+------+-------+-------+
  |   Total   |    668   |    793   |    52    |  56  | 1,569 |   7   |
  |           |          |          |          |      |       |       |
  +-----------+----------+----------+----------+------+-------+-------+

Since the results for the male and the female classes are expected to be
the same, the sexes were not separated. The combined data give 7 per cent
of crossing-over between miniature and sable.

LINKAGE OF VERMILION, SABLE, AND BAR.

Bar eye has been described by Mrs. S. C. Tice (1914). It is a dominant
sex-linked character, whose locus, lying beyond vermilion and sable, is
near the right end of the chromosome series, that is, at the end opposite
yellow.

In the first cross of a balanced series of experiments for the gens
vermilion, sable, and bar, vermilion (gray not-bar) entered from one side
([female]) and (red) sable bar from the other ([male]). The daughters were
bar and the sons vermilion. The daughters were back-crossed singly to the
triple recessive males vermilion sable (not-bar), and gave the data
included in table 14.

In the second cross, vermilion sable (not-bar) went in from one side
([female]) and (red, gray) bar from the other. The daughters were bar and
the sons were vermilion sable. Since these sons have the three recessive
factors, inbreeding of F_1 is equivalent to a triple back-cross. The
results are given by pairs in table 15. {41}

TABLE 14.--_P_1 vermilion [female] [female] × sable bar [male] [male]. B.
C. F_1 bar [female] × vermilion sable [male] [male]._

  +------+------------+-----------+------------+-----------+
  |      | v          | v    s B' | v       B' | v   s     |
  |      | ---------- | ---+----- | -----+---- | --+--+--- |
  |      |    s    B' |           |     s      |        B' |
  |      +------+-----+-----+-----+-----+------+------+----+
  |Refer-|      |     |Verm-|     |     |      |      |    |
  |ence. |Verm- |Sable|ilion|Wild-|Verm-|      |Ver-  |    |
  |      |ilion.| bar.|sable|type.|ilion|Sable.|milion|Bar.|
  |      |      |     | bar.|     | bar.|      |sable.|    |
  +------+------+-----+-----+-----+-----+------+------+----+
  |      |      |     |     |     |     |      |      |    |
  |147 I |   81 |  66 |  12 |  15 |  15 |  18  |      |    ~
  |148 I |  103 | 108 |   4 |  19 |  11 |  11  |      |    ~
  |149 I |   97 |  88 |  10 |   8 |  17 |  17  |   1  |  1 |
  |150 I |   95 |  75 |  10 |  11 |  21 |  22  |   1  |  1 |
  |151 I |  116 |  96 |  11 |  15 |  23 |  26  |      |  2 |
  | 89   |   89 |  94 |  10 |  19 |  15 |  11  |   1  |    |
  | 90   |   49 |  50 |   4 |   8 |  15 |  14  |      |    |
  | 91   |  104 |  88 |  13 |  15 |  12 |  12  |      |    |
  |      +------+-----+-----+-----+-----+------+------+----+
  |Total.|  734 | 665 |  74 | 110 | 129 | 131  |   3  |  4 |
  |      |      |     |     |     |     |      |      |    |
  +------+------+-----+-----+-----+-----+------+------+----+

  +------+------+------------------+
  |      |      |                  |
  |      |      |Cross-over values.|
  |      |      |                  |
  |      |      +------+-----+-----+
  |Refer-|Total.|      |     |     |
  |ence. |      |Verm- |     |Verm-|
  |      |      |ilion |Sable|ilion|
  |      |      |sable.| bar.|bar. |
  +------+------+------+-----+-----+
  |      |      |      |     |     |
  ~147 I |  207 |   13 |  16 |  29 |
  ~148 I |  256 |    9 |   9 |  18 |
  |149 I |  239 |    8 |  15 |  22 |
  |150 I |  236 |   10 |  19 |  27 |
  |151 I |  289 |   10 |  18 |  26 |
  | 89   |  239 |   13 |  11 |  23 |
  | 90   |  140 |    9 |  21 |  29 |
  | 91   |  244 |   11 |  10 |  21 |
  |      +------+------+-----+-----+
  |Total.|1,850 |   10 |  14 |  24 |
  |      |      |      |     |     |
  +------+------+------+-----+-----+

TABLE 15.--_P_1 vermilion sable [female] [female] × bar [male] [male]. B.
C. F_1 bar [female] × vermilion sable [male] [male]._

  +------+----------+------------+-----------+------------+
  |      | v s      |  v      B' | v  s   B' | v          |
  |      | -------- | ---+------ | -----+--- | --+---+--- |
  |      |       B' |      s     |           |     s   B' |
  |      +------+---+-----+------+-----+-----+------+-----+
  |Refer-|      |   |     |      |Verm-|     |      |     |
  |ence. |Verm- |   |Verm-|      |ilion|Wild-|Verm- |Sable|
  |      |ilion |Bar|ilion|Sable.|sable|type.|ilion.| bar.|
  |      |sable.|   | bar.|      | bar.|     |      |     |
  +------+------+---+-----+------+-----+-----+------+-----+
  |105 I |   41 | 75|  10 |   4  |   5 |  11 |      |     ~
  |106 I |   59 |122|  16 |  13  |  11 |  17 |      |     ~
  |107 I |   92 | 98|   8 |  12  |  16 |  10 |      |     |
  |116 I |  111 |149|  19 |  16  |  20 |  19 |      |   1 |
  |117 I |   92 |117|  16 |  14  |  15 |  18 |      |     |
  |126 I |   96 |160|  13 |  13  |  17 |  35 |      |     |
  |127 I |  117 |124|  13 |  25  |  24 |  30 |   1  |     |
  |      +------+---+-----+------+-----+-----+------+-----+
  |Total |  608 |845|  95 |  97  | 108 | 140 |   1  |   1 |
  +------+------+---+-----+------+-----+-----+------+-----+

  +------+------+------------------+
  |      |      |                  |
  |      |      |Cross-over values.|
  |      |      |                  |
  |      |      +------+-----+-----+
  |Refer-|Total.|      |     |     |
  |ence. |      |Verm- |     |Verm-|
  |      |      |ilion.|Sable|ilion|
  |      |      |sable.| bar.|bar. |
  +------+------+------+-----+-----+
  ~105 I |  146 |   10 |  11 |  21 |
  ~106 I |  238 |   12 |  12 |  24 |
  |107 I |  236 |    9 |  11 |  20 |
  |116 I |  335 |   11 |  12 |  22 |
  |117 I |  272 |   11 |  12 |  23 |
  |126 I |  334 |    8 |  15 |  23 |
  |127 I |  334 |   12 |  16 |  28 |
  |      +------+------+-----+-----+
  |Total |1,895 |   10 |  13 |  23 |
  +------+------+------+-----+-----+

{42}

In the third cross, vermilion (gray) bar entered from one side ([female])
and (red) sable (not-bar) from the other ([male]). The daughters are bar
and the sons vermilion bar. The daughters were back-crossed singly to
vermilion sable males and gave the data in table 16.

TABLE 16.--_P_1 vermilion bar_ [female] [female] × _sable_ [male] [male].
_B. C. F_1 bar_ [female] × _vermilion sable_ [male] [male].

  +-----------+--------------+------------+-------------+---------------+
  |           |    v   B'    |  v  s      |   v         |   v  s  B'    |
  |           |    -----     |  -+------  |   -----+--  |   -+---+--    |
  |Reference. |      s       |        B'  |       s B'  |               |
  |           +-------+------+------+-----+-------+-----+---------+-----+
  |           |  Ver- |Sable.| Ver- | Bar.| Ver-  |Sable|Vermilion|Wild-|
  |           | milion|      |milion|     |milion.|bar. |  sable  |type.|
  |           | bar.  |      |sable.|     |       |     |   bar.  |     |
  +-----------+-------+------+------+-----+-------+-----+---------+-----+
  | 129 I     |  132  |  147 |  15  | 15  |  19   |  21 |     1   |  1  ~
  | 130 I     |  194  |  168 |  21  | 17  |  28   |  25 |    ..   |  1  ~
  | 131 I     |  121  |   89 |  10  | 20  |  26   |  11 |     1   |  1  |
  | 137 I     |  139  |  113 |  19  | 12  |  33   |  14 |    ..   |  1  |
  | 138 I     |  131  |  128 |  11  | 11  |  28   |  24 |     1   |  .. |
  | 139 I     |   83  |   79 |   4  | 12  |  17   |  12 |    ..   |  .. |
  |           +-------+------+------+-----+-------+-----+---------+-----+
  |   Total.  |  800  |  724 |  80  | 87  | 151   | 107 |     3   |  4  |
  +-----------+-------+------+------+-----+-------+-----+---------+-----+

  +-----------+-------+-------------------------+
  |           |       |                         |
  |           |       |                         |
  |Reference. | Total.|  Cross-over values.     |
  |           |       +---------+-----+---------+
  |           |       |Vermilion|Sable|Vermilion|
  |           |       |sable.   |bar. |bar.     |
  +-----------+-------+---------+-----+---------+
  ~ 129 I     |   351 |    9    |  12 |   20    |
  ~ 130 I     |   454 |    9    |  12 |   20    |
  | 131 I     |   279 |   12    |  14 |   24    |
  | 137 I     |   331 |   10    |  15 |   24    |
  | 138 I     |   334 |    7    |  16 |   22    |
  | 139 I     |   207 |    8    |  14 |   22    |
  |           +-------+---------+-----+---------+
  |   Total.  | 1,956 |    9    |  14 |   22    |
  +-----------+-------+---------+-----+---------+

In the fourth cross, vermilion sable bar entered from one side, and (red
gray not-bar) wild type from the other. The daughters were bar and the sons
vermilion sable bar. The daughters were back-crossed singly to vermilion
sable males, with the results shown in table 17.

TABLE 17.--_P_1 vermilion sable bar_ [female] [female] × _wild_ [male]
[male]. _B. C. F_1 bar_ [female] × _vermilion sable_ [male] [male].

  +-----------+---------------+--------------+------------+--------------+
  |           |    v  s  B'   |    v         |  v  s      |    v   B'    |
  |           |    --------   |    -+-----   |  -----+--  |    -+-+--    |
  | Reference.|               |      s  B'   |        B'  |      s       |
  |           +---------+-----+--------+-----+-------+----+-------+------+
  |           |Vermilion|Wild-|  Ver-  |Sable|  Ver- |Bar.|  Ver- |Sable.|
  |           |  sable  |type | milion.|bar. | milion|    | milion|      |
  |           |  bar.   |     |        |     | sable.|    |  bar. |      |
  +-----------+---------+-----+--------+-----+-------+----+-------+------+
  | 132 I     |    95   | 108 |   10   |  13 |   24  | 22 |  ..   |  ..  ~
  | 133 I     |   112   | 150 |   18   |  16 |   26  | 16 |  1    |  2   ~
  | 134 I     |    84   |  95 |   14   |   7 |   15  | 16 |  ..   |  1   |
  | 135 I     |   100   |  86 |   16   |  17 |   19  | 22 |  ..   |  1   |
  | 152 I     |    73   |  88 |   12   |   8 |   14  | 18 |  ..   |  ..  |
  | 153 I     |   114   | 138 |   12   |  12 |   17  | 17 |  ..   |  ..  |
  | 154 I     |    63   |  90 |   10   |   8 |    8  | 15 |  ..   |  ..  |
  |           |         |     |        |     |       |    |       |      |
  |   Total.  |   641   | 755 |   92   |  81 |  123  |126 |  1    |  4   |
  +-----------+---------+-----+--------+-----+-------+----+-------+------+

  +-----------+------+-------------------------+
  |           |      |                         |
  |           |      |                         |
  | Reference.|Total.|  Cross-over values.     |
  |           |      +---------+-----+---------+
  |           |      |Vermilion|Sable|Vermilion|
  |           |      |sable.   |bar. |bar.     |
  +-----------+------+---------+-----+---------+
  ~ 132 I     |  272 |    9    |  17 |    25   |
  ~ 133 I     |  341 |   11    |  13 |    22   |
  | 134 I     |  232 |   10    |  14 |    22   |
  | 135 I     |  261 |   13    |  16 |    28   |
  | 152 I     |  213 |    9    |  15 |    24   |
  | 153 I     |  310 |    8    |  11 |    19   |
  | 154 I     |  194 |    9    |  12 |    21   |
  |           |      |         |     |         |
  |   Total.  |1,823 |   10    |  14 |    23   |
  +-----------+------+---------+-----+---------+

{43}

In tables 14 to 17 the calculations for the three cross-over values for
vermilion, sable, and bar are given for the separate cultures and for the
totals. The latter are here repeated.

  +-----------+-----------+---------+-----------+
  | From--    | Vermilion |  Sable  | Vermilion |
  |           |  sable.   |   bar.  |    bar.   |
  +-----------+-----------+---------+-----------+
  | Table 14  |    10     |    14   |     24    |
  |       15  |    10     |    13   |     23    |
  |       16  |     9     |    14   |     22    |
  |       17  |    10     |    14   |     23    |
  +-----------+-----------+---------+-----------+

The results of the different experiments are remarkably uniform. There can
be no doubt that the cross-over value is independent of the way in which
the experiment is made, whether any two recessives enter from the same or
from opposite sides.

TABLE 18.--_Linkage of vermilion, sable, and bar with balanced viability._

  +---------------------+---------+---------+---------+---------+-------+
  |                     | ------- | --+---- | ----+-- | --+-+-- | Total.|
  +---------------------+---------+---------+---------+---------+-------+
  | Wild-type           |   755   |   110   |   140   |    4    |       |
  | Vermilion           |   734   |    92   |   151   |    1    |       |
  | Sable               |   724   |    97   |   131   |    4    |       |
  | Bar                 |   845   |    87   |   126   |    4    |       |
  | Vermilion sable     |   608   |    80   |   123   |    3    |       |
  | Vermilion bar       |   800   |    95   |   129   |    1    |       |
  | Sable bar           |   665   |    81   |   107   |    1    |       |
  | Vermilion sable bar |   641   |    74   |   108   |    3    |       |
  |                     +---------+---------+---------+---------+-------+
  |     Total           | 5,772   |   716   | 1,015   |   21    | 7,524 |
  |     Percentage      |  76.7   |  9.53   | 13.49   | 0.28    |       |
  +---------------------+---------+---------+---------+---------+-------+

In table 18 the data from each of the four separate experiments have been
combined in the manner explained, so that viability is canceled to the
greatest extent. The amount of each kind of cross-over appears at the
bottom of the table. The total amount of crossing-over between vermilion
and sable is the sum of the single (9.53) and of the double (0.28)
cross-overs, which value is 9.8. Likewise the cross-over value for sable
bar is 13.49 + 0.28 (= 14), and for vermilion bar is 9.53 + 13.49 (= 23).
By means of these cross-over values we may calculate the coincidence
involved, which is in this case

            0.0028 × 100
  --------------------------------- = 20.8
  0.0953 + 0.0028 × 0.1349 + 0.0028

This value shows that there actually occurs only about 21 per cent of the
double cross-overs which from the values of the single cross-overs are
expected to occur in this section of the chromosome. This is the result
which is to be anticipated upon the chromosome view, for if crossing-over
is connected with loops of the chromosomes, and if these loops have an
average length, then if the chromosomes cross over at one {44} point it is
unlikely they will cross over again at another point nearer than the
average length of the loop.

The calculation of the locus for sable gives 43.0.

DOT.

In the F_2, from a cross of a double recessive (white vermilion) female by
a triple recessive (eosin vermilion pink) male, there appeared, July 21,
1912, three white-eyed females which had two small, symmetrically placed,
black, granular masses upon the thorax. These "dots" appeared to be dried
exudations from pores. It did not seem possible that such an effect could
be inherited, but as this condition had never been observed before, it
seemed worth while to mate the three females to their brothers. In the next
generation about 1 per cent of the males were dotted. From these females
and males a stock was made up which in subsequent generations showed from
10 to 50 per cent of dot. Selection seemed to have no effect upon the
percentage of dot. Although the stock never showed more than 50 per cent of
dot, yet it was found that the normal individuals from the stock threw
about the same per cent as did those that were dotted, so that the stock
was probably genetically pure. The number of males which showed the
character was always much smaller than the number of dotted females; in the
hatches which produced nearly 50 per cent of dot, nearly all the females
but very few of the males were dotted. Quite often the character showed on
only one side of the thorax.

Since this character arose in an experiment involving several eye-colors an
effort was made by crossing to wild and extracting to transfer the dot to
flies normal in all other respects. This effort succeeded only partly, for
a stock was obtained which differed from the wild type only in that it bore
dot (about 30 per cent) and in that the eyes were vermilion. Several
attempts to get the dot separated from vermilion failed. Since this was
only part of the preliminary routine work necessary to get a mutant stock
in shape for exact experimentation, no extensive records were kept.

LINKAGE OF VERMILION AND DOT.

When a dot male with vermilion eyes was bred to a wild female the offspring
were wild-type males and females. These inbred gave the data shown in table
19.

TABLE 19.--_P_1 vermilion dot [male] × wild [female] [female]. F_1
wild-type [female] [female] × F_1 wild-type [male] [male]._

  +------------+----------+-----------+-----------+-------------+---------+
  | Reference. | F_2      | Wild-type | Vermilion | Vermilion   | Dot     |
  |            | females. | [male].   | [male].   | dot [male]. | [male]. |
  +------------+----------+-----------+-----------+-------------+---------+
  |  7         |   345    |    151    |    130    |      0      |    0    |
  |  8         |   524    |    245    |    220    |      3      |    0    |
  |            +----------+-----------+-----------+-------------+---------+
  |     Total. |   869    |    396    |    350    |      3      |    0    |
  +------------+----------+-----------+-----------+-------------+---------+

{45}

Only three dot individuals appeared in F_2, but since these were males the
result indicates that the dot character is due to a sex-linked gen. These
three males had also vermilion eyes, indicating linkage of dot and
vermilion. The males show no deficiency in numbers, therefore the
non-appearance of the dot can not be due to its being semi-lethal. It
appears, therefore, that the expression of the character must depend on the
presence of an intensifying factor in one of the autosomes, or more
probably, like club, it appears only in a small percentage of flies that
are genetically pure for the character.

The reciprocal cross (dot female with vermilion eyes by wild male) was made
(table 20). The daughters were wild type and the sons vermilion. Not one of
the 272 sons showed dot. If the gen is sex-linked the non-appearance of dot
in the F_1 males can be explained on the ground that males that are
genetically dot show dot very rarely, or that its appearance is dependent
upon the intensification by an autosomal factor of the effect produced by
the sex-linked factor for dot.

TABLE 20.--_P_1 vermilion dot [female] × wild [male]._

  A = Wild-type [female].
  B = Vermilion [male].
  C = Wild-type [male].
  D = Wild-type [female].
  E = Vermilion [male].
  F = Vermilion [female].
  G = Vermilion dot [male].
  H = Vermilion dot [female].
  I = Dot [male].
  J = Dot [female].

  +--------------------++-----------------------------------------------+
  |  First generation. ||              Second generation.               |
  +----------+----+----++----------+----+----+----+----+----+---+---+---+
  |Reference.| A  | B  ||Reference.| C  | D  | E  | F  | G  | H | I | J |
  +--------------------++----------+----+----+----+----+----+---+---+---+
  |  137 C.  | 44 | 45 ||   19     |211 |198 |228 |206 | 20 | 3 | 0 | 0 |
  |  138 C.  | 77 | 62 ||   22     |266 |220 |227 |227 | 16 | 0 | 0 | 0 |
  |          |124 |124 ||   28     |143 |149 |125 |124 | 14 | 1 | 0 | 0 |
  |          | 57 | 41 ||          +----+----+----+----+----+---+---+---+
  |          |----|----||    Total.|620 |567 |570 |557 | 50 | 4 | 0 | 0 |
  |    Total.|291 |272 ||          |    |    |    |    |    |   |   |   |
  +--------------------++----------+----+----+----+----+----+---+---+---+

The F_2 generation is given in table 20. The dot reappeared in F_2 both in
females and in males, but instead of appearing in 50 per cent of both
sexes, as expected if it is simply sex-linked, it appeared in 4.0 per cent
in the females and in only 0.4 per cent in the males. The failure of the
character to be fully realized is again apparent, but here, where it is
possible for it to be realized equally in males and females, we find that
there are 50 females with dot to only 4 dot males. This would indicate that
the character is partially "_sex-limited_" (Morgan, 1914_d_) in its
realization. The dot appeared only in flies with vermilion eyes, indicating
extremely strong linkage between vermilion and dot.

The evidence from the history of the stock, together with these
experiments, shows that the character resembles club (wing) in that it is
not expressed somatically in all the flies which are homozygous for it. In
the case of club we were fortunate enough to find a constant feature {46}
which we could use as an index, but, so far as we have been able to see,
there is no such constant accessory character in the case of the dot.
Unlike club, dot is markedly sex-limited in its effect; that is, there is a
difference of expression of the gen in the male and female. This difference
recalls the sexual dimorphism of the eosin eye.

BOW.

In an F_2 generation from rudimentary males by wild females there appeared,
August 15, 1912, a single male whose wings instead of being flat were
turned down over the abdomen (fig. c). The curvature was uniform throughout
the length of the wing. A previous mutation, arc, of this same type had
been found to be a recessive character in the second group. The new
mutation, bow, is less extreme than arc and is more variable in the amount
of curvature. When the bow male was mated to wild females the offspring had
straight wings.

[Illustration: FIG. C.--Bow wing.]

TABLE 21.--_P_1 bow [male][male] × wild [female][female]._

  +------------------------------------------+
  |           First generation.              |
  +----------+-----------------+-------------+
  |Reference.|    Wild-type    |  Wild-type  ~
  |          |[female][female].|[male][male].~
  +----------+-----------------+-------------+
  |  169 C.  |       17        |     17      |
  +----------+-----------------+-------------+

  +--------------------------------------------------------+
  |              Second generation.                        |
  +----------+-----------------+-------------+-------------+
  ~Reference.|   Wild-type     |  Wild-type  |    Bow      |
  ~          |[female][female].|[male][male].|[male][male].|
  +----------+-----------------+-------------+-------------+
  |   18 I.  |       193       |     145     |     67      |
  |   21 I   |       182       |     100     |     49      |
  |          +-----------------+-------------+-------------+
  |    Total.|       375       |     245     |    116      |
  +----------+-----------------+-------------+-------------+

{47}

The F_2 ratio in table 21 is evidently the 2:1:1 ratio typical of
sex-linkage, but with the bow males running behind expectation. This
deficiency is due in part to viability but more to a failure to recognize
all the bow-winged individuals, so that some of them were classified among
the not-bow or straight wings. In favor of the view that the classification
was not strict is the fact that the sum of the two male classes about
equals the number of the females.

BOW BY ARC.

When this mutant first appeared its similarity to arc led us to suspect
that it might be arc itself or an allelomorph of arc. It was bred,
therefore, to arc. The bow male by arc females gave straight (normal)
winged males and females. The appearance of straight wings shows that bow
is not arc nor allelomorphic to arc. When made later, the reciprocal cross
of bow female by arc male gave in F_1 straight-winged females but bow
males. This result is in accordance with the interpretation that bow is a
sex-linked recessive. Further details of these last two experiments may now
be given. The F_1 (wild-type) flies from bow male by arc female were
inbred. The data are given in table 22.

TABLE 22.--_P_1 bow [male] × arc [female]._

  +--------------------------------------------+
  |            First generation.               |
  +----------+------------------+--------------+
  |Reference.|    Wild-type     |  Wild-type   ~
  |          |[female] [female].|[male] [male].~
  +----------+------------------+--------------+
  |   71 C.  |       48         |      43      |
  |   75 C.  |       28         |      27      |
  |          +------------------+--------------+
  |    Total.|       76         |      70      |
  +----------+------------------+--------------+

  +------------------------------+
  |      Second generation.      |
  +----------+---------+---------+
  ~Reference.|Straight.|  Not-   |
  ~          |         |straight.|
  +----------+---------+---------+
  |   71 C.  |   179   |   133   |
  +----------+---------+---------+

Bow and arc are so much alike that they give a single rather variable
phenotypic class in F_2. Therefore the F_2 generation is made up of only
two separable classes--flies with straight wings and flies with
not-straight wings. The ratio of the two should be theoretically 9:7, which
is approximately realized in 179:133.

If the distribution of the characters according to sex is ignored, the case
is similar to the case of the two white races of sweet peas, which bred
together gave wild-type or purple peas in F_1 and in F_2 gave 9 colored to
7 white. If sex is taken into account, the theoretical expectation for the
F_2 females is 6 straight to 2 arc, and for the F_2 males 3 straight to 1
arc to 3 bow to 1 bow-arc.

The F_1 from bow females by arc male and their F_2 offspring are given in
table 23. {48}

TABLE 23.--_P_1 bow [female] × arc [male]._

  +--------------------------------------------+
  |           First generation.                |
  |----------+------------------+--------------+
  |Reference.|    Wild-type     |     Bow      |
  |          |[female] [female].|[male] [male].|
  |----------+------------------+--------------+
  |  72 C.   |       22         |      19      ~
  |  73 C.   |       12         |      10      ~
  |   5 I.   |       22         |      21      |
  |  74 C.   |       56         |      52      |
  |          |------------------+--------------+
  |    Total.|      112         |     102      |
  +----------+------------------+--------------+

  +------------------------------+
  |         Second generation.   |
  +----------+---------+---------+
  |Reference.|Straight.|  Not-   |
  |          |         |straight.|
  +----------+---------+---------+
  ~    3 I.  |   56    |   69    |
  ~  3.1 I.  |   46    |   62    |
  |    5 I.  |   56    |   68    |
  |  5.1 I.  |   90    |  108    |
  +----------+---------+---------+
  |    Total.|  248    |  307    |
  +----------+---------+---------+

In this case the F_2 expectation is 6 straight to 10 not-straight. Since
the sex-linked gen bow entered from the female, half the F_2 males and
females are bow. The half that are not-bow consist of 3 straight to 1 arc,
so that both in the female classes and in the male classes there are 3
straight to 5 not-straight or in all 6 straight to 10 not-straight. The
realized result, 248 straight to 307 not-straight, is more nearly a 3:4
ratio, due probably to a wrong classification of some of the bow as
straight.

LEMON BODY-COLOR.

(Plate I, figure 3.)

A few males of a new mutant with a lemon-colored body and wings appeared in
August 1912. The lemon flies (Plate II, fig. 3) resemble quite closely the
yellow flies (Plate II, fig. 4). They are paler and the bristles, instead
of being brown, are black. These flies are so weak that despite most
careful attention they get stuck to the food, so that they die before
mating. The stock was at first maintained in mass from those cultures that
gave the greatest percentage of lemon flies. In a few cases lemon males
mated with their gray sisters left offspring, but the stock obtained in
this way had still to be maintained by breeding heterozygotes, as stated
above. But from the gray sisters heterozygous for lemon (bred to lemon
males) some lemon females were also produced.

LINKAGE OF CHERRY, LEMON, AND VERMILION.

In order to study the linkage of lemon, the following experiment was
carried out. Since it was impracticable to breed directly from the lemon
flies, virgin females were taken from stock throwing lemon, and were mated
singly to cherry vermilion males. Only a few of the females showed
themselves heterozygous for lemon by producing lemon as well as gray sons.
Half the daughters of such a pair are expected to be heterozygous for lemon
and also for cherry and vermilion, which went in from the father. These
daughters were mated singly to cherry vermilion males, and those that gave
some lemon sons were continued, {49} and are recorded in table 24. The four
classes of females were not separated from each other, but the total of
females is given in the table.

TABLE 24.--_P_1 lemon (het.) [female] × cherry vermilion [male] [male]. F_1
wild-type [female] × cherry vermilion [male] [male]._

  +-------+--------------+-------------+-------------+-------------+------+
  |       |  W^c     V   |  W^c l_m    |  W^c        |  W^c l_m V  |      |
  |       |  ----------  |  ---+------ | ------+---  | ---+----+---|      |
  |       |     l_m      |           V |    l_m  V   |             |      |
  |Females+-------+------+------+------+------+------+-------+-----+ Total|
  |       |Cherry |      |Cherry| Ver- |Cherry|Lemon |Cherry |Wild |[male]|
  |       | ver-  |Lemon.|lemon.|milion|      | ver- |lemon  |type.|[male]|
  |       |milion.|      |      |      |      |milion| ver-  |     |      |
  |       |       |      |      |      |      |      |milion.|     |      |
  +-------+-------+------+------+------+------+------+-------+-----+------+
  |   71  |   42  |  19  |  2   |   6  |  3   |  6   |  0    |  0  |  78  |
  |   88  |   26  |  19  |  2   |   8  |  8   |  4   |  0    |  0  |  67  |
  |   36  |   28  |   7  |  0   |   2  |  1   |  0   |  0    |  0  |  38  |
  |   51  |   12  |  22  |  0   |   4  |  4   |  4   |  0    |  0  |  46  |
  |   98  |   29  |  35  |  0   |   8  |  5   |  1   |  0    |  0  |  78  |
  |   47  |   17  |  11  |  0   |   1  |  3   |  2   |  0    |  0  |  34  |
  |   46  |   23  |  20  |  1   |   6  |  5   |  2   |  0    |  0  |  57  |
  +-------+-------+------+------+------+------+------+-------+-----+------+
  |  437  |  177  | 133  |  5   |  35  | 29   | 19   |  0    |  0  | 398  |
  +-------+-------+------+------+------+------+------+-------+-----+------+

There are three loci involved in this cross, namely, cherry, lemon, and
vermilion. Of these loci two were known, cherry and vermilion. The data are
consistent with the assumption that the lemon locus is between cherry and
vermilion, for the double cross-over classes (the smallest classes) are
cherry lemon vermilion and wild type. The number of single cross-overs
between cherry and lemon and between lemon and vermilion are also
consistent with this assumption. Since lemon flies fail to emerge
successfully, depending in part upon the condition of the bottle, the
classes involving lemon are worthless in calculating crossing-over and are
here ignored. In other words, lemon may be treated as though it did not
appear at all, _i. e._, as a lethal. The not-lemon classes--cherry,
vermilion, cherry vermilion, and wild type--give the following approximate
cross-over values for the three loci involved: Cherry lemon, 15; lemon
vermilion, 12; cherry vermilion, 27. The locus of lemon, calculated by
interpolation, is at about 17.5.

LETHAL 2.

In September 1912 a certain wild female produced 78 daughters and only 16
sons (Morgan, 1914_b_); 63 of these daughters were tested and 31 of them
gave 2 females to 1 male, while 32 of them gave 1:1 sex-ratios. This shows
that the mother of the original high sex-ratio was heterozygous for a
recessive sex-linked lethal. In order to determine the position of this
lethal, a lethal-bearing female was bred to an eosin (or white) miniature
male, and those daughters that were heterozygous for eosin, lethal, and
miniature were then back-crossed to {50} eosin miniature males. The
daughters that result from such a cross give only the amount of
crossing-over between eosin and miniature (as 29.7), but the males give the
cross-over values for eosin lethal (9.9), lethal miniature (15.4), and
eosin miniature (25.1). The data for this cross are given in table 25.

TABLE 25.--_Total data upon linkage of eosin, lethal 2, and miniature, from
Morgan, 1914b._

  +------------------------------------+
  |             Females.               |
  +--------+--------------+------------+
  |        |              |            |
  | Total. | Cross-overs. | Cross-over |
  |        |              | value.     ~
  |        |              |            ~
  +--------+--------------+------------+
  | 15,904 |    4,736     |    29.7    |
  +--------+--------------+------------+

  +-----------------------------------------------------------------------+
  |                                 Males.                                |
  +--------+--------+--------+---------+----------------------------------+
  |w^e    m|w^e l_2 |w^e     |w^e l_2 m|        Cross-over values.        |
  |--------|---+----|------+-|---+---+-+----------+-----------+-----------+
  ~    l_2 |       m|   l_2 m|         | Eosin    | Lethal 2  | Eosin     |
  ~        |        |        |         | lethal 2.| miniature.| miniature.|
  +--------+--------+--------+---------+----------+-----------+-----------+
  |  5,045 |   653  |  1,040 |    14   |    9.9   |    15.4   |    25.1   |
  +--------+--------+--------+---------+----------+-----------+-----------+

A similar experiment, in which eosin and vermilion were used instead of
eosin and miniature, is summarized in table 26.

TABLE 26.--_Total data upon the linkage of eosin, lethal 2, and vermilion,
from Morgan, 1914b._

  +------------------------------------+
  |            Females.                |
  +--------+--------------+------------+
  |        |              |            |
  | Total. | Cross-overs. | Cross-over |
  |        |              | value.     ~
  |        |              |            ~
  +--------+--------------+------------+
  |  2,656 |      729     |    27.5    |
  +--------+--------------+------------+

  +-----------------------------------------------------------------------+
  |                                Males.                                 |
  +--------+--------+--------+---------+----------------------------------+
  |w^e    v|w^e l_2 |w^e     |w^e l_2 v|        Cross-over values.        |
  |--------|---+----|------+-|---+---+-+----------+-----------+-----------+
  ~    l_2 |       v|   l_2 v|         | Eosin    | Lethal 2  | Eosin     |
  ~        |        |        |         | lethal 2.| vermilion.| vermilion.|
  +--------+--------+--------+---------+----------+-----------+-----------+
  |   902  |   124  |   227  |    6    |    10.3  |    18.5   |    27.9   |
  +--------+--------+--------+---------+----------+-----------+-----------+

Considerable data in which lethal was not involved were also obtained in
the course of these experiments and are included in the summary of the
total data given in table 27.

TABLE 27.--_Summary of all data upon lethal 2, from Morgan, 1914b._

  +--------------------+--------+--------------+------------+
  | Gens.              | Total. | Cross-overs. | Cross-over |
  |                    |        |              | values.    |
  +--------------------+--------+--------------+------------+
  | White lethal 2     |  8,011 |      767     |     9.6    |
  | White vermilion    |  6,023 |    1,612     |    26.8    |
  | White miniature    | 36,021 |   11,048     |    30.7    |
  | Lethal 2 vermilion |  1,400 |      248     |    17.7    |
  | Lethal 2 miniature |  6,752 |    1,054     |    15.4    |
  +--------------------+--------+--------------+------------+

The amount of crossing-over between eosin and lethal is about 10 per cent
and the amount of crossing-over between lethal and miniature is about 18
per cent. Since the amount of crossing-over between eosin {51} and
miniature is over 30 per cent, the lethal factor must lie between eosin and
miniature, somewhat nearer to eosin. It is impossible at present to locate
lethal 2 accurately because of a real discrepancy in the data, which makes
it appear that lethal 2 extends for a distance of about 5 units along the
chromosome from about 10 to about 15. Work is being done which it is hoped
will make clear the reason for this. For the present we may locate lethal 2
at the midpoint of its range, or at 12.5.

CHERRY.

(Plate II, figure 9.)

The origin of the eye-color cherry has been given by Safir (Biol. Bull.,
1913).

Cherry appeared (October 1912) in an experiment involving vermilion
eye-color and miniature wings. This is the only time the mutant has ever
come up, and although several of this mutant (males) appeared in Safir's
experiment, they may have all come from the same mother. It is probable
that the mutation occurred in the vermilion stock only a generation or so
before the experiment was made, for otherwise cherry would be expected to
be found also in the vermilion stock from which the mothers were taken;
however, it was not found.

A SYSTEM OF QUADRUPLE ALLELOMORPHS.

Safir has described crosses between this eye-color and red, white, eosin,
and vermilion. We conclude for reasons similar to those given by Morgan and
Bridges (Jour. Exp. Zool., 1913) for the case of white and eosin, that
cherry is an allelomorph of white and of eosin. This is not the
interpretation followed in Safir's paper, where cherry is treated as though
absolutely linked to white or to eosin. Both interpretations give, however,
the same numerical result for each cross considered by itself. Safir's data
and those which appear in this paper show that white, eosin, cherry, and a
normal (red) allelomorph form a system of quadruple allelomorphs. If this
interpretation is correct, then the linkage relations of cherry should be
identical with those of white or of eosin.

LINKAGE OF CHERRY AND VERMILION.

The cross-over value for white (eosin) and vermilion, based on a very large
amount of data, is about 31 units. An experiment of our own in which cherry
was used with vermilion gave a cross-over value of 31 units, which is a
close approximation to the cross-over value of white and vermilion. The
cross which gave this data was that of a cherry vermilion (double
recessive) male by wild females. The F_{1} wild-type flies inbred gave a
single class of females (wild-type) and the males in four classes which
show by the deviation from a 1:1:1:1 ratio the amount of crossing-over
involved. {52}

In one of the F_{2} male classes of table 28 the simple eye-color cherry
appeared for the first time (since the original mutant was vermilion as
well as cherry). Safir has recorded a similar cross with like results.

TABLE 28.--_P_{1} cherry vermilion [male] [male] × wild [female] [female].
F_{1} wild-type [female] [female] × F_{1} wild-type [male] [male]._

  +----------+---------+----------------+---------------+-------+------+
  |          |         | Non-cross-over |  Cross-over   |       |      |
  |          |         |     [male].    |    [male].    |       |      |
  |          |Wild-type+----------+-----+-------+-------+Total  |Cross-|
  |Reference.|[female] | Cherry   |Wild-|Cherry.| Ver-  |[male] |over  |
  |          |[female].|vermilion.|type.|       |milion.|[male].|value.|
  +----------+---------+----------+-----+-------+-------+-------+------+
  |  160 C   |  188    |   57     |  61 |   32  |  34   |  184  |  36  |
  |  161 C   |  256    |   85     |  93 |   40  |  52   |  270  |  34  |
  |  162 C   |  251    |   78     |  78 |   20  |  37   |  213  |  26  |
  |  163 C   |  229    |   76     |  95 |   34  |  33   |  238  |  28  |
  +----------+---------+----------+-----+-------+-------+-------+------+
  |  Total   |  924    |  296     | 327 |  126  | 156   |  905  |  31  |
  +----------+---------+----------+-----+-------+-------+-------+------+

Some cherry males were bred to wild females. The F_{1} wild-type males and
females inbred gave the results shown in table 29. Some of the cherry males
thus produced were bred to their sisters. Cherry females as well as males
resulted; and it was seen that the eye-color is the same in the males and
females, in contradistinction to the allelomorph eosin, where there is a
marked bicolorism (figs. 7, 8, Plate II). The cherry eye-color is almost
identical with that of the eosin female, but is perhaps slightly more
translucent and brighter.

TABLE 29.--_P_{1} cherry [male] [male] × wild [female] [female]. F_{1}
wild-type [female] [female] × F_{1} wild-type [male] [male]._

  +------------+---------------------+-------------------+----------------+
  | Reference. | Wild-type [female]. | Wild-type [male]. | Cherry [male]. |
  +------------+---------------------+-------------------+----------------+
  |  15 I      |        266          |       120         |     100        |
  +------------+---------------------+-------------------+----------------+

  +------------+-------------------------------------+
  |            |        First generation.            |
  | Reference. +--------------------+----------------+
  |            | White-cherry       |                |
  |            | compound [female]. | Cherry [male]. |
  +------------+--------------------+----------------+
  | 9 M        |        321         |       302      |
  +------------+--------------------+----------------+

Eosin-cherry compound was also made. An eosin female was mated to a cherry
male. The eosin-cherry daughters were darker than their eosin brothers.
Inbred they gave the results shown in table 31.

TABLE 31.--_P_1 eosin [female] × cherry [male]._

  +------------------------------------------+
  |              First generation.           |
  +------------+-------------------+---------+
  |            | Eosin-cherry      | Eosin   |
  | Reference. | compound          | [male]  |
  |            | [female][female]. | [male]. ~
  |            |                   |         ~
  +------------+-------------------+---------+
  |  43C       |         71        |    58   |
  +------------+-------------------+---------+

  +----------------------------------------------------+
  |               Second generation.                   |
  +------------+-------------------+---------+---------+
  |            | Eosin and         |         |         |
  | Reference. | eosin-cherry      | Cherry  | Eosin   |
  ~            | compound          | [male]. | [male]. |
  ~            | [female][female]. |         |         |
  +------------+-------------------+---------+---------+
  |  1I        |        154        |    99   |    62   |
  |  2I        |        174        |    74   |    77   |
  |            +-------------------+---------+---------+
  |            |        328        |   173   |   139   |
  +------------+-------------------+---------+---------+

Although in the F_2 results there are two genotypic classes of females,
namely, pure eosin and eosin-cherry compound, the eye-colors are so nearly
the same that they can not be separated. The two classes of males can be
readily distinguished; of these, one class, cherry, has the same color as
the females, while the other class, eosin, is much lighter. Such an F_2
group will perpetuate itself, giving one type of female (of three possible
genotypic compositions, but somatically practically homogeneous) and two
types of males, only one of which is like the females.

FUSED.

In a cross between purple-eyed[6] males and black females there appeared in
F_2 (Nov. 4, 1912) a male having the veins of the wing arranged as shown in
text-figure D b. It will be seen that the third and the fourth longitudinal
veins are fused from the base to and beyond the {53} point at which in
normal flies the anterior cross-vein lies. The cross-vein and the cell
normally cut off by it are absent. There are a number of other features
(see fig. D _c_) characteristic of this mutation: the wings are held out at
a wide angle from the body, the ocelli are very much reduced in size or
entirely absent, the bristles around the ocelli are usually small. The
females are absolutely sterile, not only with their own, but with any
males.

Fused males by wild females gave wild-type males and females. Inbred these
gave the results shown in table 32. The fused character reappeared only in
the F_2 males, showing that it is a recessive sex-linked character.

TABLE 32.--_P_1 fused [male] × wild [female][female]._

  +-------------------------------------------------+
  |              First generation.                  |
  +------------+-------------------+----------------+
  | Reference. | Wild-type         | Wild-type      ~
  |            | [female][female]. | [male][male].  ~
  +------------+-------------------+----------------+
  |  4I        |         66        |       43       |
  |            |                   |                |
  +------------+-------------------+----------------+

  +------------------------------------------------------------------+
  |               Second generation.                                 |
  +------------+-------------------+----------------+----------------+
  ~ Reference. | Wild-type         | Wild-type      | Fused          |
  ~            | [female][female]. | [male][male].  | [male][male].  |
  +------------+-------------------+----------------+----------------+
  |  190C      |        258        |       96       |       115      |
  |   14I      |        239        |      105       |        90      |
  |            +-------------------+----------------+----------------+
  |  Total     |        497        |      201       |       205      |
  +------------+-------------------+----------------+----------------+

The reciprocal cross was tried many times, but is impossible, owing to the
sterility of the females. Since the fused females are sterile to fused
males, the stock is kept up by breeding heterozygous females to fused
males.

By means of the following experiments the position of fused in the X
chromosome was determined. A preliminary test was made by mating with
eosin, whose factor lies near the left end of the X chromosome series.

LINKAGE OF EOSIN AND FUSED.

Fused (red-eyed) males mated to eosin (not-fused) females gave wild-type
daughters and eosin sons, which inbred gave the classes shown in table 33.

TABLE 33.--_P_1 eosin [female][female] × fused [male][male]. F_1 wild-type
[female][female] × F_1 eosin [male][male]._

  +----------+--------+-----------------+----------------+-------+--------+
  |          |        | Non-cross-over  | Cross-over     |       |        |
  |          |        | [male][male].   | [male][male].  | Total | Cross- |
  |Reference.|Females.+--------+--------+--------+-------+ males.| over   |
  |          |        | Eosin. | Fused. | Eosin  | Wild- |       | value. |
  |          |        |        |        | fused. | type. |       |        |
  +----------+--------+--------+--------+--------+-------+-------+--------+
  |  56I     |  496   |   131  |   113  |  82    |  104  |   430 |   43   |
  +----------+--------+--------+--------+--------+-------+-------+--------+

{54}

The data give 43 per cent of crossing-over, which places fused far to the
right or to the left of eosin. The latter position is improbable, since
eosin already lies very near the extreme left end of the known series.
Therefore, since 43 per cent would place the factor nearly at the right end
of the series, the next step was to test its relation to a factor like bar
that lies at the right end of the chromosome. By mating to bar alone we
could only get the linkage to bar without discovering on which side of bar
the new factor lies, but by mating to a fly that carries still another
sex-linked factor, known to lie to the left of bar, the information gained
should show the relative order of the factors involved. Furthermore, since,
by making a back-cross, both males and females give the same kind of data
(and need not be separated), the experiment was made in this way. In order
to have material for such an experiment double mutant stocks of vermilion
fused and also of bar fused were made up.

[Illustration: Fig. D.--_a_, normal wing; _b_ and _c_, fused wings. _c_
shows a typical fused wing. The most striking feature is the closure of the
cell between the third and fourth longitudinal veins with the elimination
of the cross-vein; the veins at the base of the wing differ from those in
the normal shown in a. _b_ shows the normal position in which the fused
wings are held. The fusion of the veins in _b_ is unusually complete.]

{55}

LINKAGE OF VERMILION, BAR, AND FUSED.

Males from the stock of (red) bar fused were mated to vermilion (not-bar,
not-fused) females, and produced bar females and vermilion males. The bar
F_1 daughters were back-crossed to vermilion fused males and produced the
classes of offspring shown in table 34.

TABLE 34.--P_1 _vermilion_ [female] [female] × _bar fused_ [male] [male].
_B. C. F_1 bar_ [female] × _vermilion fused_ [male] [male].

  +----------+-------------------+---------------------+------------------+
  |          | v                 | v       B'      f_u | v            f_u |
  |          | ----------------- | ----+-------------- | -----------+---- |
  |          |         B'    f_u |                     |        B'        |
  |Reference.+----------+--------+----------+----------+-----------+------+
  |          |          |        | Vermilion|          |           |      |
  |          |Vermilion.| Bar    | bar      |Wild-type.| Vermilion | Bar. |
  |          |          | fused. | fused.   |          | fused.    |      |
  +----------+----------+--------+----------+----------+-----------+------+
  |140 I     |   137    |   130  |     35   |    40    |     5     |   8  ~
  |141 I     |   144    |   137  |     38   |    41    |     4     |   2  ~
  |142 I     |   153    |   120  |     43   |    58    |     6     |   7  |
  |143 I     |   153    |    92  |     44   |    41    |     3     |   7  |
  |145 I     |    69    |    62  |     29   |    19    |     1     |  ..  |
  |146 I     |    96    |   103  |     30   |    34    |     7     |   3  |
  |156 I     |    62    |    45  |     25   |    27    |     1     |   4  |
  |157 I     |    93    |    57  |     11   |    31    |     2     |   2  |
  |          +----------+--------+----------+----------+-----------+------+
  |   Total. |   907    |   746  |    255   |   291    |    29     |  33  |
  +----------+----------+--------+----------+----------+-----------+------+

  +--------------------+--------+--------------------------------+
  | v       B'         |        |                                |
  | ----+--------+---- |        |      Cross-over values.        |
  |                f_u |        |                                |
  +-----------+--------+        +-----------+--------+-----------+
  |           |        | Total. |           |        |           |
  | Vermilion | Fused. |        | Vermilion | Bar    | Vermilion |
  | bar.      |        |        | bar.      | fused. | fused.    |
  +-----------+--------+--------+-----------+--------+-----------+
  ~     ..    |   ..   |   355  |     21    |    4   |     25    |
  ~     ..    |   ..   |   366  |     22    |    2   |     23    |
  |      1    |   ..   |   388  |     26    |    4   |     29    |
  |      3    |    1   |   344  |     26    |    4   |     28    |
  |      1    |   ..   |   181  |     27    |    1   |     27    |
  |     ..    |   ..   |   273  |     23    |    4   |     26    |
  |     ..    |   ..   |   164  |     32    |    3   |     35    |
  |     ..    |    2   |   198  |     22    |    3   |     23    |
  +-----------+--------+--------+-----------+--------+-----------+
  |      5    |    3   | 2,269  |     24    |    3   |     27    |
  +-----------+--------+--------+-----------+--------+-----------+

The data show that the factor for fused lies about 3 units to the right of
bar. This is the furthest point yet obtained to the right. The reasons for
locating fused to the right of bar are that, if it occupies such a
position, then the double cross-over classes (which are expected to be the
smallest classes) should be vermilion bar and fused, and these are, in
fact, the smallest classes. The order of factors is, then, vermilion, bar,
fused. This order is confirmed by the result that the number of cross-overs
between fused and vermilion is greater than that between bar and vermilion.

In order to obtain data to balance viability effects, the following
experiment was made:

Vermilion (not-bar) fused males were bred to (red) bar (not-fused) females.
The daughters and sons were bar. The daughters were back-crossed, singly,
to vermilion fused males and gave the results shown in table 35. Each
female was also transferred to a second culture bottle, so that for each
female there are two broods given consecutively (82, 82', etc.) in table
35.

The results given by the two broods of the same female are similar. The
values are very near to those given in the last experiment, and confirm the
conclusions there drawn. The combined data give the results shown in table
36. {56}

TABLE 35.--_P_1 bar [female] [female] × vermilion fused [male] [male]. B.
C. F_1 bar [female] × vermilion fused [male] [male]._

  A - Vermilion fused.
  B - Bar.
  C - Vermilion bar.
  D - Fused.
  E - Vermilion.
  F - Bar fused.
  G - Vermilion bar fused.
  H - Wild type.

  -------------------------------------------------------------------------
            |   v   f_u   | v    B' |v        |v  B' f_u|      |  Cross-
            |  --------   |----+----|----+----|-+---+---|      |   over
            |     B'      |     f_u |  B' f_u |         |Total.|  values.
  Reference +-------------+---------+---------+---------+      +-----------
            |   A  |   B  | C  | D  | E  | F  | G  | H  |      | C | F | A
  ----------+------+------+----+----+----+----+----+----+------+---+---+---
            |      |      |    |    |    |    |    |    |      |   |   |
    82      |  165 |  165 | 63 | 57 |  8 |  7 |  1 | .. |  466 | 26|3  | 29
    82'     |  104 |   87 | 26 | 24 | .. |  4 | .. | .. |  245 | 20|2  | 22
    83      |  128 |  164 | 51 | 39 |  6 |  4 | .. | .. |  392 | 23|3  | 26
    83'     |  100 |   94 | 28 | 30 |  4 |  4 | .. | .. |  260 | 22|3  | 25
    89      |   85 |  105 | 23 | 24 |  5 |  2 | .. | .. |  244 | 19|3  | 22
    89'     |   78 |   91 | 21 | 27 |  1 |  2 | .. |  1 |  221 | 22|2  | 23
    90      |   86 |   85 | 30 | 28 |  5 | .. | .. | .. |  234 | 25|2  | 27
    90'     |   33 |   38 | 22 | 14 |  4 |  1 | .. |  1 |  113 | 33|5  | 36
    91      |  125 |  107 | 41 | 31 |  1 |  1 | .. | .. |  306 | 24|1  | 24
    91'     |   91 |   95 | 31 | 25 |  5 |  1 | .. |  2 |  250 | 23|3  | 25
    92      |  109 |  136 | 41 | 24 |  4 |  2 | .. | .. |  316 | 21|2  | 23
    92'     |  100 |  105 | 29 | 29 | .. |  1 | .. |  1 |  265 | 22|1  | 22
    93      |   75 |   67 | 19 | 20 | .. |  1 | .. | .. |  182 | 21|1  | 22
    93'     |   68 |   94 | 31 | 17 |  1 |  1 | .. | .. |  212 | 23|1  | 24
    94      |   84 |   96 | 31 | 35 |  8 |  1 | .. | .. |  255 | 26|4  | 29
    94'     |   61 |   73 | 20 | 22 |  5 |  4 | .. | .. |  185 | 23|5  | 28
    95      |   84 |  102 | 27 | 26 |  3 |  3 | .. | .. |  245 | 22|2  | 24
    96      |  144 |  148 | 43 | 34 |  1 |  2 | .. |  1 |  373 | 21|1  | 21
    97      |   81 |   96 | 25 | 20 |  5 |  3 | .. | .. |  230 | 20|4  | 23
    98      |  107 |  112 | 39 | 33 |  1 |  2 | .. | .. |  294 | 25|1  | 26
    Firsts  |1,273 |1,383 |433 |371 | 47 | 28 |  1 |  1 |3,537 | 23|2  | 25
    Seconds |  635 |  677 |208 |188 | 20 | 18 | .. |  5 |1,751 | 23|3  | 25
  ----------+------+------+----+----+----+----+----+----+------+---+---+---
      Total.|1,908 |2,060 |641 |559 | 67 | 46 |  1 |  6 |5,288 | 23|2.3| 25
  ----------+------+------+----+----+----+----+----+----+------+---+---+---

TABLE 36.--_Linkage of vermilion, bar, and fused with balanced viability._

  +------------+----------+-----------+-----------+-----------+--------+
  |            | v B' f_u | v         | v  B'     | v     f_u |        |
  |            | -------- | --+------ | -----+--- | -+---+--- | Total. |
  |            |          |    B' f_u |       f_u |    B'     |        |
  +------------+----------+-----------+-----------+-----------+--------+
  |            |          |           |           |           |        |
  |            |   5,621  |   1,756   |    175    |    15     |  7,567 |
  | Percentage |    74.3  |   23.19   |   2.31    |     0.2   |        |
  |            |          |           |           |           |        |
  +------------+----------+-----------+-----------+-----------+--------+

Some additional data bearing on the linkage of vermilion and fused were
obtained. Males of (red) fused stock were bred to vermilion (not-fused)
females, and gave wild-type females and vermilion males, which inbred gave
the results shown in table 37.

The percentage of cross-overs between vermilion and fused is here 27, which
is in agreement with the 26 per cent of the preceding experiment.

The converse experiment, namely, red (not-fused) females by vermilion fused
males also gave, when the wild-type daughters were {57} back-crossed to
vermilion fused males, a linkage value of 27 units. Two 10-day broods were
reared from each female. The data given in table 38 show that the
percentage of crossing-over does not change as the flies get older. The
locus of fused on the basis of all of the data is at 59.5.

TABLE 37.--P_1 vermilion [female] [female] × fused [male] [male]. F_1
wild-type [female] [female] × F_1 vermilion [male] [male].

  KEY:
  A: Non-cross-over [male] [male].
  B: Cross-over [male] [male].
  C: Females.
  D: Vermilion.
  E: Fused.
  F: Vermilion fused.
  G: Wild-type.
  H: Total [male] [male].
  I: Cross-over values.

  +------------+-----+-----------+----------+-----+----+
  |            |     |     A     |    B     |     |    |
  |            |     +-----+-----+----+-----+     |    |
  | Reference. |  C  |  D  |  E  | F  |  G  |  H  | I  |
  +------------+-----+-----+-----+----+-----+-----+----+
  | 79 I       | 299 |  93 |  96 | 37 |  36 | 262 | 28 |
  | 80 I       | 245 |  93 |  60 | 28 |  27 | 208 | 26 |
  | 81 I       | 263 | 101 |  63 | 22 |  40 | 226 | 27 |
  |            +-----+-----+-----+----+-----+-----+----+
  |   Total.   | 807 | 287 | 219 | 87 | 103 | 696 | 27 |
  +------------+-----+-----+-----+----+-----+-----+----+

TABLE 38.--P_1 wild [female] [female] × vermilion fused [male] [male]. F_1
wild-type [female]    × F_1 wild-type [male] [male].

  KEY:
  A: Wild-type [female] [female].
  B: Non-Cross-over [male].
  C: Cross-over [male].
  D: Vermilion fused.
  E: Wild-type.
  F: Vermilion.
  G: Fused.
  H: Total [male] [male].
  I: Cross-over values.

  +------------+-------+------------+-----------+-----+----+
  |            |       |     B      |     C     |     |    |
  |            |       +-----+------+-----+-----+     |    |
  | Reference. |   A   |  D  |   E  |  F  |  G  |  H  | I  |
  +------------+-------+-----+------+-----+-----+-----+----+
  | 52         |    96 |  25 |   30 |  16 |  11 |  82 | 33 |
  | 52'        |   176 |  59 |   64 |  24 |  19 | 166 | 26 |
  | 53         |    60 |  20 |   22 |   9 |   6 |  57 | 26 |
  | 53'        |    76 |  21 |   27 |  11 |  10 |  69 | 31 |
  | 54         |    88 |  35 |   38 |  14 |  16 | 103 | 29 |
  | 54'        |    60 |  22 |   20 |   8 |   9 |  59 | 29 |
  | 57         |    61 |  22 |   20 |   7 |  11 |  60 | 30 |
  | 57'        |   170 |  47 |   54 |  24 |  19 | 144 | 30 |
  | 58         |   128 |  37 |   55 |  14 |  10 | 116 | 21 |
  | 58'        |   144 |  38 |   64 |  16 |  15 | 133 | 23 |
  | Firsts     |   433 | 139 |  165 |  60 |  54 | 418 | 27 |
  | Seconds    |   626 | 187 |  229 |  83 |  72 | 571 | 27 |
  |            +-------+-----+------+-----+-----+-----+----+
  |   Total    | 1,059 | 326 |  394 | 143 | 126 | 989 | 27 |
  +------------+-------+-----+------+-----+-----+-----+----+

FORKED.

On November 19, 1912 there appeared in a stock of a double recessive
eye-color, vermilion maroon, a few males which showed a novel form of the
large bristles (macrochætæ) upon the head and thorax. In this mutation
(text-fig. E) the first of several which affect the shape and distribution
of the bristles, the macrochætæ, instead of {58} being long, slender, and
tapered (see Plate 1, fig. I), are greatly shortened and crinkled as though
scorched. The ends are forked or branched, bent sharply, or merely
thickened. The bristles which are most distorted are those upon the
scutellum, where they are sometimes curled together into balls.

LINKAGE OF VERMILION AND FORKED.

[Illustration: FIG. E.--Forked bristles.]

Since forked arose in vermilion stock, the double recessive for these two
sex-linked factors could be used in testing the linkage relations of the
mutation. Vermilion forked males were crossed to wild females and gave
wild-type males and females, which inbred gave in F_2 the results shown in
table 39. Forked reappeared only in the males in the following proportion:
not-forked [female], 742; not-forked [male], 346; forked [male], 301. The
result shows that the character is a sex-linked recessive.

TABLE 39.--_P_1 wild_ [female] [female] × _vermilion-forked_ [male] [male].
_F_1 wild-type_ [female] [female] × _F_1 wild-type_ [male] [male].

  +----------+----------+----------------+---------------+--------+-------+
  |          |          | Non-cross-over | Cross-over    |        |       |
  |          |Wild-type | [male] [male]. | [male] [male].| Total  |Cross- |
  |Reference.|[female]  +--------+-------+-------+-------+ [male] | over  |
  |          |[female]. |  Ver-  |Wild-  | Ver-  |Forked.| [male].|values.|
  |          |          | milion |type.  |milion.|       |        |       |
  |          |          | forked.|       |       |       |        |       |
  +----------+----------+--------+-------+-------+-------+--------+-------+
  |  9 I     | 366      | 113    | 123   |  49   |  41   | 326    | 28    |
  | 11 I     | 376      | 116    | 150   |  42   |  31   | 339    | 22    |
  |          +----------+--------+-------+-------+-------+--------+-------+
  |    Total.| 742      | 229    | 273   |  91   |  72   | 665    | 25    |
  +----------+----------+--------+-------+-------+-------+--------+-------+

In table 39 vermilion forked and wild-type are non-cross-overs, and
vermilion and forked are cross-overs, giving a cross-over value of 25
units. The locus, therefore, is 25 units to the right or to the left of
vermilion, that is, either about 58 or 8 units from the yellow locus.

LINKAGE OF CHERRY AND FORKED.

Forked males were crossed to cherry females (cherry has the same locus as
white, which is about 1 unit from yellow) and gave wild-type females and
cherry males. These gave in F_2 the results shown in table 40. The
non-cross-overs (cherry and forked) plus the cross-overs (cherry forked and
wild type) divided into the cross-overs give a cross-over value of 46
units, which shows that the locus lies to the right of vermilion, because
if it had been to the left, the value would have been 8 (_i. e._, 33-25)
instead of 33+25=58. The difference between 58 {59} and 46 is due to the
expected amount of double crossing-over. In fact, for a distance as long as
58 an almost independent behavior of linked gens is to be expected.

TABLE 40.--_P_{1} cherry_ [female] [female] × _forked_ [male] [male].
_F_{1} wild-type_ [female] [female] × _F_{1} cherry_ [male] [male].

  +----------+--------------+---------------+-------------+-------+-------+
  |Reference.|   Females.   | Non-cross-over|  Cross-over |       |       |
  |          |              | [male] [male].|[male] [male]| Total |Cross- |
  |          +-------+------+-------+-------+-------+-----+[male] | over  |
  |          |Cherry.| Wild-|Cherry.|Forked.|Cherry |Wild-|[male].|values.|
  |          |       | type.|       |       |forked.|type.|       |       |
  +----------+-------+------+-------+-------+-------+-----+-------+-------+
  | 25       |   129 |  145 |   73  |   70  |  65   |  68 |  276  |  48   |
  | 25'      |   167 |  148 |   74  |   82  |  66   |  88 |  310  |  50   |
  | 36       |    96 |   88 |   52  |   52  |  35   |  51 |  190  |  45   |
  | 36'      |    57 |   76 |   41  |   32  |  24   |  30 |  127  |  43   |
  | 84       |    76 |   86 |   40  |   34  |  38   |  26 |  138  |  46   |
  | 84'      |    62 |   71 |   24  |   39  |  25   |  28 |  116  |  46   |
  | 85       |   114 |   86 |   43  |   78  |  41   |  53 |  215  |  44   |
  | 85'      |    98 |   95 |   48  |   63  |  52   |  46 |  209  |  47   |
  | 86       |   307 |  323 |  152  |  144  | 118   | 165 |  579  |  49   |
  | 87       |   351 |  341 |  183  |  213  | 160   | 147 |  703  |  45   |
  | 88       |   244 |  246 |  142  |  142  | 107   | 104 |  495  |  43   |
  +----------+-------+------+-------+-------+-------+-----+-------+-------+
  |Total.    | 1,701 |1,705 |  872  |  949  | 731   | 806 |3,358  |  46   |
  +----------+-------+------+-------+-------+-------+-----+-------+-------+

LINKAGE OF FORKED, BAR, AND FUSED.

This value of 58 gave the furthest locus to the right obtained up to that
time, since forked is slightly beyond rudimentary. Later, the locus for
bar-eye was found still farther to the right, and the locus for fused even
farther to the right than bar. A cross was made involving these three gens.
A forked (not-bar) fused male was bred to a (not-forked) bar (not-fused)
female and gave bar females and males. The F_1 females were back-crossed
singly to forked fused males with the result shown in table 41.

TABLE 41.--_P_1 bar_ [female] [female] × _forked fused_ [male] [male]. _B.
C. F_1 bar_ [female] × _forked fused_ [male] [male].

  +-------+------------+-------------+--------------+-------------+-------+
  |       |    f  f_u  |   f B'      |   f          |   f B' f_u  |       |
  |Refer- |    ------  |  --+-----   |   ---+---    |   -+--+---  |       |
  | ence. |      B'    |      f_u    |    B' f_u    |             |       |
  |       +------+-----+------+------+-------+------+-------+-----+ Total.|
  |       |Forked| Bar.|Forked|Fused.|Forked.| Bar  |Forked |Wild-|       |
  |       |fused.|     | bar. |      |       |fused.|bar    |type.|       |
  |       |      |     |      |      |       |      |fused. |     |       |
  +-------+------+-----+------+------+-------+------+-------+-----+-------+
  | 163   |  45  |  55 |  ..  |   1  |    4  |   2  |  ..   | ..  |  108  |
  | 164   |  71  |  90 |  ..  |  ..  |    4  |   1  |  ..   | ..  |  166  |
  | 165   |  97  | 106 |  ..  |  ..  |    2  |   4  |  ..   | ..  |  209  |
  |  11   |  21  |  35 |  ..  |  ..  |    1  |   2  |  ..   | ..  |   59  |
  |  33   |  15  |  23 |  ..  |  ..  |   ..  |   1  |  ..   | ..  |   39  |
  |       +------+-----+------+------+-------+------+-------+-----+-------+
  | Total.| 250  | 309 |  ..  |   1  |   11  |  10  |  ..   | ..  |  581  |
  +-------+------+-----+------+------+-------+------+-------+-----+-------+

{60}

The same three points were combined in a different way, namely, by mating
forked females to bar fused males. The bar daughters were back-crossed to
forked fused males and gave the results shown in table 42.

TABLE 42.--_P_1 forked_ [female] [female] × _bar fused_ [male] [male].
_B.C. F_1 bar_ [female] × _forked fused_ [male] [male].

  +------+--------------+-------------+-------------+--------------+------+
  |      |    f         |  f B' f_u   |    f  f_u   |    f B'      |      |
  |      |    ------    |  -+------   |    --+---   |    -+--+--   |      |
  |Refer-|      B' f    |             |      B'     |        f_u   |Total.|
  | ence.+-------+------+------+------+------+------+------+-------+      |
  |      |Forked.| Fused|Forked| Wild-|Forked| Bar. |Forked| Fused.|      |
  |      |       | bar. | bar  | type.|fused.|      |bar.  |       |      |
  |      |       |      |fused.|      |      |      |      |       |      |
  +------+-------+------+------+------+------+------+------+-------+------+
  |158   | 131   |  124 |   1  |  ..  |  3   |   3  | ..   |  ..   |  262 |
  |159   |  31   |   45 |  ..  |  ..  | ..   |  ..  | ..   |  ..   |   76 |
  |160   |  29   |   23 |  ..  |  ..  |  1   |   2  | ..   |  ..   |   55 |
  |161   |  24   |   11 |   1  |  ..  | ..   |  ..  | ..   |  ..   |   36 |
  |162   |  96   |   91 |   2  |  ..  |  1   |   1  | ..   |  ..   |  191 |
  |      +-------+------+------+------+------+------+------+-------+------+
  |Total.| 311   |  294 |   4  |  ..  |  5   |   6  | ..   |  ..   |  620 |
  +------+-------+------+------+------+------+------+------+-------+------+

By combining the results of tables 41 and 42 data are obtained for
cross-over values from which (by balancing the inviable classes, as
explained in table 43) the element of inviability is reduced to a minimum.

TABLE 43.

  +----------+------------+------------+------------+------------+--------+
  |          |            |            |            |            |        |
  |          |   ------   |   -+----   |   ----+-   |   -+--+-   | Total. |
  |          |            |            |            |            |        |
  +----------+------------+------------+------------+------------+--------+
  |          |            |            |            |            |        |
  |          |   1,164    |     5      |     32     |     0      | 1,201  |
  |Per cent. |    96.9    |    0.42    |     2.7    |     0      |        |
  +----------+------------+------------+------------+------------+--------+

The linkages involved in these data are very strong. The cross-overs
between forked and bar number only 5 in a total of 1,201, which gives less
than 0.5 per cent of crossing-over. There are 32 cross-overs or 2.7 per
cent between bar and fused. The value for forked fused is the sum of the
two other values, or 3.1 per cent.

LINKAGE OF SABLE, RUDIMENTARY, AND FORKED.

Rudimentary, forked, bar, and fused form a rather compact group at the
right end of the chromosome, as do yellow, lethal 1, white, abnormal, etc.,
at the zero end. The following two experiments were made to determine more
accurately the interval between rudimentary and the other members of this
group. A sable rudimentary forked {61} male mated to a wild female gave
wild-type sons and daughters. These inbred give the results shown in table
44.

TABLE 44.--_P_{1} sable rudimentary forked_ [male] × _wild_ [female].
_F_{1} wild-type_ [female] × _F_{1} wild-type_ [male] [male].

  +----------+---------+-----------------+-------------------+
  |          |         |     s  r f      |       s           |
  |          |         |     ------      |       -+----      |
  |          |         |                 |          r f      |
  |          |         +-----------+-----+-------+-----------+
  |Reference.|Wild-type|   Sable   |Wild-| Sable.|Rudimentary|
  |          |[female] |rudimentary|type.|       |  forked.  |
  |          |[female].|  forked.  |     |       |           |
  +----------+---------+-----------+-----+-------+-----------+
  |  264     |   98    |    28     | 17  |   2   |    5      ~
  |  265     |   97    |    29     | 54  |   4   |    9      ~
  |  266     |   114   |    42     | 49  |  11   |   11      |
  +----------+---------+-----------+-----+-------+-----------+
  |Total     |  309    |    99     |120  |  17   |   25      |
  +----------+---------+-----------+-----+-------+-----------+

  +----------+--------------------+--------------------+
  |          |      s r           |      s    f        |
  |          |      ---+-         |      -+--+-        |
  |          |          f         |         r          |
  |          +------------+-------+-------+------------+
  |Reference.|   Sable    |Forked.|Sable  |Rudimentary.|
  |          |rudimentary.|       |forked.|            |
  |          |            |       |       |            |
  +----------+------------+-------+-------+------------+
  ~  264     |     1      |   1   |  ..   |   ..       |
  ~  265     |    ..      |  ..   |  ..   |   ..       |
  |  266     |    ..      |   2   |  ..   |   ..       |
  +----------+------------+-------+-------+------------+
  |Total     |     1      |   3   |  ..   |   ..       |
  +----------+------------+-------+-------+------------+

There were 265 males, of which 42 were cross-overs between sable and
rudimentary and 4 between rudimentary and forked. The values found are:
sable rudimentary, 16; rudimentary forked, 1.5; sable forked, 17.

LINKAGE OF RUDIMENTARY, FORKED, AND BAR.

The three gens, rudimentary, forked, and bar, form a very compact group. A
rudimentary forked male was crossed to bar females and the daughters (bar)
were back-crossed singly to rudimentary forked males, the results being
shown in table 45.

TABLE 45.--_P_1 rudimentary forked_ [male] × _bar_ [female]. _B.C. F_1 bar_
[female] × _rudimentary forked_ [male] [male].

  +----------+---------------+---------------+-------------+--------------+
  |          |     r f       |     r   B'    |    r f B'   |     r        |
  |          |     ------    |     -+----    |    ----+-   |     -+--+-   |
  |          |         B'    |       f       |             |       f B'   |
  |          +---------+-----+-------+-------+-------+-----+-------+------+
  |Reference.| Rudim-  | Bar.| Rudim-|Forked.| Rudim-|Wild-| Rudim-|Forked|
  |          | entary  |     | entary|       | entary|type.| entary| bar. |
  |          | forked. |     |  bar. |       | forked|     |       |      |
  |          |         |     |       |       |  bar. |     |       |      |
  +----------+---------+-----+-------+-------+-------+-----+-------+------+
  |267       |    56   | 104 |  ..   |   2   |   1   |  1  |   ..  |  ..  |
  |268       |    82   |  86 |   1   |   2   |  ..   | ..  |   ..  |  ..  |
  |269       |    68   | 101 |  ..   |  ..   |  ..   |  1  |   ..  |  ..  |
  +----------+---------+-----+-------+-------+-------+-----+-------+------+
  |Total     |   206   | 291 |   1   |   4   |   1   |  2  |   ..  |  ..  |
  +----------+---------+-----+-------+-------+-------+-----+-------+------+

The cross-over values are: rudimentary forked, 1; forked bar, 0.6;
rudimentary bar, 1.6. The order of factors is rudimentary, forked, bar. On
the basis of the total data the locus of forked is at 56.5. {62}

SHIFTED.

Shifted appeared (January 1913) in a stock culture of vermilion dot. The
chief characteristic of this mutant is that the third longitudinal vein
(see text-fig. F) does not reach the margin as it does in the normal fly.
The vein is displaced toward the fourth throughout its length, and only
very rarely does it extend far enough to join the marginal vein. The
cross-vein between the third and the fourth veins is often absent because
of the shifting. The flies themselves are smaller than normal. The wings
are held out from the body at a wide angle. The two posterior bristles of
the scutellum are much reduced in size and stick straight up--a useful
landmark by which just-hatched shifted flies may be recognized, even though
the wings are not expanded.

LINKAGE OF SHIFTED AND VERMILION.

Since shifted arose in vermilion, the double recessive shifted vermilion
was available for the following linkage experiment: shifted vermilion males
by wild females gave wild-type males and females which inbred gave the data
shown in table 46.

[Illustration: FIG. F.--Shifted venation. The third longitudinal vein is
shifted toward the fourth and fails to reach the margin. Cross-vein between
third and fourth longitudinal veins is lacking.]

Disregarding the eye-color, the following is a summary of the preceding
results: wild-type [female], 1,001; wild-type [male], 437; shifted [male],
328. The result shows that shifted is a sex-linked recessive. The data of
table 46 show that the locus of shifted lies about 15 units on one side or
the other of vermilion, which from the calculated position of vermilion at
33 would give a position for shifted at either 18 or 48 from yellow.

TABLE 46.--_P_1 shifted vermilion [male] [male] × wild [female] [female].
F_1 wild-type [female] × F_1 wild-type [male] [male]._

  Key to columns:

  A:  Wild-type [female] [female].
  B:  Non-cross-over [male] [male], Shifted.
  C:  Non-cross-over [male] [male], Wild-type.
  D:  Cross-over [male] [male], Shifted.
  E:  Cross-over [male] [male], Wild-type.
  F:  Total [male] [male].
  G:  Cross-over values.

  +--------------+---------+------+------+-------+-------+-------+------+
  | Reference.   |     A   |  B   |  C   |  D    |  E    |   F   |  G   |
  +--------------+---------+------+------+-------+-------+-------+------+
  | 13           |    345  |  79  | 115  |   8   |  25   |  227  |  15  |
  | 29           |     68  |  20  |  32  |   3   |   4   |   59  |  12  |
  | 30           |    191  |  37  |  54  |   5   |  13   |  109  |  17  |
  | 31           |    151  |  41  |  65  |  17   |  13   |  136  |  22  |
  | 33           |    133  |  49  |  40  |   4   |   6   |   99  |  10  |
  | 34           |    113  |  56  |  59  |   9   |  11   |  135  |  15  |
  +--------------+---------+------+------+-------+-------+-------+------+
  |  Total.      |   1,001 | 282  |  365 |   46  |   72  |  765  |  15  |
  +--------------+---------+------+------+-------+-------+-------+------+

{63}

LINKAGE OF SHIFTED, VERMILION, AND BAR.

In order to determine on which side of vermilion shifted lies, a shifted
vermilion (not-bar) female was crossed to a (not-shifted red) bar male.
Three factors are involved, of which one, bar, is dominant. The shifted
vermilion (not-bar) stock is a triple recessive, and a three-point
back-cross was therefore possible. The daughters were bar and the sons were
shifted vermilion (the triple recessive). Inbred these gave the results
shown in table 46. The smallest classes (double cross-overs) are shifted
and vermilion bar, which places shifted to the left of vermilion at
approximately 17.8 units from yellow.

TABLE 47.--_P_1 shifted vermilion_ [female] × _bar_ [male] [male]. _F_1
bar_ [female] × _F_1 shifted vermillion_ [male] [male].

  +-------+---------------+---------------+---------------+---------------+
  |       |    s_h  v     |    s_h  B'    |   s_h v B'    |    s_h        |
  | Refer-|    ------     |    --+----    |    -----+-    |    --+-+--    |
  | ence. |        B'     |       v       |               |       v B'    |
  |       +----------+----+--------+------+---------+-----+--------+------+
  |       |Shifted   |Bar.|Shifted |Verm- |Shifted  |Wild-|Shifted.| Verm-~
  |       |vermilion.|    |  bar.  |ilion.|vermilion|type.|        | ilion~
  |       |          |    |        |      |bar.     |     |        | bar. |
  +-------+----------+----+--------+------+---------+-----+--------+------+
  |  65   |  56      |108 |   15   |  20  |  8      | 33  |  1     |  1   |
  +-------+----------+----+--------+------+---------+-----+--------+------+

  +----------+------+----------------------------+
  |          |      |                            |
  |Reference.|Total.| Cross-over values.         |
  |          |      |                            |
  |          |      +----------+---------+-------+
  ~          |      |Shifted   |Vermilion|Shifted|
  ~          |      |vermilion.|bar.     |bar.   |
  |          |      |          |         |       |
  +----------+------+----------+---------+-------+
  | 65       | 242  |  15      | 18      |  31   |
  +----------+------+----------+---------+-------+

The stock of shifted has been thrown away, since too great difficulty was
encountered in maintaining it, because, apparently, of sterility in the
females.

LETHALS SA AND SB.

The first lethal found by Miss Rawls was in a stock that had been bred for
about 3 years. While there was no _a priori_ reason that could be given to
support the view that lethal mutations would occur more frequently among
flies inbred in confinement, nevertheless a hundred females from each of
several newly caught and from each of several confined stocks were examined
for lethals (Stark, 1915). No lethals were found among the wild stocks, but
4 were found among the confined stocks. Whether this difference is
significant is perhaps open to question. The first lethal was found in
January 1913, in a stock that had been caught at Falmouth, Massachusetts,
in 1911, and had been inbred for 18 months, _i.e._, for about 50
generations. This lethal, lethal _sa_, was recessive and behaved like the
former lethals, being transmitted by half the females and causing the death
of half the sons. The position of this lethal to the X chromosome was found
as follows, by means of the cross-over value white lethal _sa_.
Lethal-bearing females were mated to white males and the lethal-bearing
daughters were again mated to white males. The white sons (894) were
non-cross-overs and the red sons (256) were cross-overs. The percentage of
crossing-over {64} is 22.2. A correction of 0.4 unit should be added for
double crossing-over, indicating that the locus is 22.6 units from white,
or at 23.7.

When the work on lethal _sa_ had been continued for 3 months, the second
lethal, lethal _sb_, was found (April 1913) to be present in a female which
was already heterozygous for lethal _sa_. It is probable that this second
lethal arose as a mutation in the father, and that a sperm whose X carried
lethal _sb_ fertilized an egg whose X carried lethal _sa_. As in the cases
of lethals 1 and 1_a_ and lethals 3 and 3_a_, this lethal, lethal _sb_, was
discovered from the fact that only a very few sons were produced, there
being 82 daughters and only 3 sons. If, as in the other cases, the number
of daughters is taken as the number of non-cross-overs and twice the number
of sons as the cross-overs, it is found that the two lethals are about 7
units apart. Since the two lethals were in different X chromosomes, all the
daughters should receive one or the other lethal, except in those few cases
in which crossing over had taken place. Of the daughters 19 were tested and
every one was found to carry a lethal. Again, if the cross-over values of
the lethals with some other character, such as white eyes, be found and
plotted, the curve should show two modes corresponding to the two lethals.
This test was applied, but the curve failed to show two modes clearly,[7]
the two lethals being too close together to be differentiated by the small
number of determinations that were made. It seems probable that lethal _sa_
and lethal _sb_ are about 5 units apart.

The position of lethal _sb_ was accurately found by continuing the
determinations with a white lethal cross-over. A white female was found
which had only one of the two lethals and the linkage of this lethal with
eosin and miniature was found as follows: A female carrying white and
lethal in one chromosome and no mutant factor in the homologous chromosome
was bred to an eosin miniature male. The white eosin daughters carried
lethal, and their sons show the amount of crossing-over between white and
lethal (15.6), between lethal and miniature (19.9), and between white and
miniature (32.9). The data on which these calculations are based are given
in table 48.

TABLE 48.--_Data on the linkage of white, lethal sb, and miniature, from
Stark, 1915_.

  +-----------+------------+------------+--------------+
  | w^e     m | w^e l_{sb} | w^e        | w^e l_{sb} m |
  | --------- | ---+------ | -------+-- | ---+-----+-- |
  | w l_{sb}  | w        m | w l_{sb} m | w            |
  |           |            |            |              |
  +-----------+------------+------------+--------------+
  | Eosin     | White      | Eosin.     | White.       ~
  | miniature | miniature. |            |              ~
  |           |            |            |              |
  +-----------+------------+------------+--------------+
  | 2,421     | 524        | 685        | 48           |
  +-----------+------------+------------+--------------+

  +--------+--------------------------------+
  |        |                                |
  |        |       Cross-over values.       |
  |        |                                |
  |        |                                |
  | Total. +----------+----------+----------+
  ~        |White     |Lethal    |White     |
  ~        |lethal    |_sb_      |miniature.|
  |        |_sb_.     |miniature.|          |
  +--------+----------+----------+----------+
  | 3,678  | 15.6     | 19.9     | 32.9     |
  +--------+----------+----------+----------+

{65}

The locus of this lethal is at 16.7; the locus of lethal _sa_ was found to
be at 23.7, so that the lethal at 16.7 is evidently the second lethal or
lethal _sb_ whose advent gave rise to the high sex-ratio. This
interpretation is in accord with the curve which Miss Stark published, for
although the mode which corresponds to lethal _sa_ is weak, the mode at
15-16 is well marked.

The two other lethals, lethals _sc_ and _sd_, which came up in the course
of these experiments by Miss Stark, are treated in other sections of this
paper.

BAR.

(Plate II, figures 12 and 13.)

The dominant sex-linked mutant called bar-eye (formerly called barred)
appeared in February 1913 in an experiment involving rudimentary and
long-winged flies (Tice, 1914). A female that is heterozygous for bar has
an eye that is intermediate between the rounded eye of the wild fly and the
narrow band of the bar stock. This heterozygous bar female is always
readily distinguishable from the normal, but can not always be separated
from the pure bar. Bar is therefore nearly always used as a dominant and
back-crosses are made with normal males.

Bar is the most useful sex-linked character so far discovered, on account
of its dominance, the certainty of its classification, and its position
near the right end of the X chromosome. The locus of bar at 57 was
determined on the basis of the data of table 65.

NOTCH.

A sex-linked dominant factor that brings about a notch at the ends of the
wings appeared in March 1913, and has been described and figured by Dexter
(1914, p. 753, and fig. 13, p. 730). The factor acts as a lethal for the
male. Consequently a female heterozygous for notch bred to a wild male
gives a 2:1 sex-ratio; half of her daughters are notch and half normal; the
sons are only normal. The actual figures obtained by Dexter were 235 notch
females, 270 normal females, and 235 normal males.

The location of notch in the X chromosome was not determined by Dexter, but
the mutant has appeared anew three or four times and the position has been
found by Bridges to be approximately at 2.6. {66}

DEPRESSED.

Several mutations have appeared in which the wings are not flat. Of these
the first that appeared was curved (second chromosome), in which the wings
are curved downward throughout their length, but are elevated and held out
sidewise from the body; the texture is thinner than normal. The second of
these wing mutants to appear was jaunty (second chromosome), in which the
wings turn up sharply at the tip; they lie in the normal position. The
third mutant, arc (second chromosome), has, as its name implies, its wings
curved like the arc of a circle. The fourth mutant, bow (first chromosome,
fig. C), is like arc, but the amount of curvature is slightly less. The
fifth mutant, depressed (first chromosome, fig. G), has the tip of its
wings turned down instead of up, as in jaunty, but, as in jaunty, the wing
is straight, except near the tip, where it bends suddenly. These stocks
have been kept separate since their origin, and flies from them have seldom
been crossed to each other, because in the succeeding generations it would
be almost impossible to make a satisfactory classification of the various
types. But that they are genetically different mutations is at once shown
on crossing any two, when wild-type offspring are produced. For instance,
bow and arc are the two most nearly alike. Mated together (bow [male] by
arc [female]), they give in F_1 straight-winged flies which inbred give in
F_2 9 straight to 7 not-straight (_i.e._, bow, arc, and bow arc together).

Depressed wings first appeared (April 1913) among the males of a culture of
black flies. They were mated to their sisters and from subsequent
generations both males and females with depressed wings were obtained which
gave a pure stock. This new character proved to be another sex-linked
recessive.

LINKAGE OF DEPRESSED AND BAR.

Depressed (not-bar) males mated to (not-depressed) bar females gave bar
daughters. Two of these were back-crossed singly to depressed males and
gave the results shown in table 49. Males and females were not separated,
since they should give the same result.

TABLE 49.--_P_1 depressed_ [female] [female] × _bar_ [female] [female].
_B.C. F_1 bar_ [female] × _depressed_ [male] [male].

  +----------+--------------------+-------------------+-------+-----------+
  |          | Non-cross-overs.   | Cross-overs.      |       |           |
  +----------+-------------+------+-----------+-------+       |           |
  |Reference.|  Depressed. | Bar. | Depressed | Wild- | Total.| Cross-over|
  |          |             |      | bar.      | type. |       | values.   |
  +----------+-------------+------+-----------+-------+-------+-----------+
  | 66 I     |    48       |   51 |   21      |   41  |  161  |   39      |
  | 67 I     |    85       |  104 |   44      |   70  |  303  |   38      |
  +----------+-------------+------+-----------+-------+-------+-----------+
  |    Total.|   133       |  155 |   65      |  111  |  464  |   38      |
  +----------+-------------+------+-----------+-------+-------+-----------+

{67}

[Illustration: FIG. G.--Depressed wing.]

LINKAGE OF CHERRY, DEPRESSED, AND VERMILION.

The linkage value 38 (see table 49) indicates that depressed is somewhere
near the opposite end of the series of sex-linked factors from bar. The
locus could be more accurately determined by finding the linkage relations
of depressed with gens at its end of the chromosome. Accordingly, depressed
females were crossed to cherry vermilion males. F_1 gave wild-type females
and depressed males. The daughters bred again to cherry vermilion males
gave the results shown in table 50. The data only suffice to show that the
locus of depressed is about midway between cherry and vermilion, or at
about 15 units from yellow.

The F_1 males in the last experiment did not have their wings as much
depressed as is the condition in stock males, and in F_2 most of the
depressed winged males were of the F_1 type, although a few were like those
of stock. This result suggests that the stock is a double recessive,
_i. e._, one that contains, in addition to the sex-linked depressed, an
autosomal factor that intensifies the effect of the primary sex-linked
factor.

TABLE 50.--_P_1 depressed [female] × cherry vermilion [male] [male]._

  +-------------------++---------------------------------------+
  |                   ||             Second generation.        |
  |       First       |+----------+--------+-------------------+
  |    generation.    ||          |        |       w^c  v      |
  +---------|---------+|          |        |      -------      |
  |         |         ||          |        |        d_p        |
  |  Wild-  |Depressed||          |        +---------+---------+
  |  type   | [male]  ||Reference.|        |         |         ~
  |[female] | [male]. ||          |[female]|Cherry   |         ~
  |[female].|         ||          |[female]|vermilion|Depressed|
  |         |         ||          |        |         |         |
  |         |         ||          |        | [male]. | [male]. |
  +---------+---------++----------+--------+---------+---------+
  |   21    |    31   ||     19 I |   59   |     23  |   24    |
  +---------+---------++----------+--------+---------+---------+

  +---------------------------------------------------------+
  |                  Second generation.                     |
  +-------------------+-------------------------------------+
  |     w^c d_p       |     w^c          |    w^c d_p  v    |
  |     --+-----      |     -----+--     |   --+----+---    |
  |            v      |       d_p  v     |                  |
  +---------+---------+--------+---------+---------+--------|
  ~         |         |        |         |         |        |
  ~ Cherry  |         | Cherry |Depressed|Cherry   | Wild-  |
  |depressed|Vermilion|        |vermilion|depressed| type   |
  |         |         |        |         |vermilion|        |
  | [male]. |[male].  |[male]. | [male]. | [male]. | [male].|
  +---------+---------+--------+---------+---------+--------+
  |    6    |    6    |   5    |    5    |      0  |    0   |
  +---------+---------+--------+---------+---------+--------+

{68}

CLUB.

In May 1913 there were observed in a certain stock some flies which,
although mature, did not unfold their wings (text-fig. H_a_). This
condition was at first found only in males and suspicion was aroused that
the character might be sex-linked. When these males were bred to wild
females the club-shaped wings reappeared only in the F_2 males, but in
smaller number than expected for a recessive sex-linked character. The
result led to the further suspicion that not all those individuals that are
genetically club show club somatically. These points are best illustrated
and proven by the following history of the stock:

[Illustration: FIG. H.--Club wing. _a_ shows the unexpanded wings of club
flies; _c_ shows the absence of the two large bristles from the side of the
thorax present in the normal condition of the wild, b.]

Club females were obtained by breeding F_2 club males to their F_2
long-winged sisters, half of which should be heterozygous for club. {69}
5,352; wild-type [male], 4,181; club [male], 236. The wild-type males
include, of course, those club males that have expanded wings (potential
clubs).

Club females by wild males gave in the F_2 generation (mass cultures):
wild-type [female], 1,131; wild-type [male], 897; club [female], 57; club
[male], 131.

It is noticeable that there were fewer club females than club males,
equality being expected, which might appear to indicate that the club
condition is more often realized by the male than by the female, but later
crosses show that the difference here is not a constant feature of the
cross.

Long-winged males from club stock (potential clubs) bred to wild females
gave in F_2 the following: wild-type [female], 521; wild-type (and
potential club) [male], 403; club [male], 82.

Club females by club males of club stock gave in F_2: potential club
[female], 126; potential club [male], 78; club [female], 95; club [male],
81. These results are from 8 pairs. The high proportion of club is
noticeable.

Potential club females and males from pure club stock (_i. e._, stock
derived originally from a pair of club) gave in F_2 the following:
potential club [female], 1,049; potential club [male], 666; club [female],
450; club [male], 453.

GENOTYPIC CLUB.

Accurate work with the club character was made possible by the discovery of
a character that is a constant index of the presence of homozygous club.
This character is the absence of the two large bristles (text-fig. H_c_)
that are present on each side of the thorax of the wild fly as shown in
figure Hb. All club flies are now classified by this character and no
attention is paid to whether the wings remain as pads or become expanded.

LINKAGE OF CLUB AND VERMILION.

The linkage of club and vermilion is shown by the cultures listed in table
51, which were obtained as controls in working with lethal III. The
cross-over value is shown in the male classes by the cross-over fraction
276/1463 or 19 per cent.

LINKAGE OF YELLOW, CLUB, AND VERMILION.

The data just given in table 51 show that club is 19 units from vermilion,
but in order to determine in which direction from vermilion it lies, the
crossing-over of club to one other gen must be tested. For this test we
used yellow, which lies at the extreme left of the chromosome series. At
the same time we included vermilion, so that a three-point experiment was
made.

Females that were (gray) club vermilion were bred to yellow (not-club red)
and gave wild-type daughters and club vermilion sons. These inbred gave the
results of table 52.

The data from the males show that the locus of club is about midway between
yellow and vermilion. This conclusion is based on the {70} evidence that
yellow and club give 18 per cent of crossing-over, club and vermilion 20
per cent, and yellow and vermilion 35 per cent. The double cross-overs on
this view are yellow club (3) and vermilion (3). The females furnish
additional data for the linkage of club and vermilion. The value calculated
from the female classes alone is 20 units, which is the same value as that
given by the males.

TABLE 51.--_P_1 club_ [female] [female] × _vermilion_ [male] [male]. _F_1
wild-type_ [female] × _F_1 club_ [male].

  +----------+--------+-----------------+-----------------+-------+-------+
  |          |        | Non-cross-over  | Cross-over      |       |       |
  |          |        | [male] [male].  | [male] [male].  |       |       |
  |          |        +------+----------+----------+------+ Total |Cross- |
  |Reference.|Females.| Club.|Vermilion.|Club      |Wild- |[male] | over  |
  |          |        |      |          |Vermilion.|type. |[male].|values.|
  |          |        |      |          |          |      |       |       |
  +----------+--------+------+----------+----------+------+-------+-------+
  | 137      |    75  |  17  |    39    |      6   |  11  |    73 |  23   |
  | 138      |    64  |  24  |    32    |      6   |   8  |    70 |  20   |
  | 139      |    56  |  10  |    31    |      4   |   3  |    48 |  15   |
  | 140      |    74  |  13  |    39    |      3   |   5  |    60 |  13   |
  | 144      |    97  |  30  |    40    |     10   |  13  |    93 |  25   |
  | 145      |    63  |  15  |    29    |      4   |   6  |    54 |  19   |
  | 146      |   126  |  44  |    46    |      9   |   9  |   108 |  15   |
  | 106      |    92  |  33  |    34    |      6   |  10  |    83 |  19   |
  | 107      |    55  |  31  |    25    |      7   |   3  |    66 |  15   |
  | 108      |    86  |  29  |    32    |      7   |  10  |    78 |  22   |
  | 109      |   103  |  25  |    36    |      4   |   9  |    74 |  18   |
  |          |    83  |  30  |    34    |      6   |   9  |    79 |  19   |
  |          |    77  |  18  |    26    |      7   |   8  |    59 |  25   |
  |          |    67  |  20  |    21    |      6   |   7  |    54 |  24   |
  |          |   126  |  32  |    60    |     15   |  13  |   120 |  23   |
  |          |    63  |  21  |    28    |      7   |  10  |    66 |  26   |
  |          |   114  |  45  |    71    |      9   |   7  |   132 |  12   |
  |          |    46  |  18  |    18    |      3   |   3  |    42 |  14   |
  |          |   111  |  35  |    56    |      6   |   7  |   104 |  13   |
  |          +--------+------+----------+----------+------+-------+-------+
  |    Total.| 1,578  | 490  |   697    |    125   | 151  | 1,463 |  19   |
  +----------+--------+------+----------+----------+------+-------+-------+

TABLE 52.--_P_1 club vermilion_ [female] [female] × _yellow_ [male] [male].
_F_1 wild-type_ [female] [female] × _F_1 club vermilion_ [male] [male].

  +------------+-----------------------------------+
  |            | F_2 females.                      |
  |            +-----------------+-----------------+
  |            | Non-cross-overs.|  Cross-overs.   |
  |            |                 |                 |
  |            |                 |                 |
  |Reference.  +-----------+-----+------+----------+
  |            | Club      |Wild-| Club.|Vermilion.~
  |            | vermilion.|type.|      |          ~
  +------------+-----------+-----+------+----------+
  |  99        |    44     | 52  |  13  |    7     |
  | 100        |    38     | 58  |   6  |   12     |
  | 101        |    30     | 32  |   6  |   12     |
  | 102        |    44     | 55  |  20  |   13     |
  | 103        |   ...     |...  |  ... |   ...    |
  |            +-----------+-----+------+----------+
  |    Total.  |   156     |197  |  45  |   44     |
  +------------+-----------+-----+------+----------+

  +-----------------------------------------------------------------------+
  |       F_2 males.                                                      |
  +------------------+-----------------+----------------+-----------------+
  |     y            |       y c_l v   |      y   v     |      y  c_l     |
  |     ------       |       -+-----   |      ---+-     |      -+--+-     |
  |     c_l  v       |                 |       c_l      |           v     |
  +-------+----------+-----------+-----+----------+-----+------+----------+
  ~Yellow.|Club      |Yellow club|Wild-|Yellow    |Club.|Yellow|Vermilion.|
  ~       |vermilion.|vermilion. |type.|vermilion.|     |club. |          |
  +-------+----------+-----------+-----+----------+-----+------+----------+
  |  35   |   27     |    2      |  9  |    8     |  11 |   0  |     1    |
  |  43   |   23     |    1      | 15  |   11     |  14 |   0  |     0    |
  |  19   |   24     |    6      |  5  |   10     |   3 |   1  |     0    |
  |  48   |   38     |   12      | 14  |    8     |  15 |   1  |     1    |
  |  43   |   32     |    7      | 16  |   13     |   7 |   1  |     1    |
  +-------+----------+-----------+-----+----------+-----+------+----------+
  | 188   |  144     |   28      | 59  |   50     |  50 |   3  |     3    |
  +-------+----------+-----------+-----+----------+-----+------+----------+

{71}

LINKAGE OF CHERRY, CLUB, AND VERMILION.

The need for a readily workable character whose gen should lie in the long
space between cherry and vermilion has long been felt. Cherry and vermilion
are so far apart that there must be considerable double crossing-over
between them. But with no favorably placed character which is at the same
time viable and clearly and rapidly distinguishable, we were unable to find
the exact amount of double crossing-over, and hence could not make a proper
correction in plotting the chromosome. Club occupies just this favorable
position nearly midway between cherry and vermilion. The distances from
cherry to club and from club to vermilion are short enough so that no error
would be introduced if we ignored the small amount of double crossing-over
within each of these distances.

It thus becomes important to know very exactly the cross-over values for
cherry club and club vermilion. The experiment has the form of the yellow
club vermilion cross of table 52, except that cherry is used instead of
yellow. Cherry is better than yellow because it is slightly nearer club
than is yellow and because the bristles of yellow flies are very
inconspicuous. In yellow flies the bristles on the side of the thorax are
yellowish brown against a yellow background, while in gray-bodied flies the
bristles are very black against a light yellowish-gray background.

For the time being we are able to present only incomplete results upon this
cross. In the first experiment cherry females were crossed to club
vermilion males and the wild-type daughters were back-crossed to cherry
club vermilion, which triple recessive had been secured for this purpose.
Table 53 gives the results.

TABLE 53.--_P_{1} cherry_ [female] [female] × _club vermilion_ [male]
[male]. _B. C. F__{1} _wild-type_ [female] × _cherry club vermilion_ [male]
[male].

  +--------+-------------------+-------------------+-------------------+
  |        |  w^c              |  w^c   c_l     v  |  w^c           v  |
  |        |  ---------------  |  ----+----------  |  -----------+---  |
  | Refer- |       c_l      v  |                   |        c_l        |
  | ence.  +-------------------+---------+---------+-------------------+
  |        |         |         |         |         |         |         |
  |        |         | Club    | Cherry  |         | Cherry  |         |
  |        | Cherry. | ver-    | club    |  Wild-  | ver-    |  Club.  |
  |        |         | milion. | ver-    |  type.  | milion. |         |
  |        |         |         | milion. |         |         |         |
  +--------+---------+---------+---------+---------+---------+---------+
  |        |         |         |         |         |         |         ~
  |  163   |    68   |    68   |     4   |    10   |    21   |    13   ~
  |  164   |    99   |    67   |    13   |    21   |    21   |    12   |
  |  165   |    23   |    37   |     9   |     7   |    15   |     2   |
  |  166   |   107   |    86   |    14   |    28   |    31   |    43   |
  |  167   |    42   |    49   |     7   |    11   |    12   |    11   |
  |  168   |    40   |    30   |     6   |    15   |    16   |     8   |
  |        +---------+---------+---------+---------+---------+---------+
  | Total. |   379   |   337   |    53   |    92   |   116   |    89   |
  +--------+---------+---------+---------+---------+---------+---------+

  +-------------------+---------+----------------------------+
  |  w^c   c_l        |         |                            |
  |  ----+------+---  |         |     Cross-over values.     |
  |                v  |         |                            |
  +---------+---------+         +----------------------------+
  |         |         |         |        |         |         |
  |         |         | Total.  |        | Club.   | Cherry  |
  | Cherry  | Ver-    |         | Cherry | ver-    | ver-    |
  | club.   | milion. |         | club.  | milion. | milion. |
  |         |         |         |        |         |         |
  +---------+---------+---------+--------+---------+---------+
  ~         |         |         |        |         |         |
  ~    1    |    0    |   185   |    8   |   19    |    26   |
  |    1    |    0    |   234   |   15   |   15    |    29   |
  |    0    |    2    |    95   |   19   |   25    |    35   |
  |    3    |    3    |   315   |   15   |   25    |    37   |
  |    2    |    2    |   136   |   16   |   20    |    30   |
  |    0    |    0    |   115   |   18   |   21    |    39   |
  +---------+---------+---------+--------+---------+---------+
  |    7    |    7    | 1,080   |   15   |   20    |    32   |
  +---------+---------+---------+--------+---------+---------+

{72}

A complementary experiment was made by crossing cherry club vermilion
females to wild males and inbreeding the F_1 in pairs. Table 54 gives the
results of this cross.

TABLE 54.--_P_{1} cherry club vermilion_ [male] [male]. [female] [female] ×
_wild_ [male] [male]. _F_{1} wild-type_ [female] × _F_{1} cherry club
vermilion_ [male] [male].

  +----------+-----------------+------------------+-----------------+
  |          |  w^c   c_l   v  |  w^c             |  w^c  c_l       |
  |          |  -------------  |  ----+--------   |  ---------+---  |
  |          |                 |        c_l   v   |              v  |
  |          +-----------+-----+-------+----------+------+----------+
  |Reference.|Cherry club|Wild-|Cherry.|  Club    |Cherry|Vermilion.|
  |          |vermilion. |type.|       |vermilion.| club.|          |
  +----------+-----------+-----+-------+----------+------+----------+
  | 188      |    60     |  76 |  12   |    8     |  12  |   29     ~
  | 189      |   228     | 314 |  48   |   44     |  50  |   60     ~
  | 197      |    68     |  81 |  23   |   13     |   9  |   22     |
  +----------+-----------+-----+-------+----------+------+----------+
  |Total.    |   356     | 471 |  83   |   65     |  71  |  111     |
  +----------+-----------+-----+-------+----------+------+----------+

  +----------------+------+----------------------------+
  |  w^c         v |      |                            |
  |  ----+----+--- |      |     Cross-over values.     |
  |       c_l      |      |                            |
  +----------+-----+Total.+------+----------+----------+
  |  Cherry  |Club.|      |Cherry|Club      |Cherry    |
  |vermilion.|     |      |club. |vermilion.|vermilion.|
  +----------+-----+------+------+----------+----------+
  ~    2     |  1  |  200 |  11  |   22     |  30      |
  ~    1     |  8  |  753 |  13  |   16     |  27      |
  |    2     |  0  |  218 |  17  |   15     |  31      |
  +----------+-----+------+------+----------+----------+
  |    5     |  9  |1,171 |  14  |   17     |  28      |
  +----------+-----+------+------+----------+----------+

The combined data of tables 53 and 54 give 14.2 as the value for cherry
club. All the data thus far presented upon club vermilion (886 cross-overs
in a total of 4,681), give 19.2 as the value for club vermilion. The locus
of club on the basis of the total data available is at 14.6.

GREEN.

In May 1913 there appeared in a culture of flies with gray body-color a few
males with a greenish-black tinge to the body and legs. The trident pattern
on the thorax, which is almost invisible in many wild flies, was here quite
marked. A green male was mated to wild females and gave in F_2 a close
approach to a 2:1:1 ratio. The green reappeared only in the F_2 males, but
the separation of green from gray was not as easy or complete as desirable.
From subsequent generations a pure stock of green was made. A green female
by wild male gave 138 wild-type females and 127 males which were greenish.
This green color varies somewhat in depth, so that some of these F_1 males
could not have been separated with certainty from a mixed culture of green
and gray males.

The results of these two experiments show that green is a sex-linked
melanistic character like sable, but the somatic difference produced is
much less than in the case of sable, so that the new mutation, although
genetically definite, is of little practical value. We have found several
eye-colors which differed from the red color of the wild fly by very small
differences. With some of these we have worked successfully by using
another eye-color as a developer. For example, the double recessive
vermilion "clear" is far more easily distinguished from vermilion than is
clear from red. But it is no small task to make up the stocks {73}
necessary for such a special study. In the case of green we might perhaps
have employed a similar method, performing all experiments with a common
difference from the gray in all flies used.

CHROME.

In a stock of forked fused there appeared, September 15, 1913, three males
of a brownish-yellow body-color. They were uniform in color, without any of
the abdominal banding so striking in other body-colors. Even the tip of the
abdomen lacked the heavy pigmentation which is a marked secondary sexual
character of the male. About 20 or more of these males appeared in the same
culture. This appearance of many males showing a mutant character and the
non-appearance of corresponding females is usual for sex-linked characters.
In such cases females appear in the next generation, as they did in this
case when the chrome males were mated to their sisters in mass cultures.
Since both females and males of chrome were on hand, it should have been an
easy matter to continue the stock, but many matings failed, and it was
necessary to resort to breeding in heterozygous form. The chrome, however,
gradually disappeared from the stock. Such a difficult sex-linked mutation
as this could be successfully handled (like a lethal) if it could be mated
to a double recessive whose members lie one on each side of the mutant, but
in the case of chrome this was not attempted soon enough to save the stock.

LETHAL 3.

In the repetition of a cross between a white miniature male and a vermilion
pink male (December 1913), the F_2 ratios among the males were seen to be
very much distorted because of the partial absence of certain classes
(Morgan 1914_c_). While it was suspected that the disturbance was due to a
lethal, the data were useless for determining the position of such a
lethal, from the fact that more than one mother had been used in each
culture. From an F_2 culture that gave practically a 2:1 sex-ratio,
vermilion females were bred to club males. Several such females gave
sex-ratios. Their daughters were again mated to vermilion males. Half of
these daughters gave high female sex-ratios and showed the linkage
relations given in table 55.

TABLE 55.--_Linkage data on club, lethal 3, and vermilion, from Morgan,
1914c_.

  +----------+-----------------------------------------------------------+
  |          |                             Males.                        |
  |          +-----------+--------------+------------------+-------------+
  |          |  c_1      |  c_1 l_3 v   |  c_1  v          |  c_1 l_3    |
  | Females. |  -------  |  --+------   |  ----+-          |  --+--+--   |
  |          |    l_3 v  |              |    l_3           |         v   |
  |          +-----------+--------------+------------------+-------------+
  |          |  Club.    |  Wild-type.  |  Club vermilion. |  Vermilion. |
  +----------+-----------+--------------+------------------+-------------+
  |  588     |   182     |    28        |    11            |    1        |
  +----------+-----------+--------------+------------------+-------------+

{74}

Lethal 3 proved to lie between club and vermilion, 13 units from club and 5
from vermilion. The same locus was indicated by the data from the cross of
vermilion lethal-bearing females by eosin miniature males. The complete
data bearing on the position of lethal 3 is summarized in table 56. On the
basis of this data lethal 3 is located at 26.5.

TABLE 56.--_Summary of linkage data on lethal 3, from Morgan, 1914c_.

  +---------------------+--------+--------+------------+
  |     Gens.           | Total. | Cross- | Cross-over |
  |                     |        | overs. | values.    |
  +---------------------+--------+--------+------------+
  | Eosin lethal 3      | 1,327  |  268   |    20.2    |
  | Eosin vermilion     | 1,327  |  357   |    27.0    |
  | Eosin miniature     | 3,374  |  967   |    29.0    |
  | Club lethal 3       |   222  |   29   |    13.0    |
  | Club vermilion      |   877  |  161   |    18.4    |
  | Lethal 3 vermilion  | 1,549  |  105   |     6.8    |
  | Lethal 3 miniature  | 1,481  |  138   |     9.3    |
  | Vermilion miniature | 1,327  |   31   |     2.3    |
  +---------------------+--------+--------+------------+

LETHAL 3a.

In January 1914 a vermilion female from a lethal 3 culture when bred to a
vermilion male gave 71 daughters and only 3 sons; 34 of these daughters
were tested, and every one of them gave a 2:1 sex-ratio. The explanation
advanced (Morgan 1914_c_) was that the mother of the high ratio was
heterozygous for lethal 3, and also for another lethal that had arisen by
mutation in the X chromosome brought in by the sperm. On this
interpretation the few males that survived were those that had arisen
through crossing-over. The rarity of the sons shows that the two lethals
were in loci near together, although here of course in different
chromosomes, except when one of them crossed over to the other. As
explained in the section on lethal 1 and 1_a_ the distance between the two
lethals can be found by taking twice the number of the surviving males
(2+3) as the cross-overs and the number of the females as the
non-cross-overs. But the 34 daughters tested were also non-cross-overs,
since none of them failed to carry a lethal. The fractions (6+0)/(71+34) =
6/105 give 5.7 as the distance between the lethals in question. In the case
of lethals 3 and 3_a_ another test was applied which showed graphically
that two lethals were present. Each of the daughters tested showed, by the
classes of her sons, the amount of crossing-over between white and that
lethal of the two that she carried. These cross-over values were plotted
and gave a bimodal curve with modes 7 units apart. It had already been
shown that the locus of one of the two lethals was at 26.5, and since the
higher of the two modes was at about 23, it corresponds to lethal 3. The
data and the curve show that the lethals 3 and 3_a_ are about 7 units
apart, _i. e._, lethal 3_a_ lies at about 19.5. {75}

LETHAL 1b.

A cross between yellow white males and abnormal abdomen females gave
(February 1914) regular results in 10 F_2 cultures, but three cultures gave
2 [female] : 1 [male] sex-ratios (Morgan, 1914_b_, p. 92). The yellow white
class, which was a non-cross-over class in these 10 cultures, had
disappeared in the 3 cultures. Subsequent work gave the data summarized in
table 57. At the time when the results of table 57 were obtained it did not
seem possible that two different lethals could be present in the space of
about 1 unit between yellow and white, and this lethal was thought to be a
reappearance of lethal 1 (Morgan, 1912_b_, p. 92). Since then a large
number of lethals have arisen, one of them less than 0.1 unit from yellow,
and at least one other mutation has taken place between yellow and white,
so that the supposition is now rather that the lethal in question was not
lethal 1. Indeed, the linkage data show that this lethal, which may be
called lethal 1_b_, lies extraordinarily close to white, for the distance
from yellow was 0.8 unit and of white from yellow on the basis of the same
data 0.8. There was also a total absence of cross-overs between lethal 1_b_
and white in the total of 846 flies which could have shown such
crossing-over. On the basis of this linkage data alone we should be obliged
to locate lethal 1_b_ at the point at which white itself is situated,
namely, 1.1, but on _a priori_ grounds it seems improbable that a lethal
mutation has occurred at the same locus as the factor for white eye-color.
Farther evidence against this supposition is that females that have one X
chromosome with both yellow and white and the other X chromosome with
yellow, lethal, and white are exactly like regular stock yellow white
flies. The lethal must have appeared in a chromosome which was already
carrying white and yet did not affect the character of the white. We
prefer, therefore, to locate lethal 1_b_ at 1.1-.

TABLE 57.--_Summary of all linkage data upon lethal 1b, from Morgan,
1914b_.

  +-------------------------+---------+--------+---------------+
  |     Gens.               |  Total. | Cross- | Cross-over    |
  |                         |         | overs. | values.       |
  +-------------------------+---------+--------+---------------+
  | Yellow lethal 1_b_      |   744   |   6    |  0.81         |
  | Yellow white            | 2,787   |  23    |  0.82         |
  | Lethal 1_b_ white       |   846   |   0    |  0.0          |
  +-------------------------+---------+--------+---------------+

FACET.

Several autosomal mutations had been found in which the facets of the
compound eye are disarranged. One that was sex-linked appeared in February
1914. Under the low power of the binocular microscope the facets are seen
to be irregular in arrangement, instead of being arranged in a strictly
regular pattern. The ommatidia are more nearly circular than hexagonal in
outline, and are variable in size, some being considerably larger than
normal. The large ones are also darker than {76} the smaller, giving a
blotched appearance to the eye. The short hairs between the facets point in
all directions instead of radially, as in the normal eye. The irregular
reflection breaks up the dark fleck which is characteristic of the normal
eye. The shape of the eye differs somewhat from the normal; it is more
convex, smaller, and is encircled by a narrow rim destitute of ommatidia.

Facet arose in a back-cross to test the independence of speck (second
chromosome) and maroon (third chromosome). One of the cultures produced,
among the first males to hatch, some males which showed the facet
disarrangement. None of the females showed this character. The complete
output was that typical of a female heterozygous for a recessive sex-linked
character: not-facet [female] [female] (2), 112; not-facet [male] [male]
(1), 57; facet [male] [male] (1), 51.

Of the three characters which were shown by the F_2 males, one, facet, is
sex-linked, another, speck, is in the second chromosome, and maroon is in
the third chromosome. All eight F_2 classes are therefore expected to be
equal in size, and each pair of characters should show free assortment,
that is, 50 per cent. The assortment value for facet speck is 48, for speck
maroon 52, and for facet maroon 48, as calculated from the F_2 males of
table 58.

TABLE 58.--_P_1 speck maroon_ [male] × _wild_ [female] [female]. _B.C. F_1
wild-type_ [female] × _speck maroon_ [male].

  +----------+----------------------------+
  |          |        F_2 females.        |
  |Reference.+-------+-----+------+-------+
  |          |Speck  |Wild-|Speck.|Maroon.|
  |          |maroon.|type.|      |       ~
  |          |       |     |      |       ~
  +----------+-------+-----+------+-------+
  |     66   |  31   |  30 |   26 |   25  |
  +----------+-------+-----+------+-------+

  +----------------------------------------------------------+
  |                          F_2 males.                      |
  +------+-------+-------+-----+-------+------+------+-------+
  |Facet.|Speck  |Facet  |Wild-|Facet  |Speck.|Facet |Maroon.|
  ~      |maroon.|speck  |type.|maroon.|      |speck.|       |
  ~      |       |maroon.|     |       |      |      |       |
  +------+-------+-------+-----+-------+------+------+-------+
  |  14  |  14   |  14   |  10 |   11  |   17 |  12  |  17   |
  +------+-------+-------+-----+-------+------+------+-------+

LINKAGE OF FACET, VERMILION AND SABLE.

In order to determine the location of facet in the first chromosome, one of
the facet males which appeared in culture 66 was crossed out to vermilion
sable females. Three of the wild-type daughters were back-crossed to
vermilion sable males. The females of the next generation should give data
upon the linkage of vermilion and sable, while the males should show the
linkage of all three gens, facet, vermilion, and sable. The offspring of
these three females are classified in table 59.

The cross-over fraction for vermilion sable as calculated from the females
is 19/194. The cross-over value corresponding to this fraction is 10 units,
which was the value found in the more extensive experiments given in the
section on sable.

It will be noticed that the results in the males of culture 150 are
markedly different from those of the other two pairs. While the sable males
are fully represented, their opposite classes, the gray males, are {77}
entirely absent. This result is due to a lethal factor, lethal 5, which
appeared in this culture for the first time.

The males of the two cultures 149 and 151 give the order of gens as facet,
vermilion, sable; that is, facet lies to the left of vermilion and toward
yellow. The cross-over values are: facet vermilion 40; vermilion sable 12;
facet sable 42. Since yellow and vermilion usually give but 34 per cent of
crossing-over, this large value of 40 for facet vermilion shows that facet
must lie very near to yellow.

TABLE 59.--_P_1 facet_ [male] × _vermilion sable_ [female] [female]. _B.C.
F_1 wild-type_ [female] × _vermilion sable_ [male] [male].

  +----------+----------------------------------+
  |          |           F_2 females.           |
  |          +----------------+-----------------+
  |          |                |                 |
  |          |Non-cross-overs.|   Cross-overs.  |
  |          |                |                 |
  |Reference.+---------+------+----------+------+
  |          |Vermilion|Wild- |Vermilion.|Sable.|
  |          |sable.   |type. |          |      ~
  |          |         |      |          |      ~
  +----------+---------+------+----------+------+
  | 149      |  16     |  29  |    3     |  3   |
  | 150      |  13     |  17  |    2     |  2   |
  | 151      |  37     |  63  |    7     |  2   |
  |          +---------+------+----------+------+
  |   Total. |  66     | 109  |   12     |  8   |
  +----------+---------+------+----------+------+

  +--------------------------------------------------------------------+
  |                         F_2 males.                                 |
  +----------------+---------------+-----------------+-----------------+
  |   f_a          |   f_a v s     |     f_a  s      |      f_a v      |
  |   ------       |   --+----     |     ----+-      |      --+--+--   |
  |      v s       |               |        v        |            s    |
  +------+---------+---------+-----+------+----------+----------+------+
  |Facet.|Vermilion|Facet    |Wild-|Facet |Vermilion.|Facet     |Sable.|
  ~      |sable.   |vermilion|type.|sable.|          |vermilion.|      |
  ~      |         |sable.   |     |      |          |          |      |
  +------+---------+---------+-----+------+----------+----------+------+
  | 17   | 10      |  8      | 12  |  2   |  ..      |  2       |  1   |
  | ..   | 10      |  9      | ..  |  1   |  ..      | ..       | ..   |
  | 38   | 23      | 12      | 26  |  2   |   8      |  4       |  1   |
  +------+---------+---------+-----+------+----------+----------+------+
  | 55   | 43      | 29      | 38  |  5   |   8      |  6       |  2   |
  +------+---------+---------+-----+------+----------+----------+------+

LINKAGE OF EOSIN, FACET, AND VERMILION.

In order to obtain more accurate information on the location of facet, a
facet male was mated to an eosin vermilion female. The F_1 females were
mated singly to wild males and they gave the results shown in table 60. The
F_2 females were not counted, since they do not furnish any information.
The evidence of table 60 places facet at 1.1 units to the right of eosin,
or at 2.2.

TABLE 60.--_P_1 eosin vermilion_ [female] × _facet_ [male]. _F_1 wild-type_
[female] × _wild_ [male].

  +----------+-----------------+-----------------+-----------------+
  |          |     w^c   v     |   w^c f_a       |     w^c         |
  |          |     -------     |   --+----       |     ----+-      |
  |          |       f_a       |         v       |      f_a v      |
  |Reference.+----------+------+------+----------+------+----------+
  |          |Eosin     |Facet.|Eosin |Vermilion.|Eosin.|Facet     |
  |          |vermilion.|      |facet.|          |      |vermilion.|
  |          |          |      |      |          |      |          |
  +----------+----------+------+------+----------+------+----------+
  | 512      |  43      |  43  | ..   |   1      |  13  |  16      ~
  | 513      |  28      |  35  | ..   |   2      |  19  |   5      ~
  | 514      |  18      |  31  |  1   |  ..      |  17  |  11      |
  | 515      |  18      |  60  | ..   |  ..      |  20  |  15      |
  | 516      |  10      |  31  | ..   |  ..      |   7  |  12      |
  | 517      |  24      |  34  | ..   |  ..      |  10  |  12      |
  | 518      |  44      |  38  |  1   |   1      |  23  |  22      |
  +----------+----------+------+------+----------+------+----------+
  |    Total.| 185      | 272  |  2   |   4      | 109  |  93      |
  +----------+----------+------+------+----------+------+----------+

  +----------------+------+----------------------------+
  |   w^c f_a v    |      |                            |
  |   --+---+--    |      |     Cross-over values.     |
  |                |      |                            |
  +----------+-----+Total.+------+----------+----------+
  |Eosin     |Wild-|      |Eosin |Facet     |Eosin     |
  |facet     |type.|      |facet.|vermilion.|vermilion.|
  |vermilion.|     |      |      |          |          |
  +----------+-----+------+------+----------+----------+
  ~  ..      | ..  | 116  | .... | ....     | ....     |
  ~  ..      | ..  |  89  | .... | ....     | ....     |
  |  ..      | ..  |  78  | .... | ....     | ....     |
  |  ..      | ..  | 113  | .... | ....     | ....     |
  |  ..      | ..  |  60  | .... | ....     | ....     |
  |  ..      | ..  |  80  | .... | ....     | ....     |
  |  ..      |  1  | 130  | .... | ....     | ....     |
  +----------+-----+------+------+----------+----------+
  |  ..      |  1  | 666  | 1.05 | 30.5     | 31.3     |
  +----------+-----+------+------+----------+----------+

{78}

LETHAL SC.

The third of the lethals which Miss Stark found (Stark, 1915) while she was
testing the relative frequency of occurrence of lethals in fresh and inbred
wild stocks arose in April 1914 in stock caught in 1910. Females
heterozygous for this lethal, lethal _sc_, were mated to white males and
the daughters were back-crossed to white males. Half of the daughters gave
lethal sex-ratio, and these gave 1,405 cross-overs in a total of 3,053
males, from which the amount of crossing-over between white and lethal _sc_
has been calculated as 46 per cent.

By reference to table 65 it is seen that white and bar normally give only
about 44 per cent of crossing-over in a two-locus experiment; lethal _sc_
then is expected to be situated at least as far to the right as bar.
Females heterozygous for lethal _sc_ were therefore crossed to bar males,
and their daughters were tested. The lethal-bearing daughters gave 144
cross-overs in a total of 1,734 males, that is, bar and lethal _sc_ gave
8.3 per cent of crossing-over. Lethal _sc_ therefore lies 8.3 units beyond
bar or at about 66.5. The cross-over value sable lethal _sc_ was found to
be 23.5 (387 cross-overs in a total of 1,641 males) which places the lethal
at 43+23.5, or at 66.5. We know from other data that there is enough double
crossing-over in the distance which gives an experimental value of 23.5 per
cent, so that the true distance is a half unit longer or the locus at 67.0
is indicated by the 1,641 males of the sable lethal experiment. In a
distance so short that the experimental value is only 8.3 per cent there
is, as far as we have been able to determine, no double crossing-over at
all, or at most an amount that is entirely negligible, so that a locus at
57+8.3 or 65.3 is indicated by the 1,734 males of the bar lethal
experiment. To get the value indicated by the total data the cases may be
weighted, that is, the value 65.3 may be multiplied by 1,734, and 67.0 may
be multiplied by 1,641. The sum of these two numbers divided by the sum of
1,734 and 1,641 gives 66.2 as the locus indicated by all the data
available. This method has been used in every case where more than one
experiment furnishes data upon the location of a factor. In constructing
the map given in diagram I rather complex balancings were necessary.

LETHAL SD.

The fourth lethal which Miss Stark found (May 1914) in the inbred stocks of
_Drosophila_ has not been located by means of linkage experiments. It is
interesting in that the males which receive the lethal factor sometimes
live long enough to hatch. These males are extremely feeble and never live
more than two days. There is, as far as can be seen, no anatomical defect
to which their extreme feebleness and early death can be attributed. {79}

FURROWED.

In studying the effect of hybridization upon the production of mutations in
_Drosophila_, F. N. Duncan found a sex-linked mutation which he called
"furrowed eye" (Duncan 1915). The furrowed flies are characterized by a
foreshortening of the head, which causes the surface of the eye to be
thrown into irregular folds with furrows between. The spines of the
scutellum are stumpy, a character which is of importance in classification,
since quite often flies occur which have no noticeable disturbance of the
eyes.

The locus of furrowed was determined to be at 38.0 on the basis of the data
given in table 61.

TABLE 61.--_Data on the linkage of furrowed, from Duncan, 1915_.

  +------------+-------------------------------------------+------+
  |   Gens.    |               F_2 males.                  |      |
  +------------+---------+---------+-----------+-----------+      +
  |            | w^e m   | w^e f_w | w^e m f_w | w^e       |Total.|
  |            | ------- | --+---- | -----+--- | --+--+--  |      |
  |            |     f_w |     m   |           |     m f_w |      |
  |            +---------+---------+-----------+-----------+------+
  |Eosin,      |         |         |           |           |      |
  |  miniature,|         |         |           |           |      |
  |  furrowed  |   142   |    59   |     4     |    3      | 208  |
  |            +=========+=========+===========+===========+======+
  |            | f_w     | f_w s f | f_w  f    | f_w s     |      |
  |            | ------- | --+---- | ----+-    | --+--+--  |      |
  |            |    s  f |         |   s       |        f  |      |
  |            |---------+---------+-----------+-----------+------+
  |Furrowed,   |         |         |           |           |      ~
  |  sable,    |         |         |           |           |      ~
  |  forked    |   166   |     9   |    31     |    3      | 209  |
  |            +=========+=========+===========+===========+======+
  |            | v    B' | v f_w   | v         | v f_w B'  |      |
  |            | ------- | -+----- | ----+--   | -+---+--  |      |
  |            |  f_w    |      B' |  f_w B'   |           |      |
  |            +---------+---------+-----------+-----------+------+
  |Vermilion,  |         |         |           |           |      |
  |  furrowed, |         |         |           |           |      |
  |  bar       |   188   |     9   |    43     |    0      | 240  |
  +------------+---------+---------+-----------+-----------+------+

  +------------------------------+
  |      Cross-over values.      |
  +----------+---------+---------+
  |Eosin     |Miniature|Eosin    |
  |miniature.|furrowed.|furrowed.|
  |          |         |         |
  +----------+---------+---------+
  |          |         |         |
  |          |         |         |
  | 29.8     |  30.4   |  30.3   |
  +==========+=========+=========+
  |Furrowed  |Sable    |Furrowed |
  |sable.    |forked.  |forked.  |
  |          |         |         |
  +----------+---------+---------+
  ~          |         |         |
  ~          |         |         |
  |  5.7     |  16.3   |  19.1   |
  +==========+=========+=========+
  |Vermilion |Furrowed |Vermilion|
  |furrowed. |bar.     |bar.     |
  |          |         |         |
  +----------+---------+---------+
  |          |         |         |
  |          |         |         |
  |  3.8     |  21.6   |  17.9   |
  +----------+---------+---------+

ADDITIONAL DATA FOR YELLOW, WHITE, VERMILION, AND MINIATURE.

Considerable new work has been done by various students upon the linkage of
the older mutant characters, namely, yellow, white, vermilion, and
miniature. We have summarized these new data, and they give values very
close to those already published. We have included in the white miniature
data those published by P. W. Whiting (Whiting 1913). {80}

TABLE 62.--_Data upon the linkage of yellow, white, vermilion, and
miniature_ (_contributed by students_).

  +--------------------+-----------------+-------------+-------+----------+
  | Gens.              | Non-cross-overs.| Cross-overs.|       |          |
  +--------------------+-----------------+-------------+       |          |
  |                    | w           m   | w           |Total. |Cross-over|
  |                    | -------------   | -----+----- |       |values.   |
  |                    |                 |           m |       |          |
  |                    +-----------------+-------------+-------+----------+
  |White miniature.    | 6,219[8] 7,378  | 3,754 3,337 |20,688 |   34.2   |
  |                    +=================+=============+=======+==========+
  |                    | w               | w         m |       |          |
  |                    | -------------   | -----+----- |       |          |
  |                    |             m   |             |       |          |
  |                    +-----------------+-------------+-------+----------+
  |                    | 1,651   1,116   |   671 1,047 | 4,485 |   38.3   |
  |                    +=================+=============+=======+==========+
  |                    | y               | y         m |       |          |
  |                    | -------------   | -----+----- |       |          |
  |                    |             m   |             |       |          |
  |                    +-----------------+-------------+-------+----------+
  |Yellow miniature.   |   761     923   |   421   653 | 2,758 |   39     |
  |                    +=================+=============+=======+==========+
  |                    | v               | v         m |       |          |
  |                    | -------------   | -----+----- |       |          |
  |                    |             m   |             |       |          |
  |                    +-----------------+-------------+-------+----------+
  |Vermilion miniature.| 1,685   1,460   |    32    36 | 3,213 |    2.1   |
  |                    +=================+=============+=======+==========+
  |                    | y           w   | y           |       |          |
  |                    | -------------   | -----+----- |       |          |
  |                    |                 |           w |       |          |
  |                    +-----------------+-------------+-------+----------+
  |Yellow white.       | 1,600   1,807   |    10     7 | 3,424 |    0.5   |
  |                    +=================+=============+=======+==========+
  |                    | y           v   | y           |       |          |
  |                    | -------------   | -----+----- |       |          |
  |                    |                 |           v |       |          |
  |                    +-----------------+-------------+-------+----------+
  |Yellow vermilion.   |   509     587   |   328   284 | 1,708 |   35.8   |
  |                    +=================+=============+=======+==========+
  |                    | w          B'   | w           |       |          |
  |                    | -------------   | -----+----- |       |          |
  |                    |                 |          B' |       |          |
  |                    +-----------------+-------------+-------+----------+
  |White bar.          |   198     272   |   168   166 |   804 |   42     |
  |                    +=================+=============+=======+==========+
  |                    | b_1             | b_1       r |       |          |
  |                    | -------------   | -----+----- |       |          |
  |                    |             r   |             |       |          |
  |                    +-----------------+-------------+-------+----------+
  |Bifid rudimentary.  |   142      15   |    12   116 |   285 |   45     |
  |                    +=================+=============+=======+==========+
  |                    | r               | r         f |       |          |
  |                    | -------------   | -----+----- |       |          |
  |                    |             f   |             |       |          |
  |                    +-----------------+-------------+-------+----------+
  |Rudimentary forked. |    73     211   |   ...     4 |   288 |    1.4   |
  +--------------------+-----------------+-------------+-------+----------+

{81}

NEW DATA CONTRIBUTED BY A. H. STURTEVANT AND H. J. MULLER.

Data from several experiments upon sex-linked characters described in this
paper have been contributed by Dr. A. H. Sturtevant and Mr. H. J. Muller,
and are given in table 63.

TABLE 63.--_Data contributed by A. H. Sturtevant and H. J. Muller._

  +---------------------+-----------------------------------+------+
  |Gens.                |              Classes.             |      |
  +---------------------+--------+--------+--------+--------+      |
  |                     | y w    | y   b_1| y w b_1| y      |Total.|
  |                     | -------| -+-----| ---+---| -+--+--|      |
  |                     |     b_1|  w     |        |  w b_1 |      |
  |                     +--------+--------+--------+--------+------+
  |Yellow white × bifid.| 233 254|  1   2 | 10   6 | ..  .. |  506 |
  |                     +========+========+========+========+======+
  |                     | y      | y v B' | y    B'| y v    |      |
  |                     | -------| -+-----| ---+---| -+--+--|      |
  |                     |    v B'|        |    v   |      B'|      |
  |Yellow × vermilion   +--------+--------+--------+--------+------+
  |bar.                 | 99 101 | 60  55 | 49  48 |  9  14 |  435 |
  |                     +========+========+========+========+======+
  |                     | w b_1  | w     f| w b_1 f| w      |      |
  |                     | -------| -+-----| ---+---| -+--+--|      |
  |                     |       f|    b   |        |   b_1 f|      |
  |                     +--------+--------+--------+--------+------+
  |White bifid × forked.| 84  77 |  9   6 | 65  59 |  1   5 |  306 |
  |                     +========+========+========+========+======+
  |                     | v m    | v     s| v m   s| v      |      |
  |                     | -------| -+-----| ---+---| -+--+--|      |
  |                     |       s|    m   |        |    m  s|      |
  |Vermilion miniature  +--------+--------+--------+--------+------+
  |× sable.             | 152 111|  4   2 |  5  12 |  ..  ..|  286 |
  |                     +========+========+========+========+======+
  |                     | s    r | s     f| s  r  f| s      |      ~
  |                     | -------| -+-----| ----+--| -+--+--|      ~
  |                     |       f|      r |        |    r  f|      |
  |Sable rudimentary ×  +--------+--------+--------+--------+------+
  |forked.              | 143 195| 26  27 |  4   3 |  ..  ..|  398 |
  +---------------------+--------+--------+--------+--------+------+
  |                  WHITE BIFID × RUDIMENTARY.                    |
  +---------------------+-----------------------------------+------+
  |  F_{2} females.     |            F_{2} males.           |      |
  +--------+------------+--------+--------+--------+--------+      |
  |w   b_1 | w          | w b_1  | w   r  | w b_1 r| w      |Total.|
  |------- | --+---     | -------| -+---  | -----+-| +---+- |      |
  |        |    b_1     |      r |   b_1  |        |  b_1 r |      |
  +--------+------------+--------+--------+--------+--------+------+
  |228 335 | 15  11     | 150 66 |  2  10 | 29  135|  2   1 |  395 |
  +--------+------------+--------+--------+--------+--------+------+
  |             WHITE BIFID × MINIATURE RUDIMENTARY.               |
  +--------+------------+--------+--------+--------+--------+------+
  |w   b_1 | w          |        |        |        |        |      |
  |------- | --+---     | ------ |  -+--- | ---+---| -----+-|-+-+--|
  |        |    b_1     |        |        |        |        |      |
  +--------+------------+--------+--------+--------+--------+------+
  |  344   |   31       |  109   |     2  |    58  |    41  |   2  |
  +--------+------------+--------+--------+--------+--------+------+

  +--------------------------------------+
  |          Cross-over values.          |
  +------------+------------+------------+
  | Yellow     |  White     | Yellow     |
  |  white.    |  bifid.    | bifid.     |
  |            |            |            |
  +------------+------------+------------+
  |   0.6      |   3.2      |   3.8      |
  +============+============+============+
  | Yellow     |Vermilion   | Yellow     |
  |vermilion.  |   bar.     |   bar.     |
  |            |            |            |
  +------------+------------+------------+
  |   32       |  28        |  49        |
  +============+============+============+
  |  White     |  Bifid     |  White     |
  |  bifid.    | forked.    | forked.    |
  |            |            |            |
  +------------+------------+------------+
  |    7       |  42        |  45        |
  +============+============+============+
  | Vermilion  |Miniature   |Vermilion   |
  | miniature. |  sable.    |  sable.    |
  |            |            |            |
  +------------+------------+------------+
  |    2.1     |   6        |   8.1      |
  +============+============+============+
  ~  Sable     |Rudimentary |  Sable     |
  ~rudimentary.|  forked.   | forked.    |
  |            |            |            |
  +------------+------------+------------+
  |    13.3    |   1.8      |  15        |
  +------------+------------+------------+
  |      WHITE BIFID × RUDIMENTARY.      |
  +--------------------------------------+
  |           Cross-over values.         |
  +------------+------------+------------+
  |   White    |  Bifid     |   White    |
  |   bifid.   |rudimentary.|rudimentary.|
  |            |            |            |
  +------------+------------+------------+
  |     3.8    |  42.3      |  44.5      |
  +------------+------------+------------+
  | WHITE BIFID × MINIATURE RUDIMENTARY. |
  +------------+------------+------------+
  |            |            |            |
  |  -+--+-    |  ---+-+-   | -+-+-+-    |
  |            |            |            |
  +------------+------------+------------+
  |     0      |      6     |    1       |
  +------------+------------+------------+

{82}

SUMMARY OF THE PREVIOUSLY DETERMINED CROSS-OVER VALUES.

The data of the earlier papers, namely, Dexter, 1912; Morgan, 1910_c_,
1911_a_, 1911_f_, 1912_f_, 1912_g_; Morgan and Bridges, 1913; Morgan and
Cattell, 1912 and 1913; Safir, 1913; Sturtevant, 1913 and 1915; and Tice,
1914, have been summarized in a recent paper by Sturtevant (Sturtevant,
1915) and are given here in table 64. Our summary combines three summaries
of Sturtevant, viz, that of single crossing-over and two of double
crossing-over.

TABLE 64.--_Previously published data summarized from Sturtevant, 1915_.

  +------------------------+--------+-------------+------------+
  |     Factors.           | Total. | Cross-overs.| Cross-over |
  |                        |        |             | values.    |
  +------------------------+--------+-------------+------------+
  | Yellow white.          | 46,564 |    498      |    1.07    |
  | Yellow vermilion.      | 10,603 |  3,644      |   33.4     |
  | Yellow miniature.      | 18,797 |  6,440      |   34.3     |
  | Yellow rudimentary.    |  2,563 |  1,100      |   42.9     |
  | Yellow bar.            |    191 |     88      |   46.1     |
  | White vermilion.       | 15,257 |  4,910      |   32.1     |
  | White miniature.       | 41,034 | 13,513      |   32.8     |
  | White rudimentary.     |  5,847 |  2,461      |   42.1     |
  | White bar.             |  5,151 |  2,267      |   44.0     |
  | Vermilion miniature.   |  5,329 |    212      |    4.0     |
  | Vermilion rudimentary. |  1,554 |    376      |   24.1     |
  | Vermilion bar.         |  7,514 |  1,895      |   25.2     |
  | Miniature rudimentary. | 12,567 |  2,236      |   17.8     |
  | Miniature bar.         |  3,112 |    636      |   20.4     |
  | Rudimentary bar.       |    159 |      7      |    4.4     |
  +------------------------+--------+-------------+------------+

{83}

SUMMARY OF ALL DATA UPON LINKAGE OF GENS IN CHROMOSOME I.

In table 65 all data so far secured upon the sex-linked characters are
summarized. These data include the experiments previously published in the
papers given in the bibliography and the experiments given here. The data
from experiments involving three or more loci are calculated separately for
each value and included in the totals.

TABLE 65.--_A summary of all linkage data upon chromosome I_.

  +----------------------------+----------+--------------+------------+
  |     Gens.                  |  Total.  | Cross-overs. | Cross-over |
  |                            |          |              | values.    |
  +----------------------------+----------+--------------+------------+
  | Yellow lethal 1.           |     131  |      1       |  0.8       |
  | Yellow lethal 1_b_.        |     744  |      6       |  0.8       |
  | Yellow white.              |  81,299  |    875       |  1.1       |
  | Yellow abnormal.           |  15,314  |    299       |  2.0       |
  | Yellow bifid.              |   3,681  |    201       |  5.5       |
  | Yellow club.               |     525  |     93       | 17.7       |
  | Yellow vermilion.          |  13,271  |  4,581       | 34.5       |
  | Yellow miniature.          |  21,686  |  7,559       | 34.3       |
  | Yellow sable.              |   1,600  |    686       | 42.9       |
  | Yellow rudimentary.        |   2,563  |  1,100       | 42.9       |
  | Yellow bar.                |     626  |    300       | 47.9       |
  | Lethal 1 white.            |   1,763  |      7       |  0.4       |
  | Lethal 1 miniature.        |     814  |    323       | 39.7       |
  | Lethal 1_b_ white.         |     846  |      0       |  0.0       |
  | White facet.               |     666  |      7       |  1.1       |
  | White abnormal.            |  16,300  |    277       |  1.7       |
  | White bifid.               |  23,595  |  1,260       |  5.3       |
  | White lethal 2.            |   8,011  |    767       |  9.6       |
  | White club.                |   2,251  |    321       | 14.3       |
  | White lethal _sb_.         |   3,678  |    572       | 15.6       |
  | White lemon.               |     241  |     35       | 14.5       |
  | White depressed.           |      59  |     12       | 20.3       |
  | White lethal _sa_.         |   1,150  |    256       | 22.2       |
  | White vermilion.           |  27,962  |  8,532       | 30.5       |
  | White reduplicated.        |     418  |    121       | 28.9       |
  | White miniature.           | 110,701  | 31,071       | 33.2       |
  | White furrowed.            |     208  |     63       | 30.3       |
  | White sable.               |   2,511  |  1,032       | 41.2       |
  | White rudimentary.         |   6,461  |  2,739       | 42.4       |
  | White forked.              |   3,664  |  1,676       | 45.7       |
  | White bar.                 |   5,955  |  2,601       | 43.6       |
  | White fused.               |     430  |    186       | 43.3       |
  | White lethal _sc_.         |   3,053  |  1,406       | 46.0       |
  | Facet vermilion.           |     852  |    278       | 32.6       |
  | Facet sable.               |     186  |     80       | 43.0       |
  | Bifid vermilion.           |   2,724  |    849       | 31.1       |
  | Bifid miniature.           |     219  |     67       | 30.6       |
  | Bifid rudimentary.         |     899  |    384       | 42.7       |
  | Bifid forked.              |     306  |    130       | 42.5       |
  | Lethal 2 vermilion.        |   1,400  |    248       | 17.7       |
  | Lethal 2 miniature.        |   6,752  |  1,054       | 15.4       |
  | Club lethal 3.             |     222  |     29       | 13.0       |
  | Club vermilion.            |   5,558  |  1,047       | 18.8       |
  | Lethal _sb_ miniature.     |  3,678   |    733       | 19.9       |
  | Lemon vermilion.           |     241  |     29       | 12.0       |
  {84}
  | Shifted vermilion.         |   1,007  |    155       | 15.5       |
  | Shifted bar.               |     242  |     76       | 31.4       |
  | Depressed vermilion.       |      59  |     10       | 17.0       |
  | Depressed bar.             |     464  |    176       | 38.0       |
  | Lethal 3 vermilion.        |   1,549  |    105       |  6.8       |
  | Lethal 3 miniature.        |   1,481  |    138       |  9.3       |
  | Vermilion dot.             |      57  |      0       |  0.0       |
  | Vermilion reduplicated.    |     667  |     11       |  1.7       |
  | Vermilion miniature.       |  10,155  |    317       |  3.1       |
  | Vermilion furrowed.        |     240  |      9       |  3.8       |
  | Vermilion sable.           |   9,209  |    929       | 10.1       |
  | Vermilion rudimentary.     |   1,554  |    376       | 24.1       |
  | Vermilion forked.          |     665  |    163       | 24.5       |
  | Vermilion bar.             |  23,522  |  5,612       | 23.9       |
  | Vermilion fused.           |   9,252  |  2,390       | 25.8       |
  | Reduplicated bar.          |     583  |    120       | 20.6       |
  | Miniature furrowed.        |     208  |      7       |  3.4       |
  | Miniature sable.           |   1,855  |    125       |  6.7       |
  | Miniature rudimentary.     |  12,786  |  2,284       | 17.9       |
  | Miniature bar.             |   3,112  |    636       | 20.5       |
  | Furrowed sable.            |     209  |     12       |  5.7       |
  | Furrowed forked.           |     209  |     40       | 19.1       |
  | Furrowed bar.              |     240  |     43       | 17.9       |
  | Sable rudimentary.         |     663  |     95       | 14.3       |
  | Sable forked.              |     872  |    140       | 16.0       |
  | Sable bar.                 |   7,524  |  1,036       | 13.8       |
  | Sable lethal _sc_.         |   1,641  |    387       | 23.6       |
  | Rudimentary forked.        |   1,456  |     20       |  1.4       |
  | Rudimentary bar.           |     664  |     15       |  2.3       |
  | Forked bar.                |   1,706  |      8       |  0.5       |
  | Forked fused.              |   1,201  |     37       |  3.1       |
  | Bar fused.                 |   8,768  |    222       |  2.5       |
  | Bar lethal _sc_.           |   1,734  |    144       |  8.3       |
  +----------------------------+----------+--------------+------------+

       *       *       *       *       *


{85}

BIBLIOGRAPHY.

BRIDGES, CALVIN B.

    1913. Non-disjunction of the sex-chromosomes of _Drosophila_. Jour.
    Exp. Zool., 15, p. 587, Nov. 1913.

    1914. Direct proof through non-disjunction that the sex-linked gens of
    _Drosophila_ are borne by the X chromosome. Science, 40, p. 107, July
    17, 1914.

    1915. A linkage variation in _Drosophila_. Jour. Exp. Zool., 19, p. 1.
    July 1915.

    1916. Non-disjunction as proof of the chromosome theory of heredity.
    First instalment, Genetics I, p. 1-52; second instalment, Genetics I,
    No. 2, 107-164.

CHAMBERS, R.

    1914. Linkage of the factor for bifid wing. Biol. Bull. 27, p. 151,
    Sept. 1914.

DEXTER, JOHN S.

    1912. On coupling of certain sex-linked characters in _Drosophila_.
    Biol. Bull. 23, p. 183, Aug. 1912.

    1914. The analysis of a case of continuous variation in _Drosophila_ by
    a study of its linkage relations. Am. Nat., 48, p. 712, Dec. 1914.

DUNCAN, F. N.

    1915. An attempt to produce mutations through hybridization. Am. Nat.,
    49, p. 575, Sept. 1915.

HOGE, M. A.

    1915. The influence of temperature on the development of a Mendelian
    character. Jour. Exp. Zool., 18, p. 241.

MORGAN, T. H.

    1910a. Hybridization in a mutating period in _Drosophila_. Proc. Soc.
    Exp. Biol. and Med., p. 160, May 18, 1910.

    1910b. Sex-limited inheritance in _Drosophila_. Science 32, p. 120,
    July 22, 1910.

    1910c. The method of inheritance of two sex-limited characters in the
    same animal. Proc. Soc. Exp. Biol. and Med., 8, p. 17.

    1911a. An alteration of the sex-ratio induced by hybridization. Proc.
    Soc. Exp. Biol. and Med., 8, No. 3.

    1911b. The origin of nine wing mutations in _Drosophila_. Science, 33,
    p. 496, Mar. 31, 1911.

    1911c. The origin of five mutations in eye-color in _Drosophila_, and
    their mode of inheritance. Science, April 7, 1911, 33, P. 534.

    1911d. A dominant sex-limited character. Proc. Soc. Exp. Biol. and
    Med., Oct. 1911.

    1911e. Random segregation _versus_ coupling in Mendelian inheritance.
    Science, 34, p. 384, Sept. 22, 1911.

    1911_f_. An attempt to analyze the constitution of the chromosomes on
    the basis of sex-linked inheritance in _Drosophila_. Jour. Exp. Zool.,
    11, p. 365, Nov. 1911.

    1912a. Eight factors that show sex-linked inheritance in _Drosophila_.
    Science, Mar. 22, 1912.

    1912c. Heredity of body-color in _Drosophila_. Jour. Exp. Zool., 13, p.
    27, July 1912.

    1912d. The masking of a Mendelian result by the influence of the
    environment. Proc. Soc. Exp. Zool. and Med., 9, p. 73.

    1912e. The explanation of a new sex-ratio in _Drosophila_. Science, 36,
    p. 718, No. 22, 1912.

    1912_f_. Further experiments with mutations in eye-color of
    _Drosophila_. Jour. Acad. Nat. Sci. Phil., Nov. 1912.

    1912_g_. A modification of the sex-ratio and of other ratios through
    linkage. Z. f. ind. Abs. u. Veterb. 1912.

    1914a. Another case of multiple allelomorphs in _Drosophila_. Biol.
    Bull. 26, p. 231, Apr. 1914.

    1914b. Two sex-linked lethal factors in _Drosophila_ and their
    influence on the sex-ratio. Jour. Exp. Zool., 17, p. 81, July 1914.

    1914c. A third sex-linked lethal factor in _Drosophila_. Jour. Exp.
    Zool., 17, p. 315, Oct. 1914.

    1914d. Sex-limited and sex-linked inheritance. Am. Nat., 48, P. 577,
    Oct. 1914.

    1915a. The infertility of rudimentary-winged females of _Drosophila_.
    Am. Nat., 49, p. 40, Apr. 1915.

    1915b. The rôle of the environment in the realization of a sex-linked
    Mendelian character in _Drosophila_. Am. Nat., 49, p. 385, July 1915.

{86}

MORGAN, T. H., and C. B. BRIDGES.

    1913. Dilution effects and bicolorism in certain eye-colors of
    _Drosophila_. Jour. Exp. Zool., 15, p. 429, Nov. 1913.

MORGAN, T. H., and ELETH CATTELL.

    1912. Data for the study of sex-linked inheritance in _Drosophila_.
    Jour. Exp. Zool., July, 1912.

    1913. Additional data for the study of sex-linked inheritance in
    _Drosophila_. Jour. Exp. Zool., Jan. 1913.

MORGAN, T. H., and H. PLOUGH.

    1915. The appearance of known mutations in other mutant stocks. Am.
    Nat., 49, p. 318, May 1915.

MORGAN, STURTEVANT, MULLER, and BRIDGES. The mechanism of Mendelian
heredity. Henry Holt & Co., 1915.

MORGAN, T. H., and S. C. TICE.

    1914. The influence of the environment on the size of the expected
    classes. Biol. Bull., 26, p. 213, Apr. 1914.

RAWLS, ELIZABETH.

    1913. Sex-ratios in _Drosophila ampelophila_. Biol. Bull. 24, p. 115,
    Jan. 1913.

SAFIR, S. R.

    1913. A new eye-color mutation in _Drosophila_ and its mode of
    inheritance. Biol. Bull. 25, p. 47, June 1913.

STARK, M. B.

    1915. The occurrence of lethal factors in inbred and wild stocks of
    DROSOPHILA. Jour. Exp. Zool., 19, p. 531-538. Nov. 1915.

STURTEVANT, A. H.

    1913. The linear arrangement of six sex-linked factors in _Drosophila_
    as shown by their mode of association. Jour. Exp. Zool., Jan. 1913.

    1915. The behavior of the chromosomes as studied through linkage. Z. f.
    Ind. Abs. u. Vereb. 1915.

TICE, S. C.

    1914. A new sex-linked character in _Drosophila_. Biol. Bull., Apr.,
    1914.

WHITING, P. W.

    1913. Viability and coupling in _Drosophila_. Am. Nat., 47, p. 508,
    Aug. 1913.

       *       *       *       *       *


DESCRIPTIONS OF PLATES.

              PLATE I.

  FIG. 1. Normal [female].

  FIG. 2. Sable [female].

  FIG. 3. Lemon [male].

  FIG. 4. Abnormal abdomen [female].

  FIG. 5. Abnormal abdomen [female].

  FIG. 6. Yellow [female].

              PLATE II.

  FIG. 7. Eosin, miniature, black [male].

  FIG. 8. Eosin, miniature, black [female].

  FIG. 9. Cherry.

  FIG. 10. Vermilion.

  FIG. 11. White.

  FIG. 12. Bar (from above).

  FIG. 13. Bar (from side).

  FIG. 14. Spot [female] (abdomen from above).

  FIG. 15. Spot [female] (abdomen from side).

  FIG. 16. Spot [male] (abdomen from above).

  FIG. 17. Spot [male] (abdomen from side).

[Illustration]

[Illustration]

       *       *       *       *       *


Notes

[1] For a fuller discussion see "The Mechanism of Mendelian Heredity" by
Morgan, Sturtevant, Muller, and Bridges. Henry Holt & Co., 1915.

[2] _B. C._ here and throughout stands for back-cross.

[3] The first dark body-color mutation "black" (see plate II, figs. 7, 8)
had appeared much earlier (Morgan 1911_b_, 1912_c_). It is an autosomal
character, a member of the second group of linked gens. Still another dark
mutant, "ebony," had also appeared, which was found to be a member of the
third group of gens.

[4] Wherever reference numbers are given, these denote the pages in the
note-books of Bridges upon which the original entries for each culture are
to be found.

[5] In addition to these expected F_1 wild-type females there occurred 13
females of an eye-color like that of the mutant pink. So far as was seen
none of the F_1 males differed in eye-color from the expected eosin
vermilion. Since the eosin vermilion and sable stocks were unrelated and
neither was known to contain a "pink" as an impurity, these "pinks" must be
due to mutation of an unusual kind. That these "pinks" were really products
of the cross is proven by the result of crossing one of them to one of her
eosin vermilion brothers, for she showed herself to be heterozygous for
eosin, vermilion, and sable.

_F_1 "pink" (Ref. 51 C) [female] × F_1 eosin vermilion [male]._

  +------+---------------+----------------+---------------+---------------+
  |      |   Wild-type.  |Eosin vermilion.|     Eosin.    |   Vermilion.  |
  |Refer-+-------+-------+--------+-------+--------+------+--------+------+
  |ence. |[female]|[male]|[female]|[male] |[female]|[male]|[female]|[male]|
  +------+-------+------+---------+-------+--------+------+--------+------+
  |59 C  |   59  |  38  |    43   |   40  |   15   |   9  |   16   |  17  |
  +------+-------+------+---------+-------+--------+------+--------+------+

In addition to the combinations of eosin and vermilion, sable also appeared
in its proper distribution though no counts were made. The four smaller
classes are cross-overs between eosin and vermilion. Since no "pinks"
appeared the color is recessive, and the brother was not heterozygous for
it.

Two other "pink" females mated to wild males gave similar results in their
sons.

_F_1 "pink" [female] × wild [male]._

  +------------+---------+---------+---------+-------+---------+
  |            |         |         |  Eosin  |       |         |
  |            |Wild-type|Wild-type|Vermilion| Eosin |Vermilion|
  | Reference. |[female].| [male]. | [male]. |[male].| [male]. |
  +------------+---------+---------+---------+-------+---------+
  | 61 C       |   101   |    33   |    37   |    9  |    11   |
  +------------+---------+---------+---------+-------+---------+

These F_1 flies should all be heterozygous for "pink." A pair of wild-type
flies which were mated gave a 3 : 1 ratio--wild type 51 to "pink" 18. From
the "pinks" which appeared in this cross a stock was made which was lost
through sterility. Females tested to males of true pink were also sterile,
so that no solution can be given of the case.

[6] Purple is an eye-color whose gen is in the second chromosome.

[7] The curve published by Miss Stark included by mistake 6 cultures from
the succeeding generations, and these coming from only one of the lethals
(lethal _sb_) increase its mode so that the mode of the other lethal
(lethal _sa_) becomes submerged. If these cultures are taken out the curve
shows two modes more clearly.

[8] The figures to the left in each double column correspond to the symbols
above the heavy line, as, in the first example 6,219 white miniature. The
similar figure to the right corresponds to the symbol below the heavy line.
If no symbols are present below, as in the first example, the column to the
right should be read wild-type.

       *       *       *       *       *


Changes made against printed original.

Page 24. "two contrary classes, eosin vermilion and bar": 'eosin bar and
vermilion' in original.

Page 59. "The bristles which are most distorted": 'disorted' in original.

Pages 69-70. One or more lines are missing before "5,352".

Ibid. "The data just given in table 51": 'table 50' in original.

Page 75. "lethal 3_a_ lies at about 19.5.": 'lethal 3' in original.

Page 77. Table 58, last "Facet": 'Fecet' in original.