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SEX-LINKED INHERITANCE IN

DROSOPHILA

T. H. MORGAN and C. B. BRIDGES

WASHINGTON
Published by the Carnegie Institution of Washington

1916

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SEX-LINKED INHERITANCE IN

DROSOPHILA

T. H. MORGAN and C. B. BRIDGES

WASHINGT
Published by the Carnegie Insti

1916

THIS BOOK IS DUE ON THE DATE
INDICATED BELOW AND IS SUB-
JECT TO AN OVERDUE FINE AS
POSTED AT THE CIRCULATION
DESK.

FEB 2 0 1980

MAfT^

1980

\CQry

NOV 2 c 1980

mRTTWT

<;nM/9.7B

CARNEGIE INSTITUTION OF WASHINGTON
Publication No. 237.

^o,»$ of thl$ 800k
•^»''e rirst issued
IViAr8 1916

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

CONTENTS.

PACE.

Part I. Introductory S

Mendel's law of segregation 5

Linkage and chromosomes 5

Crossing-over 7

The Y chromosome and non-disjunction °

Mutation in Drosophila amfelophila ^°

Multiple allelomorphs "

Sex-linked lethals and the sex ratio H

Influence of the environment on the realization of two sex-linked characters i6

Sexual polymorphism ^7

Fertility and sterility in the mutants '°

Balanced inviability "

How the factors are located in the chromosomes 20

The sex-linked factors of Drosophila ^^

, Map of chromosome X

Nomenclature *

Part II. New data ^5

White ^5

Rudimentary \

Mmiature

Vermilion

Yellow ^7

Abnormal abdomen '

r^ ■ 20

Losm o

Bifid •■•.•• ;,; f^

Linkage of bifid with yellow, with white, and with vermilion Z9

Linkage of cherry, bifid, and vermilion 3°

Reduplicated legs '

Lethal 1 ^\

Lethal \a 11

Spot "

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 +

Linkage of vermilion, sable, and bar +

Dot ^

Linkage of vermilion and dot **

Bow f

Bow by arc *^

Lemon body-color • • *„

Linkage of cherry, lemon, and vermilion +

Lethal 2 *^

Cherry ■

A system of quadruple allelomorphs •>

Linkage of cherry and vermilion 5

Compounds of cherry ^'

Fused "

Linkage of eosin and fused ^*

Linkage of vermilion, bar, and fused 5

4 CONTENTS.

Part II. New Data — Continued. page.

Forkfd rg

Linkage of vermilion and forked eg

linkaKt of cherry and forked eg

Linkage of forked, bar, and fused 60

Linkage of sable, rudimentary, and forked 61

Linkage of rudimentary, forked, and bar 62

Shifted g.

Linkage of shifted and vermilion 6^

Linkage of shifted, vermilion, and bar 64

Lethals ja and sb g.

Bar

^''"■-^- ■ '.'.'.'.'. 66

Depressed /r_

Linkage of depressed and bar 57

Linkage of cherry, depressed, and vermilion 68

Club '.'.'.'.'.'.'.'.'.'.'..'.'. 69

Gcnotypic club _q

Linkage of club and vermilion -q

Lmkage of yellow, club, and vermilion ' ' 70

Linkage of cherry, club, and vermilion 72

Green

Chrome

Lethal 3 ''^

Lethal }a 74

Lethal lb '.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'..'. ^6

Facet '' ,

Linkage of facet, vermilion, and sable

Lmkage of eosin, facet, and vermilion ... _o

Lethal /f 7»

Lethal sd '.'.'.'.'.'.'.'.'.'.'.'.'.'.'. ''^

Furrowed '°

.Additional data for yellow, white, vermilion, and miniature 80

New data contributed by A. H. Sturtevant and H. J. Muiler 82

Summary of the previously determined cross-over values g^

Summary of all data upon linkage of gens in chromosome I c.

Bibliography.. °4

PART I. INTRODUCTORY.
MENDEL'S LAW OF SEGREGATION.

Although the ratio of 3 to i in which contrasted characters reappear
in the second or F2 generation is sometimes referred to as Mendel's Law
of Heredity, the really significant discovery of Mendel was not the
3 to I ratio, but the segregatio7i 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 zvhen the hybrid forms germ-cells
the factors segregate frojn each other zvithout 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 i 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 Fo 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 Fo generation.

6 SEX-LINKED INHERITANCE IN DROSOPHILA.

Such cases of linkage have been described in several forms, but no-
where on so extensive a scale as in the pomace fly, Drosophila ampelo-
phila. 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 th^ number of linked groups corresponds to the 7iumber 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 them-
selves 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 distribu-
tion of the X cliromosomes must also contain the chromosome in ques-
tion. 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 pro-
duces a female; any egg fertilized by a Y sperm produces a male.
I he scheme of mheritance is as follows.

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-jinked inheritance all sons show the characters
of their_mother^ A^maletransmits his sex-linked character tq^ his
daughters, who. show it if dominant ancl conceal it Tf 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

INTRODUCTORY. 7

from the male. Many other combinations show the same relations.
In the case of non-disjunction, to be given later, there is direct experi-
mental 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-hnked. 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
influencethe distribution of sex, since like parts are simplyinterchanged.
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 Hnkage. 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 'w'ild
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 awaythat a piece of one chromosome hasbeen exchanged
for the homologous piece of the other. The same conclusion is reached
if the cross is made in such awaythat the same two sex-linked characters
enter, but, one from the mother and the other from the father. 1 he
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 kmd
showing normal characters. Here again the results must be due to
interchange between the two X's in the hybrid female. The number oj

8 SEX-LINKED INHERITANCE IN DROSOPHILA.

the sons due to exchange in the two foregoing crosses is ahvays the samey
although they are of contrary classes. Clearly, then, the interchange
takes place irrespective of the way in which the factors enter the cross.
W'c call those classes that arise through interchange between the chro-
mosonus "cross-over classes" or merely "cross-overs." The phenom-
enon of holding together we speak, of as linkage.

I^v taking a ninnher of factors into consideration at the same time
it has been shown that crossing-over involves large pieces of the chromo-
somes. 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.

\\ hile 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 condi-
tion 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 '\' 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.)

INTRODUCTORY. 9

Ordinarily all the sons and none of the daughters show the recessive
sex-linked characters of the mother when the father carries the domi-
nant allelomorph. The peculiarity of non-disjunction is that some-
times a female produces a daughter like herself or a son like the
father, although the rest of the offspring are perfectly regular. ¥ov
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 reduc-
tion 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 role is
proved by the fact that all the XO males have been found to be abso-
lutely 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 percent). 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 reduc-
tion. 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 X\ "li , and like-

lO SEX-MNKFD INHKRITANCE IN DROSOPHILA.

wise ftniaks with the formulas XX, XXY, and XXYY, that the Y is
without ert'cct 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 twoX'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
^' 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
normallv 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 male-
ness and femaleness already determined by some other agent.

Cvtological examination has demonstrated the existence of one
XXY"\' 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 excep-
tions 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 a^npelophila than in some of
the other species oi 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 /). ampdophila. 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.

\ he mutants of Drosophila have been referred to by Baur as "muta-
tions 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

INTRODUCTORY. I I

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 Cuenot 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 Cuenot 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 ol course
that allelomorph of these factors present in the wild fly and which
when present gives the red color. In this instance the normal allelo-

12 SEX-LINKED INHERITANCE IN DROSOPHILA.

niorph donun.itcs all the other three, hut in mice the mutant factor for
yellow donunates the wild or "normal" allelomorph.

'Ihe other svstem of multiple allelomorphs in the first chromosome is
a tnple system made up of yellow (body-color), spot (on abdomen),
and their normal allelomorph — the factor in the normal fly that stands

or Kf'TV-

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 allelo-
morphs of each other. First, if they are allelomorphs only two can
exist in the same individual; and, m the case of sex-linked characters,
while two ma\ exist in the same female, only one can exist in the male,
for he contains but one X chromosome. Second, all the allelomorphs
should pive 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 the\ form a system (]uite apart from linkage and in relation to
normal allelomorphism. It may be worth while, therefore, to discuss
this (juestion more at length, especially because Drosophila is one of
the best cases known for such a discussion.

I he 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 chromo-
some 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 R of b enters the combination with a. Since b is completely
linked to A ami 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.

I here is no way from Mendelian data by which this difference
between a true caseof multiple allelomorphs and one of complete linkage
Lis just illustrated) can be determined. There is, however, a difl^erent
line of attack which, in a case like that o{ 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 there-
fore not be repeated here. It must suffice to say that if two mutant

INTRODUCTORY. 1 3

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.^ 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 mul-
tiple 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 muta-
tive changes in the same fundamental element that affects the char-
acter 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 fac-
tor 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 diflPerence is striking. There are other eflects
also that follow in the wake of the white factor. Now, it is quite
conceivable that in some specific case one of the eflFects 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.

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

14 SEX-LIN K.HD INHERITANCE IN DROSOPHILA.

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 hving in the protected life that our cultures afford. Were
they thrown into open competition with wild forms, or, better said,
were thev 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 Ufe
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 con-
taining 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 descen-
dants the 2 : I sex-ratio.

\\ hether 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

INTRODUCTORY. I5

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 : i 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 reces-
sive 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 : i sex-ratio, and by the
fact that nearly all the sons that do survive showthe 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 ot 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 (i9i4<:), and again by Stark (191 5),
and it has been found to confirm the explanation. The daughters of

l6 SEX-LINKEI) INHERITANCE IN DROSOPHILA.

such n fcmnle shoiiki all (exceprin-i; a rare one due to crossing-over)
mvf 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 hetero-
zygous 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;
tin- 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, 191 2r/, 191 5/^) and for reduplication of the legs
(Hoge, 191 5). 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 char-
acter 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 m 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 eflfect 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 char-
acter 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

Kaleigh

INTRODUCTORY. I 7

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 homo-
zygous 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 abnor-
mal, 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 mut,ant, 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 F2 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 simu-
lation, 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 phdodtce
there is one type of male, yellow, and two types of females, yellow and

l8 SEX-LINKED INHERITANCE IN DROSOPHILA.

w hitf. In Colias nirydice 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 heterozy-
gous for the sc.\ 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 poly-
morphism 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 oflFspring 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.
Tiuir 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
tiure 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.

INTRODUCTORY.

BALANCED INVIABILITY.

The determination of the cross-over values of the factors was at Hrst
hindered because of the poor viabihty of some of the mutants. If the
viabiHty of each mutant type could be determined in relation to the
viability of the normal, "coefficients of viability" could serve as cor-
rections 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 larvae 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 w^ere
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 com-
petition in mass cultures the pair-breeding method reduced the dis-
turbances 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 accu-
rately 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 dis-
proportionately 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, tor
three pairs of gens there are eight classes, and in order that each ot
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 SEX-LINKED INHERITANCE IN DROSOPHILA.

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 F2 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 Fo. But wMien crossed to pink no double recessive
appears in F;., 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, it 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,
descnbmg 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 111 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
resultmg percentages, or cross-over values, are used as measures of the
distances between loci. Fhus 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 i unit apart, and that yellow and bifid are about
6 units apart. We can therefore construct a diagram with yellow as

INTRODUCTORY.

The sex-linked factors of Drosophila.

Gen.

White

Rudimentary
Miniature. . .
Vermilion. . .

Yellow

Abnormal . . .

Eosin

Bifid

Reduplicated

Lethal 1

Lethal la*. . .

Spot*

Sable*

Dot*

Bow*

Lemon*

Lethal 2

Cherry

Fused*

Forked*

Shifted*

Lethal sa. . . .

Bar

Notch

Depressed* . .
Lethal sb. . . .

Club*

Green*

Chrome* ....

Lethal 3

Lethal 3a. . . .
Lethal li*...

Facet*

Lethal sc . . . .
Lethal sd. . . .
Furrowed. . . .

Part affected.

Eye-color. . .

Wings

Eye-color. . .
Body-color.
Abdomen. . .
Eye-color. . .

Wings

Legs

Life

Body-color.
Body-color .

Thorax

Wings

Body-color .

Life

Eye-color. . .
Venation . . .
Bristles. . . .
Venation . . .

Life

Eye-shape. .

Wing

Life

Wings

Body-color. .
Body-color. .

Life

Eye

Life

Eye

Figure

II
A

7-8

14-17

9
D

E
F

12-13

Symbol.

r
m

ys
s

•2
WC

Sh
Isa

N'
dp

isb
Cf

I16

Isc

Isd
fw

^ocus.

I . I

55-1
36.1
33.0

0.0

2.4

I .

34-
o.

3.
o.

43-
33

■3
■7
■7
■3
.0
.0

175
12.5 =

I . I

59-5
56. 5
17.8
23.7
57. o
2.6
18.0
16.7
14.6

26.5

195
I

2.2

66.2

I —

38.

Datef

ound.

May

1910

June

1910

Aug.

1910

Nov.

1910

Jan.

1911

July

1911

Aug.

1911

Nov.

1911

Nov.

1911

Feb.

1912

Mar.

1912

April

1912

July

1912

July

1912

Aug.

1912

Aug.

1912

Sept.

1912

Oct.

[912

Nov.

1912

Nov.

[912

Jan.

1913

Jan.

913

Feb.

913

Mar.

913

.April

913

April 1

913

May 1

913

May 1

913

Sept. 1

913

Dec. 1

913

Jan. 1

914

Feb. 1

914

Feb. 1

914

April 1

914

May 1

914

Nov. 1

914

Found by.

Morgan.

Wallace.

Morgan.

Hoge.

Rawls.

Cattell.

Bridges.

Wallace.

Morgan.

Safir.

Bridges.

Stark.

Tice.

Dexter.

Bridges.

Stark.

Morgan.

Bridges.

Morgan.

Morgan

Morgan.

Bridges.

Stark.

Duncan.

the zero, with white at i, 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 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 pomts.

Conversely, when a long distance is indicated on the chromosome
diagram, the actual cross-over value found by experiment (;. e., the

SEX-LINKED INHERITANCE IN DROSOPHILA.

6.3 - - Bifid

12.5 - ■

M.G - -

10.7
17.5
17.8
18.0
19.5

23.7

26.5

YeUoat, spot

thai I
Lethal lb
■Whilg. eosin, cherry

Facet
Alinormal
Notch
Lethal la

33.0
33.±

34.7

36.1

Lethal II

l^ethal sb

Club
Lemon
Shifted
Depressed
Lethal Ilia

Lethal sa
Lethal III

• Vermilion
Dot

Reduplicated
Miniature

38.0 - - Furrowed

43.0 - - Sable

55.1
57.0
.59.5

Rudimentary

Forked
Bar

Fused-

66.2 - - Lethal sc

Diagram I.

INTRODUCTORY.

percentage of cross-overs) will be less than the diagram indicates,
because the diagram has been corrected for double crossing-over.

Diagram I has been constructed upon the basis of all the data sum-
marized in table 65 (p. 84) for the first or X chromosome. It shows the
relative positions of the gens of the sex-linked characters oi Drosophila.
One unit of distance corresponds to i 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 dis-
proportionately 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, some-
what 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 i, white, facet, abnormal, notch, and bifid) which
lie close to yellow were the first to be calculated and plotted. Ihe
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 w^eighted. 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 vermihon 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, mmiature,
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.

SEX-LINKED INHERITANCE IN DROSOPHILA.

NOMENCLATURE.

'Ihe 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 mini-
ature. I'he symbolsforthedominant 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 there-
fore add a subletter in such cases, as bifid (b;), fused (f J, and lethal 2
(I2). 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' for spot, w'' for eosin, and w" for cherry. The normal
allelomorphs of the mutant gens are indicated by the converse letter,
as \' for not-vermilion, B, for not-bifid, and b' for not-bar. In the
table headings the normal allelomorphs are indicated by position alone

w*"" B'
without the use of a symbol. Thus the symbol

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 1 ttt 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 ver-

. . w^ B'
milion m a mother which was of the composition ;

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.

PART II. NEW DATA.
WHITE.

(Plate II, figure ii.)

The recessive character white eye-color, which appeared in May
1910, was the first sex-linked mutation in Drosophila (Morgan, 1910^,
1910^). Soon afterwards (June 1910) rudimentary appeared, and the
two types were crossed (Morgan, 1910c). 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, thougl^ 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 sepa-
ration, white was extensively used in linkage experiments, especially
with miniature and yellow (Morgan, 191 1^; Morgan and Cattell, 191 2
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 l.i 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 pubHshed 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-
Hnked character in Drosophila (Morgan, i9ior). Its viability has
always been very poor; in this respect it is one of the very poorest ol
the sex-Hnked characters. The early linkage data (Morgan, 1911^)
derived from mass cultures have all been discarded. By breedmg from
a single Fi female in each large culture bottle it has been possible to
obtain results which are fairly trustworthy (Morgan, 191 2^:; Morgan
and Tice, 1914). These data appear in table 65, which summarizes
all the published data.

SEX-LINKED INHERITANCE IN DROSOPHILA.

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

Fig. a. — a, nulimentary 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 develop-
ment in the early growth stage of the eggs (Morgan, 1915a).

MINIATURE.

(Plate II, figures 7 and 8.)

The recessive sex-linked mutant miniature wings appeared in August
1910 (Morgan, 1911^ and 19x2^). The viability of miniature is fair,
and this stock has been used in linkage experiments more than any

NEW DATA.

other, with the single exception of white. While the win^s of miniatu
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 lo.)

The recessive sex-Hnked mutant vermilion eye-color (Morgan, igiic
and 1912^) 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 ap-
peared in January 1911 (Morgan, 1911c and 191213). Its first appear-
ance 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 chromo-
some, 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 sepa-
rated 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 i.i units distant from yellow and is generally
preferred.

ABNORMAL ABDOMEN.

(Plate I, figure 4.)

The dominant sex-linked character abnormal abdomen appeared in
July 191 1 (Morgan, 1911^). It was soon found that the realization ot
the abnormal condition depended greatly upon the nature of the envi-
ronment (Morgan, 1912). Recently a very extensive study of this
character has been published (Morgan, 191 5). As this case has been
reviewed in the introduction, there is little further to be said here.

SEX-LINKED INHERITANCE IN DROSOPHILA.

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 (191 5^). These data,
which in general include only the abnormal classes, are summarized
in table i.

Table 1, — Linkage data, from Morgan, 1915b.

Gens.

Total.

Cross-
overs.

Cross-over
values.

Yellow white. . .

28,018

15,314
16,300

334
299
277

1.2
2.0
1-7

Yellow abnormal

White abnormal

EOSIN.

(Plate II, figures 7 and 8.)

The recessive sex-linked mutation eosin eye-color appeared in
August 1911 m a culture of white-eyed flies (Morgan 1912a) The
eye-color is different in the male and female, the male being a light
pinkish yellow, while the female is a rather dark vellowish pink. Eosin
is allelomorphic to white and the white-eosin compound or heterozygote
has the color of the eosin male. There is probablv no special sig-
nihcance 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 i.i. Eosin is the most useful character among
all those in the left end of the chromosome.

BIFID.

The sex-hnked wing mutant bifid, which appeared in November
191 1, 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
ot 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

1 he stock of bifid was at first extremely varied in the amount of this
truncation By selection a stock was secured which showed only verv
greatly reduced wings like those shown in figures a, b. Another stock
(hgs. f , d) was secured by outcrossing and selection which showed wines
ot 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 vem. We believe that the extreme
reduction in size seen in the one stock was due to an added modifier of

NEW DATA.

the nature of beaded, since this could be eliminated by outcrossing
and selection.

c ^^-^TTTf-r- ;■-' ■~i.;\-ii-nrrF'

Fig B -Bifid wing, c and d show the typical condition of bifid wings. ^U the longitudinal v^^^^^^
are fused into'a heavy stalk at the base of the wing, a shows the JVP'-^P-^-^-^^^^^^J^^J
the bifid wings are held. The small size of the wings in a and h 1. due to the action o.
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 determming the chromosome locus of bihd 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, /p/j.

Gens.

Yellow bifid . .
White bifid . . .
Bifid vermilion

Total.

3,175

20,800

2,509

Cross-
overs.

182

1,127

806

Cross-over
values.

S-8

5-3

32.1

SEX-LINKED INHERITANCE IN DROSOPHILA.

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. V 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. — Pi ch

frry vermilion 9 9 X bifid cTcf.
zvhite bifid cf cf .

B. C.

^Fi«

ild-ty-pe

9 X

Refer-
ence.

Fi females.

Fi males.

Non-cross-
overs.

Cross-overs.

V w^ b

yv\ bi . V

bi'v

White-
cherry

Bifid.

White-
cherry
bifid.

Wild-
type.

Cherry

ver-
milion.

R-r , 1 Cherry

^'^^i bifid.

Ver-
milion.

Cherry.

Bifid
ver-
milion.

vermilion. ^"

262
263

Total.

40
47

46
45

I
3

2
3

45
30

1
38; 3
50 I

2
3

13
10

88 4 5

*H. C. here and throughout stands for back-cross.

Both males and females give a cross-over value of 5 units for cherry
bifid, which is the value determined b}' 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.

NEW DATA.

REDUPLICATED LEGS.

In November 191 2 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 con-
dition 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 redupli-
cated 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, iQij.

Gens.

Total.

Cross-
overs.

Cross-over
values.

White reduolicated

418
667

S83

121
II

120

29.0

1-7
20.6

Reduplicated vermilion

Reduolicated bar

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,
/. f., from a three-locus experiment.

LETHAL I.

In February 191 2 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 : i sex-ratios, while the other half gave normal i : i sex-ratios.

SEX-LINKED INHERITANCE IN DROSOPHILA.

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, 191 2(?). 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 : i 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, 1914Z?): white?, 457; red?, 433; white cf, 370; redcf, 2.
It is evident from these data that there must be present in the sex-chro-
mosome 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 i) to various
other sex-linked gens was determined (Morgan 1914Z?), and is summa-
rized in table 5. On the basis of these data it is found that the gen
lethal I lies 0.4 unit to the left of white, or at 0.7.

Table 5. — Summary of linkage dataupon lethal i , from Morgayi, 1914b, pp. 81-Q2.

Gens.

Total.

Cross-
overs.

Cross-over
values.

Yellow lethal 1

Yellow miniature

Lethal 1 white

Lethal 1 miniature

White miniature

131
131
1,763
814
994

323
397

0.8

34 4

0.4

39-7
39-9

LETHAL la.

In the second generation of the flies bred by Miss Rawls, one female
gave (March 191 2) only 3 sons, although she gave 312 daughters. It
was not known for some time (see lethals 3 and T,a) what was the
cause of this extreme rarity of sons. It is now apparent, however,
that this mother carried lethal i in one X and in the other X a new
lethal which had arisen by mutation. The new lethal was very close
to lethal I, as shown by the rarity of the surviving sons, which are
cross-overs between lethal i and the new lethal that we may call lethal
\a. I here is another class of cross-overs, namely, those which have
lethal I and get lethal \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 (3X2) 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,

NEW DATA. 33

we may take 300 as the number of the males which died. Tht-re
must have been, then, about 2 percent of crossing-over, which makes
lethal la lie about 2 units from lethal i. This location of lethal
I a is confirmed by a test that Miss Rawls made of the daughters (jf
the high-ratio female. Out of 98 of these daughters none repeated the
high sex-ratio and only 2 gave i 9 : i cf ratios. The two daughters
which gave i : i 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 (2X2) and taking
98 — 2 as the number of non-cross-overs. The cross-over fraction

^ gives 2.6 as the distance between the two lethals. Lethal

300-f96

I a is probably to the right of lethal i at 0.7-1-2.6 = 3.3.

SPOT.

(Plate II, figures 14 to 17.)

In April 191 2 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^). 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, 2. 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 F2 have, in addition to the spot on the abdo-
men, 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. 1 he 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 F2 of the cross of spot by black one yellow black fly appeared,
although none are expected, on the assumption that spot and yellow

34 SEX-LINKED INHERITANCE IN DROSOPHILA.

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^ a fly appeared (July
19, 191 2) whose body-color difi^ered 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 heterozy-
gous for ordinary recessive black, but at the same time that his dark
color must be due to another kind of black. The gray Fi flies when
mated together gave a series of gray and dark flies in F2 about as follows:
In the females 3 grays to i 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 Fj males were the new blacks. Suspected males were
bred (singly) to wild females, and the F2 dark males, from those cultures
that gave the closest approach to a 2 gray 9 : i graycf : i darkcf , were
bred to their sisters in pairs in order to obtain sable females and males.
I hus 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 eff'ects.
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 Fo, as seen in table 6. The reciprocal
cross gave the results shown in table 7.

'The first dark body-color mutation "black" (see plate II, figs. 7, 8) had appeared much earlier
(.Morgan igiib, 1912c;. 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.

NEW DATA.

The Fi 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.— Pi wild 9 9 X sable cT. fi zvild-type 9 9 X Fi zcild-ty

■type cfcf.

Reference.*

Wild-type 9 •

Wild-type cf .

Sable cf .

88 C

143 C

146 C

Total . . .

218

24s
200

100
108
"5

72
82

663

323

224

' 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.

Table 7. — Pi sable 9 X wild cTcf . Fi wild-type 9 X A sable cT.

Reference.

Wild-type 9 .

Wild-type cf ■

Sable 9 .

Sable cf.

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 F2 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 (F3) of the F2 yellow sable males by F2 yellow females, pure stock
of the double recessive yellow sable was made up and used in the
crosses to test linkage.

In color the yel!ow 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 Fi consisted of
wild-type males and females. These inbred gave the results shown m
table 8.

36 SEX-LINKED INHERITANCE IN DROSOPHILA.

Table 8.— Pi a-ild 9 9 X yellow sable cTcf • fi wild-type 9 9 X

F\ wild-type cf cT .

Reference.

Wild-
type 9 .

Non-cross-over cf.

Cross-over cf •

Total
males.

Cross-over
value.

Yellow sable.

Wild-type.

Yellow.

Sable.

44 I

202

384

no
104

75
71

36
60

264
293

42
45

45 I

Total

676

214

lOI

146

557

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

Table 9.— Pi wild-type 9 9 X yellow sable d'd'. B.C.Fi wild-type 9 X

yellow sable cf cf .

Reference.

Non-cross-overs.

Cross-overs.

Total.

Cross-over
value.

Wild-type.

Yellow sable.

Yellow.

Sable.

31I

49 I

Total . .

108
265

175

58
161

56
169

273
770

373

226

219

225

1.043

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 cros.s-overs and absolute linkage o 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
T,6oo 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 rela-
tions to the other sex-linked factors.

NEW DATA.

LINKAGE OF CHERRY AND SABLE.

The origin of cherry eye-color (Plate II, fig. 9) has been given by
Safir (Biol. Bull., 191 3). From considerations which will be dis-
cussed 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. — Pi cherry 9 9 X sahle cf cf. Fi wild-type 9 X Pi cherry d'cf .

Reference.

Wild-
type 9.

Cherry

Non-cross-over cT.

Cross-over cT.

Total
males.

Cross-
over
value.

Cherry.

Sable.

Cherry sable.

Wild-type.

24 I

55 I

S5'I

Total . . .

lOI

105

131

20
38
29

43
48
30

156
201
142

43
42

291

330

166

125

121

499

The percentage of crossing-over between cherry and sable is 42.
Since cherry is one point from yellowy 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
(z. e.y to the right). Sable will he 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) vermiHon (gray) males, the daughters were wild
type and the males were eosin sable. Inbred these gave the classes
shown in table 11.

SEX-LINKED INHERITANCE IN DROSOPHILA.

Table 11. — Pi eosin sable 9 X vermilion cf cf . Fi wild-type 9 9 X Fi eosin

sable d'd^.

Reference.

Fj females.

Fi males.

W^ S

W<^

we s

V s

W* V s

Eosin
sable.

Wild- r-^ •„
Losm.
type.

Sable.

Eosin
sable.

Ver-
milion.

Eosin
ver-
milion.

Sable.

Eosin.

Ver-
milion
sable.

Eosin
ver-
milion
sable.

Wild-
type.

26 I ... .
26'!....

Total .

I3i
96

171 113
146 86

109
78

127
74

163
128

75
76

76
59

37
18

2
4

228

317

199

187

201

291

151

135

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 indi-
cated 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.

\v V' s

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

Non-cross-overs
Single cross-overs

Double cross-overs

s /Eosin sable.

V \Vermilion.

2^ fEosin vermilion.

s {Sable.

/.E

osin.

v s [Vermilion sable.

_^[ i_s JEosIn vermilion sable.

^ \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 vermiHon (gray) female

NEW DATA.

was mated to (red red) sable males and gave 202 wild-type^ females
and 184 eosin vermilion males. Two Fi pairs gave the results shown
in table 12 (the four classes of females not being separated).

Table 12. — Pi eosin vermilion 9 9 X sable cf cf . Fi wild-type 9 X Fi

eosin vermilion cf cT .

Reference.

females.

Fj males.

W« V

W^ s
V

W^ v.s

V S

Eosin
ver-
milion

cT.

Sable

Eosin
sable

cf.

Ver-
milion

d'.

Eosin

vermilion

sable

Wild-
type

Eosin

Ver-
milion
sable

59 C...

61 c...

Total..

133

lOI

33
26

7
8

16
II

5
7

234

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

^In addition to these expected Fi 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 Fj 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 heterozy-
gous for eosin, vermilion, and sable.

Fi "pink" {Ref. 5/ C) 9 X Fi eosin vermilion d.

Reference.

Wild-type.

Eosin vermilion.

Eosin.

\'ermilion.

59 C

In addition to the combinations of eosin and vermilion, sable also appeared in its proper dis-
tribution, 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 hetero-
zygous for it.

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

Fi "pink" 9

X tvild d.

Reference.

Wild-type 9-

Wild-type d.

Eosin vermilion cf .

Eosin d.

Vermilion c?".

61 C

lOI

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

SEX-LINKED INHERITANCE IN DROSOPHILA.

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 Hes about 3 units to the right of vermihon, 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.— Pi u-ild 9 9 X miniature sable d'd'. B. C. Fi wild-type 9 9 X

miniature sable cf cf •

Reference.

Non-cross-overs.

Cross-overs.

Total.

Cross-
over
value.

Miniature sable.

Wild-type.

Miniature.

Sable.

38I

43I

46I

Total . .

245
191

232

283
236
274

IS
13

17
18
21

560
458

SSI

7
8

668

793

i,S69

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 domi-
nant 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 ( 9 ) and (red) sable bar from the other ic^). 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
( 9 ) and (red, gray) bar from the other. The daughters were bar and
the sons were vermilion sable. Since these sons have the three reces-
sive factors, inbreeding of Fi is equivalent to a triple back-cross. The
results are given by pairs in table 15.

NEW DATA.

Table 14. — Pi vermilion 9 9 X sable bar cf cf . B. C. Fi bar 9 X vermilion

sable cTcf.

Reference.

s B'

VjS B'

V |B'

' 'B'

Total.

Cross-over values.

Ver-
milion.

Sable
bar.

Ver-
milion
sable
bar.

Wild-
type.

Ver-
milion
bar.

Sable.

Ver-
milion
sable.

Bar.

Ver-
milion
sable.

Sable
bar.

Ver-
milion
bar.

147 I....

148 I....

149 I....

150 I

isii....

Total .

103

116

104

108
88

75
96

SO
88

4
10
10
II
10

4
13

15
19

8
II
IS
19

IS
II

17
21

23
IS
IS
12

18
II

17
22
26
II

I
I

I
I
2

207
256
239
236
289
239
140
244

16
9
IS
19
18
II
21
10

29
18
22

27
26
23
29
21

734

66s

129

131

4 1.850

Table 15. — Pi vermilion sable 9 9 X bar c^<f. B. C. Fi bar 9 X vermilion

sable cf c?'.

V s

V S

hM:

Total.

Cross

-over values.

Reference.

Ver-

Ver-
milion

WilH-

Ver-

Sable

Ver-

Sable

Ver-

milion

Har

milion

Sable.

sable
bar.

bar.

milion

bar.

milion

sable.

bar.

type.

milion.

sable.

bar.

105 I... .

146

106 I....

122

238

107 I. ...

236

116I....

III

149

335

117I....

117

272

126 I....

160

334

127I....
Total .

117

124

334

608

84s

108

140

1.895

SEX-LINKED INHERITANCE IN DROSOPHILA.

In the third cross, vermiHon (gray) bar entered from one side ( 9 )
and (red) sable (not-bar) from the other (cT). The daughters are
bar and the sons vermiHon bar. The daughters were back-crossed
singly to vermilion sable males and gave the data in table i6.

Table 16. — Pi vermilion bar 9 9 X sable cf cf.

sable cTcf,

B. C. Fi bar $ X vermilion

V S

V s

Reference.

Total.

Cross

-over values.

Ver-
milion

Sable.

! Ver-
milion

Bar.

Ver-

Sable
bar.

Ver-

milion

sable

bar.

Wild-

Ver-
milion

Sable

Ver-
milion

bar.

sable.

mihon.

type.

sable.

bar.

129 I....

132

147

351

130I

194

168

454

131 I

121

279

137 I.. ••

139

331

138I....

131

128

334

139I....
Total.

«3

207

800

724

151

107

1,956

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.

T.\BLE

17.—

Pi vermilion sable bar

9 9 X tvild cf cT

'. B.

C.F,

bar 9 X

vermilion

sable cf cf •

V s B'

V S

V. .B'

Reference.

's B'

-+-H

Total.

Cross

-over values.

Ver-
milion
sable
bar.

Wild-
type.

Ver-
milion.

Sable
bar.

Ver-
milion
sable.

Bar.

Ver-
milion
bar.

Sable.

Ver-
milion
sable.

Sable
bar.

Ver-
milion
bar.

132I....

108

272

133 I...

112

ISO

341

134I....

84 95

232

I3SI...

100

261

152I..,.

213

153! •••

114

I3«

310

154! •
Total .

194

■'

641 755

123

126

1,823

NEW DATA.

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.

Table 14. . . .

15 ••••

16....

17....

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

Vermilion

755
734
724
845
608
800
66s
641

110
92
97
87
80

95
81

140

151
131
126
123
129
107
108

4
4

I
I
3

Sable

Bar

Vermilion sable

Vermilion bar

Sable bar

Vermilion sable bar. . . .
Total

5,772
76.7

716
9-53

1,015
13-49

0.28

7,524

Percentage

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

0.0028X100 o

; = 20 . O

0.0953+0.0028x0. 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. I his 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

SEX-LINKED INHERITANCE IN DROSOPHILA.

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 Fo, from a cross of a double recessive (white vermilion) female
by a triple recessive (eosin vermilion pink) male, there appeared, July
21, 191 2, 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 i 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 t!he char-
acter 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 ver-
milion. 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. — Pi vermilion dot cf X zvild 9 9. Fi wild-type 9 9

X Fi zvild-type cf cf.

Reference.

Fj females.

Wild-type cT.

Vermilion cf.

Vermilion

dot cf.

Dot cf .

Total .

345
524

151

24s

130
220

0
3

0
0

869

396

350

NEW DATA.

Only three dot individuals appeared in Fo, but since these were males
the result indicates that the dot character is due to a sex-linked ^en.
These three males had also vermilion eyes, indicating linkaj;e 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
vermiUon. Not one of the 272 sons showed dot. If the gen is sex-
linked the non-appearance of dot in the Fi 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. — Pi vermilion dot 9 X ^vild cf .

First generation.

Second generation.

Reference.

Wild-
type

Ver-
milion

Reference.

Wild-
type
9.

Wild- Ver-
tvpe milion

d'. ! 9.

Ver- ; Ver-
milion milion

d. dot 9.

Ver-
milion
dot cf.

Dot

d'.

137 c...

138C. ...
Total .

44 45

77 62

124 ; 124

57 41

Total .

211

266
143

198 228
220 227
149 1 125

206 20
227 16
124 14

3
0

0
0
0

620

567

570

557

291 1 272

The F2 generation is given in table 20. The dot reappeared in V>
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. 1 his would
indicate that the character is partially ''sex-limited'' (Morgan, I9I4(/)
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

SEX-LINKED INHERITANCE IN DROSOPHILA.

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 Fo generation from rudimentary males by wild females there
appeared, August 15, 191 2, 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.

Fig. C. — Bow wing.

Table 21.— Pi bow d'd' X zvild 9 9

First generation.

Second generation.

Reference.

Wild-type
99.

Wild-type

Reference.

Wild-type
9 9.

Wild-type

Bow

cfcT.

169 C. . .

18I

21 I

Total .

193
182

145

100

375

245

116

NEW DATA.

The F2 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, there-
fore, 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 Fi straight-winged
females but bow males. This result is in accordance with the inter-
pretation that bow is a sex-Hnked recessive. Further details of these
last two experiments may now be given. The Fi (wild-type) flies from
bow male by arc female were inbred. The data are given in table 22.

Table 22. — Pi bou; d^ X arc 9 .

First generation.

Second generation.

Reference.

Wild-type

9 9.

Wild-tvpe

Reference.

Straight.

Not-
straight.

71 c...

75 C. . . .
Total .

48
28

71C....

179

133

Bow and arc are so much alike that they give a single rather variable
phenotypic class in F2. Therefore the F2 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 Fi and in F-.. gave 9
colored to 7 white. If sex is taken into account, the theoretical expec-
tation for the F2 females is 6 straight to 2 arc, and for the Y2 males 3
straight to i arc to 3 bow to i bow-arc.

The Fi from bow females by arc male and their F2 oft'spring arc given
in table 23.

SEX-LINKED INHERITANCE IN DROSOPHILA.

Table 23. — Pi bow 9 X arc d^.

First generation.

Second generation.

Reference.

Wild-type
9 9.

Bow cf cf-

Reference.

Straight.

Not-
straight.

72 C...
73C....

5I....
74C....

Total .

22
12
22
S6

19
10
21

3I....
3.1I....

5.1I....

Total .

56
46

108

112

102

248

307

In this case the F2 expectation is 6 straight to 10 not-straight. Since
the sex-Hnked gen bow entered from the female, half the F2 males
and females are bow. The half that are not-bow consist of 3 straight to
I 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 off'spring,
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.

NEW DATA.

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. — Pj lemon (hei.) 9 X cherry vermilion cf cf . Fi zcild-type 9 X

cherry vermilion cf d^.

Females.

! w^i

in
V

-4-

u. V

W^.!,,,

Total

l.n

' 1

■T^

r>. 1

Cherry

ver-
milion.

Lemon.

Cherry
lemon.

Ver-
milion.

Cherry.

Lemon

ver-
milion.

Cherry

lemon

vermilion.

Wild
type.

O'er.

s ,

3 1

437

177

133

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
smgle cross-overs betw^een 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.y 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. I he
locus of lemon, calculated by interpolation, is at about 17.5

LETHAL 2.

In September 191 2 a certain wild female produced 78 daughters and
only 16 sons (Morgan, 1914Z'); 63 of these daughters were tested and
31 of them gave 2 females to i male, while ^z of them gave i : i
sex-ratios. This shows that the mother of the original high sex-ratio
was heterozygous for a recessive sex-linked lethal. In order to deter-
mine the position of this lethal, a lethal-bearing female was bred to an
eosin (or white) miniature male, and those daughters that were hetero-
zygous for eosin, lethal, and miniature were then b;ick-crossed to

SEX-LINKED INHERITANCE IN DROSOPHILA.

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.

T.-VBLE 25. — Total data upon linkage of eosin, lethal 2, and miniature, from

Morgan, igi4.b.

'emales.

Total.

Cross-
overs.

cross-
over
value

Males.

W^ m wMo w^'

w^ lo m

Cross-over values.

lo m

4-^+-

Eosin
lethal 2.

Lethal 2
miniature.

Eosin
miniature.

15,904

4,736 j 29.7

5.045

653

1,040

9 9

15 4

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, igi^b.

Females. \ Males.

Total.

Cross-
overs.

Cross- ^e y

W^ I5 W^

w^ !„ V

Cross-over values.

Eosin
lethal 2.

Lethal 2 Eosin
vermilion, vermilion.

value. *2

V I2 V

2,656

729

j
275 i 902

124

227

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, fro?n Morgan, iQi4b.

Gens.

Total.

Cross-overs.

Cross-over
values.

White lethal 2

White vermilion

8,011
6,023
36,021
1,400
6,752

767
1,612

1 1 , 048

248

1,054

9.6

26.8
30.7
177
15 4

White miniature

Leth:il 2 vermilion

Lethal 2 miniature

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

NEW DATA.

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 191 2) 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 vermiHon. 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 ot a
cherry vermilion (double recessive) male by wild females. 1 he I'l
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.

SEX-LINKED INHERITANCE IN DROSOPHILA.

In one of the Fo 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. — Pi cherry vermilion cf d^ X zt'ild 9 9- Fi wild-type 9 9

X Fi zc'ild-type cf cf •

Reference.

Wild-
type 9 9 •

Non-cross-over cT.

Cross-over c?.

Total.

Cross-
over
value.

Cherry

ver-
milion.

Wild-
type.

Cherry.

Ver-
milion.

160C

188
256
251
229

57
8S
78
76

93
78
95

32
40
20
34

34
52
37
33

184

270
213
238

26
28

161C

162 C

163 C

Total

924

296

327

126

156

905

Some cherry males were bred to wild females. The Fi 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 allelo-
morph 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. — Pi cherry cT cT X wild 9 9. Fi wild-type 9 9 X Pi wild-typed^ o^.

Reference.

Wild-type 9 •

Wild-type cf .

Cherry cf .

15I

266

120

100

COMPOUNDS OF CHERRY.

In order to examine the effect of the interaction of cherry and white
in the same individual (i. e., white-cherry compound) cherry females
were crossed to white males. This cross should give white-cherry
females and cherry males. These white-cherry females were found
(table 30) to be very much lighter than their brothers, the cherry males.
The color of the pure cherry females and males is the same, but the
substitution of one white for one cherry lowers the eye-color of the
female below that of the cherry male. In eosin the white also lowers
the eye-color of the compound female about in the same proportion as
in the case of cherry. In the eosin the female starts at a higher degree
of pigmentation than the male and dilution seems to bring her down

NEW DATA.

to the level of the male. But this coincidence of color between eosin
male and white-eosin compound female is probably without significance,
as shown by the results with cherry.

Table 30.— Pi cherry 9 9 X white d^ d" .

Reference.

First generation.

White-cherry
compound 9 .

Cherry d^.

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. — Pi eosin 9 X cherry cf .

First generation.

Second generation.

Reference.

Eosin-cherry
compound

9 9.

Eosin cf cf.

Reference.

Eosin and

eosin-cherry

compound 9 9-

Cherry cf.

Eosin (f.

43 C

I I

154
174

99
74

328

173

139

Although in the F2 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 F2 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^ males and black females there ap-
peared in F2 (Nov. 4, 191 2) 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

'Purple is an eye-color whose gen is in the second chromosome.

SEX-LINKED INHERITANCE IN DROSOPHILA.

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 Fo males, showing that it is a recessive sex-
linked character.

Table 32.— Pi fused & X zvild 9 9 .

First generation.

Second generation.

Reference.

Wild-type
9 9.

Wild-type

Reference.

Wild-type

9 9.

Wild-type

cfcf.

Fused cfcf.

190 C

14I

Total

258
239

105

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 chromo-
some 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. — Pj eosin 9 9 X fused cf cf . Fj tvild-type 9 9 X Fi eosin cf cT-

Reference.

Females.

Non-cross-over d'cf.

Cross-over cf cf •

Total
males.

Cross-
over
value.

Eosin.

Fused.

Eosin fused.

Wild-type.

S6I

496

131

113

104

430

NEW DATA.

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 improhable,
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 Hke 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

Fig. D. — a, normal wing; b and c, fused wings, c shows a typical fused wing. The nio-<t !»trikinK
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 po.sition in which the fused wings arc held. The fusion of
the veins in b is unusually complete.

which side of bar the new factor lies, but by mating to a Hy that carries
still another sex-linked factor, known to lie to the left of bar, the infor-
mation 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 sucii an experi-
ment double mutant stocks of vermilion fused and also of bar fused
were made up.

SEX-LINKED INHERITANCE IN DROSOPHILA.

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 Fi daughters were back-crossed to vermilion fused
males and produced the classes of offspring shown in table 34.

Table 34. — Pi zermilion 9 9 X bar fused cf cT. B.C. Fibar 9 X vermilion

fused cf cf-

B'fu

V B' f„

■^

Total.

Cross

-over values.

Reference.

Ver-
milion.

Bar
fused.

Ver-
milion

bar
fused.

Wild-
type.

Ver-
milion
fused.

Bar.

Ver-
milion
bar.

Fused.

Ver-
milion
bar.

Bar
fused.

Ver-
milion
fused.

140 I...

137

130

355

141 I.

^ 144

137

366

142 I.

153

120

388

143 I.

153

344

145 I.

181

146 I.

103

273

156 1.

. .

164

157 I.

198

Tot:

i\.

907

746

255

291

2,269

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 3 5. 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.

NEW DATA.

Table 35. — Pi bar 9 9 X vermilion fused cfcf . B. C. Fi bar 9 X vermilion

fused cp cf ■

Reference.

V B'

.B'

Total.

' fu

B'"fu

— 1 1 —

Cross-over values.

Ver-
milion
fused.

Bar.

Ver-
milion
bar.

Fused.

Ver-
milion.

Bar

fused.

Ver-
milion

bar
fused.

Wild-
type.

Ver-
milion
bar.

Bar

fused.

Ver-
milion
fused.

82'

83'

89'

90'

91'

92'

93'

94'

Firsts. . .
Seconds .

Total .

i6s

104
128
100

85
78
86

33
125

91
109

100

75
68

84
61

84
144

107

1,273

635

i6s

164

105

107

95
136
105

94
96

73
102
148

112

1.383

677

63
26

23
21

41
31
41
29
19
31
31
20

27
43
25
39
433
208

57
24
39
30
24
27
28
14
31

24
29
20
17

35
22
26

34
20

33
371
188

6
4

5
4

5
3

7
4
4
4

I
I

I
2

I
I
I
I

3
2

466

245
392
260

244
221

234
"3

306
250
316
265
182
212

255
1 8s

245

373
230

294
3.537
1.75 1

26
20
23

33
24
23
21
22
21

23
26

23
22
21

20
25
23
23

3
2

3
3
3

2
2

3
2

I
I
I
4
5
2
I

I
2
3

22
26

22
23
27
36

22
22

24
29
28

24
21

23
26

25
25

1,908

2,060

641

559

5,288

2.3

Table 36. — Linkage of vermilion, bar, and fused with balanced viability.

Percentage. . .

B'fu

5,621

74-3

B'fu

1,756
23.19

V B'

17s

2.31

0.2

Tot

ai.

7,567

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 expermient.

The converse experiment, namely, red (not-fused) temaks by
vermilion fused males also gave, when the wild-type daugiiters were

SEX-LINKED INHERITANCE IN DROSOPHILA.

back-crossed to vermilion fused males, a linkage value of 27 units.
Two lo-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. — Pj vermilion 9 9 X fused cf cf . Fi wild-type 9 9

X Fi vermilion cf cf •

Reference.

Females.

Non-cross-over cf cf .

Cross-over cf cf.

Total

cTd^.

Cross-
over
values.

Vermilion.

Fused.

Vermilion
fused.

Wild-
type.

79I

80 I

81 I

Total .

299

245
263

93
93

lOI

60
63

37
28
22

36
27
40

262
208
226

28
26
27

807

287

219

103

696

Table 38. — Pi wild 9 9 X vermilion fused cf cf . F\ wild-type 9

X Fi wild-type cf cf .

Reference.

Wild-
type
9 9.

Non-cross-over cf.

Cross-over cf.

Total

cfcf.

Cross-
over
values.

Vermilion
fused.

Wild-
type.

Vermilion.

Fused.

52'

S3'

54'

57'

58'

Firsts. . . .
Seconds. .

Total .

176

170

128

144

433
626

59
20
21

35
22
22
47
37
38
139
187

30
64
22

27
38
20
20
54
55
64
i6s
229

7
24

14
16
60
83

9
II

15
54
72

82
166

57
69

103

144

116

133
418
571

33
26
26
31
29
29
30
30
21
23
27
27

1. 059

326

394

126

989

FORKED.

On November 19, 191 2, 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 (macrochaetae) 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 macrochaetae, instead of

NEW DATA.

being long, slender, and tapered (see Plate I, 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 disorted are those upon the
scutellum, where they are sometimes curled
together into balls.

LINKAGE OF VERMILION AND FORKED.

Since forked arose in vermilion stock, the
double recessive for these two sex-linked fac-
tors could be used in testing the linkage rela-
tions of the mutation. Vermilion forked males
were crossed to wild females and gave wild-
type males and females, which inbred gave
in F2 the results shown in table 39. Forked
reappeared only in the males in the following
proportion: not-forked 9,742; not-forked c?*, ^'^ e.
346; forked cf, 301. The result shows that the character is a se.x-
linked recessive.

Table 39. — Pi wild 9 9 X vermilion-jorked cf cf . Fi ivild-type 9 9

X F\ wild-type cf cT.

-Forked bristles.

Reference.

Wild-
type
99.

Non-cross-over cTcf.

Cross-over cTcf-

Total

Cross-
over
values.

Vermilion
forked.

Wild-
type.

Vermilion.

Forked.

Ill

Total .

366
376

113
1x6

123
ISO

49
42

41
3J

326
339

28
22

742

229

273

66s

In table 39 vermilion forked and wild-type are non-cross-overs, and
vermilion and forked are cross-overs, giving a cross-over value ot 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 i unit from yellow) and gave wild-type
females and cherry males. These gave in Fo 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 (z. e., 33-25) instead of 33 + 25 = 58. The difference between 58

SEX-LINKED INHERITANCE IN DROSOPHILA.

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.— Pi f/z^rry 9 9 X forked d^d". Fi wild-type 9 9 XFi cherry cfd".

Reference.

Females.

Non-cross-over d' cT .

Cross-over cTcf .

Total

Cross-
over
values.

Cherry.

Wild-
type.

Cherry.

Forked.

Cherry
forked.

Wild-
type.

25'

36'

84'

85'

Total .

129

167

114

98
307

351
244

145

148

323

341

246

73
74

41
40

152

183

142

144

213

142

6S
66

35
24
38

52
Ii8
160
107

68
88

30
26
28

53
46

165

147

104

276
310
190
127
138
116

215

209

579
703

495

48
50
45
43
46
46
44
47
49
45
43

1,701

1.705

872

949

731

806

3.358

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 Fi females were back-crossed singly to forked fused males with the
result shown in table 41.

Table 41.— Pi bar9 9X forked fused cf cf . B. C. Fi bar 9

X forked fused cT cf .

Reference.

f fu

f B'

' fu

B''f„

iA^

Total.

Forked
fused.

Bar.

Forked
bar.

Fused.

Forked.

Bar

fused.

Forked
bar fused.

Wild-
type.

163

164

i6s

Total.

45
71
97
21

106

35
23

4
4

2
I

4
2
I

108

166

209

250

309

581

NEW DATA.

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 4:2.— P^ forked 9 9 X har fused d^cf . B. C. F, bar 9 X forked

fused cf cf .

Reference.

B'f

f,B' U

B''

f B'

Total.

Forked.

Fused
bar.

Forked
bar fused.

Wild-
type.

Forked
fused.

Bar.

Forked
bar.

Fused.

158

159

160

161

162

Total.

131

31
29
24
96

124

23
II
91

3
I
I

3
2

262
76

191

311

294

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' mini-
mum.

Table 43,

■

Total.

Per cent

1,164
96.9

0.42

32
2.7

0
0

1,201

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 FORK ID.

Rudimentary, forked, bar, and fused form a rather compact group
at the right end of the chromosome, as do yellow, lethal i, white,
abnormal, etc., at the zero end. The following two cxpennu-nts were
made to determine more accurately the interval between rudmientary
and the other members of this group. A sable rudiiiuntary forked

SEX-LINKED INHERITANCE IN DROSOPHILA.

male mated to a wild female gave wild-type sons and daughters.
These inbred give the results shown in table 44.

Table 44. — Pi sable rudimentary forked cf X wild 9 . Fi wild-type 9

X F\ wild-type cT & •

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.-

-Pi rudimentary forked cf X bar 9 • B.C. Fi bar 9
X rudimentary forked cf cf •

Reference.

r f

'■^

r f|B'

'f'fi'

Rudi-
mentary
forked.

Bar.

Rudi-
mentary
bar.

Forked.

Rudi-
mentary
forked
bar.

Wild-
type.

Rudi-
mentary.

Forked
bar.

267

268

269

Total.

56
82
68

104
86

lOI

I
I

206

291

The cross-over values are: rudimentary forked, i; 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.

NEW DATA.

SHIFTED.
Shifted appeared (January 1913) in a stock culture of vermilion dot.
The chief characteristic of this mutant is that the third lonj^itudinal
vein (see text-fig. f) does not reach the margin as it does in the nor-
mal 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 ver-
milion males by wild females gave wild-type
males and females which inbred gave the data
shown in table 46.

Disregarding the eye-color, the following
is a summary of the preceding results: wild-
type?, 1,001; wild-typed^, 437; shifted cf,
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, fig. f.— shiited venation. The

1 • , P , , , 1 . • c third longitudinal vein is shitted

which from the calculated position Ot Ver- toward the fourth and fail!* to

milion at 3'? would give a position for shifted reach the margin. Cros.^-vcin

. , "^^ r^ r II between third and fourth longi-

at either 18 or 48 trom yellow.

Table 46. — Pi shifted vermilion cf cf X ■'^ild 9 9

X Fi wild-type cf cf .

tudinal veins is lacking.
F, uild-type 9

Reference.

Wild-
type

9 9.

Non-cross-over cTcf-

Cross-over cf cT-

Total

Cross-
over
values.

Shifted
vermilion.

Wild-
type.

Shifted.

Vermilion.

Total .

345
68
191
151
133
"3

37
41
49
56

32
54
6S
40

8
3
5

17
4
9

4
13

227

59
109
136

99
"35

>5
12

22
10

1,001

282

36s

765

SEX-LINKED INHERITANCE IN DROSOPHILA.

LINKAGE OF SHIFTED, VERMILION, AND BAR.

In order to determine on which side of vermiHon shifted lies, a shifted
vermilion (not-bar) female w^as 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 ver-
miHon at approximately 17.8 units from yellow.

Table 47. — Pi shifted vermilion 9 X bar d'd'. Fi bar 9 X Fi shifted

vermilion cf cf.

Sh V

Refer- I
ence. Shifted j

ver- jBar.
i milion.

65.... i 56

108

Shifted
bar.

iVep
mil-
ion.

Sh v^B-

Shifted

ver- Wild-
milion

bar.

type.

V B'

Ver-
Shifted. milion
bar.

Cross -over values.

Total.

Shifted

ver-
milion.

242

Ver

milion

bar

Shifted
bar.

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, 191 5). No lethals
were found among the wild stocks, but 4 were found among the confined
stocks. Whether this difi^erence is significant is perhaps open to ques-
tion. 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 say
was recessive and behaved like the former lethals, being transmitted
by half the females and causing the death of half the sons. The posi-
tion of this lethal in 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

NEW DATA.

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 191 3) 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 i and la and lethals 3 and 3^7, this lethal, lethal
j-^,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,^ the two lethals being too
close together to be differentiated by the small number of determina-
tions 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 homol-
ogous chromosome was bred to an eosin miniature male. 1 he white
eosin daughters carried lethal, and their sons show the amount of
crossing-over between white and lethal (15.6), between lethal and mini-
ature (19.9), and between white and miniature (32.9). I he data on
which these calculations are based are given in table 48.

Table 48. — Data on the linkage of zvhite, lethal sb, and miniature,

from Stark, iQij.

w^ m

yv] Isb

W^ ,

w^. Ish . m

Total.

Cross-over values.

—

W Isb

w m

w Isb ' m

Eosin
miiii.iture.

White
miniature.

Eosin.

White.

White Lethal sb ' White
lethal J*, miniature, miniature.

2,421

524 685

3.678

1
15.6 1 199 329

iThe curve published by Miss Stark included by mistake 6 cultures from the succo<-<linK Rf"-
erations, and these coming from only one of the lethals (lethal sh) increase its mod.- so th.it the
mode of the other lethal (lethal sa) becomes submerged. If these cultnn-i are taken <.ut tlie
curve shows two modes more clearly.

66 SEX-LINKED INHERITANCE IN DROSOPHILA.

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 sh whose advent gave rise to the high sex-ratio. This interpre-
tation 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 191 3 in an experiment involving rudi-
mentary and long-winged flies (Tice, 1914). A female that is hetero-
zygous 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 hetero-
zygous 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 w^ngs 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 : i 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.

NEW DATA.

DEPRESSED.

Several mutations have appeared in which the winj!,s 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 chro-
mosome), 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. Ihese 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 difi^erent mutations is at once
shown on crossing any two, when wild-type oftsprmg are produced.
For instance, bow and arc are the two most nearly alike. Mated
together (bow cf by arc 9 ), they give in Fi straight-winged flies which
inbred give in F2 9 straight to 7 not-straight {i. e., bow, arc, and bow arc
together).

Depressed wings first appeared (April 191 3) 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 DEPRESSKD 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.— Pi depressed 9 9 X bar 9 9 . B. C. F, bar 9 X depressed <?&.

Non-cross-overs.

Cross-overs.

rotai.

Cross-over
values.

Reference.

Depressed.

Bar.

Depressed
bar.

Wild-
type.

66 I

67 I

Total..

48
85

104

21
44

161

303

39
38

133

iSS

III

464

SEX-LINKED INHERITANCE IN DROSOPHILA.

LINKAGE OF CHERRY, DEPRESSED, AND VERMILION.

The linkage value 38 (see table 49) indicates that depressed is some-
where near the opposite end of the series of sex-linked factors from bar.
The locus could be more accurately determined by finding the link-
age relations of depressed with gens at its end of the chromosome.
Accordingly, depressed females were crossed to cherry vermilion males.
Fi gave wild-type females and depressed males. The daughters bred
again to cherry vermilion males gave the results shown in table 50.

Fig. G. — Depressed wing.

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 Fi males in the last experiment did not have their wings as much
depressed as is the condition in stock males, and in Fo most of the
depressed winged males were of the Fi 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\ depressed 9 X cherry vermilion d^cf •

First
generation.

Wild-; De-
type pressed

9 9.; d^cf.

Second generation.

Refer-
ence.

19 I

y/c

w^.d,,

9 9 i Cherry j

j ver- '
I milion

De-
pressed

Cherry

de-
pressed

Ver-
milion

w'^dp^v

! De- :^^/;7

^•1 I pressed . Wild

Cherry ] "^ pressed

cf.

ver-
milion

6".

ver-
milion

type

NEW DATA.

CLUB.

In May 1913 there were observed in a certain stock some flies which,
although mature, did not unfold their wings (text-fig. Ha). This con-
dition 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 1% males, bur
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 illus-
trated and proven by the following history of the stock:

Club females were obtained by breeding Fo club males to their F2
long-winged sisters, half of which should be heterozygous for club.

Fig. H. — Club wing, a shows the une.^panded wings of club flies; c shows the absence oi tlie two
large bristles from the side of the thorax present in the normal condition of the wild. b.

When the F2 club females and club males were bred together, instead of
only clubs being produced, long-winged flies also appeared. In fact,
only about a third of the offspring showed the club character.

Club females bred to wild males gave some club males in I'l (although
most of the males had long wings), and in Fo some of the females and
some of the males were club. In all essential points club shows the
characteristic features of a sex-linked recessive, except that it is reahzt-d
in only a small proportion of the individuals that are genetically club.

These general statements are substantiated by the following data:
Club male by wild female gave in 20 F2 mass cultures, wild-type Q ,

JO SEX-LINKED INHERITANCE IN DROSOPHILA.

5,352; wild-t\'pe cf, 4,181; club cf , 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 F2 generation (mass cultures) :
wild-type 9, 1,131; wild-type 0^,897; club 9, 57; club cf, 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 Fo the following: wild-type 9, 521; wild-type (and
potential club) c:f,403; club 6^,82.

Club females by club males of club stock gave in F2: potential club 9 ,
126; potential club cT, 78; club 9, 95; club cf, 81. These results are
from 8 pairs. The high proportion of club is noticeable.

Potential club females and males from pure club stock {i. <?., stock
derived originally from a pair of club) gave in F2 the following: potential
club 9, 1,049; potential club cf , 666; club 9,450; club cf , 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
homozjT^gous club. This character is the absence of the two large
bristles (text-fig. hc) that are present on each side of the thorax of
the wild fly as shown in figure h^. 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

r • 276

traction — zr- or lo per cent.
1463 ^ ^

LINKAGE OF YELLOW, CLUB, AND VERMILION.

The data just given in table 50 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 chro-
mosome series. At the same time we included vermilion, so that a
three-point experiment was made.

Females that were (gray) club vermiHon 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 evi-

NEW DATA.

dence 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 ver-
milion. The value calculated from the female classes alone is 20 units,
which is the same value as that given by the males.

T.-\BLE 51. — Pi club 9 9 X vermilion cf cT. Fi zvild-type 9 X f 1 club d" ■

Reference.

Females.

Non-cross-over cT cf .

Cross-over cfcT.

Total

Cross-
over
values.

Club.

Ver-
milion.

Club
vermilion.

Wild-
type.

137

138

139

140

144

145

146

106

107

108

109

Total .

75
64
56
74
97
63
126
92

55
86

103
83
77
67

126

63
114

46
III

17
24
10

'5
44

33
31
29

25
30
18
20
32
21

45
18

39
32
31
39
40
29
46
34
25
32
36

34
26
21
60

71
18

4
9
6

7
7
4
6

7
6
15
7
9
3
6

8
3
5

13
6

10
9
9
8

7
13
10

7
3

73
70
48
60
93

54
108

83
66

78
74
79
59
54

120
66

132
42

104

23
20

«S
13

25
«9
15
19
'5
22
18
19
25
24
23
26
12
14
13

1,578

490

697

125

151

'.463

Table 52. — Pi club vermilion 9 9 X yellow cTcf. fi 'icild-type 9 9

X F\ club vermilion cf cf •

F2 females.

Fj males.

Non-cross- i„

y.ci

V V

: Cross

-overs.

H —

•+-

=H-

~^ —

Refer-
ence.

overs.

Ci V

Club
ver-

wiid-iciub.

Ver-

Yellow.

Club
ver-

Yellow
club

Wild-

Yellow
ver-

Club.

Yellow
club.

Ver-
milion.

milion.

type.

milion.

ver-

^P^i milion.

milion.

99 •••

ICXD. . . .

58 i 6

43 23

lOI. . . .

32 6

5 «o

102. .. .

55 , 20

103. . . .

Total.

....

156

197 1 45

188

144

59 50

3 !

SEX-LINKED INHERITANCE IN DROSOPHILA.

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
crossmg-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 cross-
ing-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. — Pi cherry 9 9 X club vermilion cf cf. B. C. Fi wild-type 9

X cherry club vermilion cf cf.

Refer-
ence.

Ci V

W^ Ci V

■^

W'jCi

Total.

Cross

-over values.

Cherry.

Club
ver-
milion.

Cherry
club
ver-
milion.

Wild-
type.

Cherry

ver-
milion.

Club.

Cherry
club.

Ver-
mil-
ion.

Cherry
club.

Club
ver-
milion.

Cherry

ver-
milion.

163..

I8S

164..

234

165..

166..

107

315

167..

136

168..
Total.

115

379

337

116

1,080

NEW DATA.

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

Table 54. — Pi cherry club vermilion cfcf'. 9 9 X wild (f<f.
Fi zcild-type 9 X Fi cherry club verinilion cf cf .

W" C\ V

Ci V

Reference. Cherry
I club

ver-
'milion.

Wild-
type.

Club
Cherry.! ver-
imilion

Cherry
club.

W^ Ci

W^ V

I I

Ver-
1-

Cherry
ver- Club,

ion. milion.

Total

Cherry
club.

Cross-

over values.

Club Cherry
i ver- I ver-
milion, milion.

i88 6o 76 12

189 228 314 48

197 68 81 23

Total. 356 471 I 83

44
13

29
60

III

8
o

200

753
218

II
»3

22
16

30
27

1,171

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 F2 a close approach to a 2 : i : i ratio. The green reap-
peared only in the F2 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 Fi 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 reces-
sive 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

SEX-LINKED INHERITANCE IN DROSOPHILA.

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, 191 3,
three males of a brownish-yellow body-color. They were uniform in
color, w^ithout 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 F2 ratios among the males
were seen to be very much distorted because of the partial absence of
certain classes (Morgan 1914c). While it was suspected that the
disturbance was due to a lethal, the data were useless tor determining
the position of such a lethal, from the fact that more than one mother
had been used in each culture. From an F2 culture that gave practi-
cally 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. — Liyikage data on club, lethal 3, and vermilion, from Morgan, 1914c,

Females.

Males.

I3 V

Cljls V

C, V

I3 '

Club.

Wild-type.

Club vermilion.

Vermilion.

S88

182

NEW DATA.

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-hearing females by eosin miniature
males. The complete data bearing on the position of lethal 3 is sum-
marized 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, 1014c.

Gens.

Total.

Cross-
overs.

Cross-over
values.

Eosin lethal 3

1,327
1,327

3,374
222

877
1,549
1,481

1,327

268

357
967

29
161
105
138

3«

20.2
27.0
29.0
13.0
18.4
6.8

9 3

2-3

Eosin vermilion

Eosin miniature

Club lethal 3

Club vermilion

Lethal 3 vermilion

Lethal 3 miniature

Vermilion miniature

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 : i sex-rario.
The explanation advanced (Morgan 1914c) was that the mother of rhe
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 i and \a the distance
between the two lethals can be found by taking twice the number of the
surviving males (2-f3) 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

\ = give t;.7 as the distance between the lethals in question.

71 + 34 105 ^ ^ ^

In the case of lethals 3 and 3^ 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 ot crossing-over
between white and that lethal of the two that she earned. 1 hese
cross-over values were plotted and gave a bi modal 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 3a are about 7 units apart, /. r., lethal 3 lies
at about 19.5.

SEX-LINKED INHERITANCE IN DROSOPHILA.

LETHAL lb.

A cross between yellow white males and abnormal abdomen females
gave (February 1914) regular results in 10 Fo cultures, but three
cultures gave 2 9 : I cf sex-ratios (Morgan, 1914^, 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 pres-
ent in the space of about i unit between yellow and white, and this lethal
was thought to be a reappearance of lethal i (Morgan, igiib, 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 i. Indeed, the linkage data show
that this lethal, which may be called lethal lb, 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 lb 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 lb at the point
at which white itself is situated, namely, i.i, 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. Further evidence against this sup-
position 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 aff'ect the character of the white. We prefer, therefore, to
locate lethal ib at i . i — .

Table 57. — Summary of all linkage data upon lethal ib, from Morgan, igi4b.

Gens.

Total.

Cross-overs.

Cross-over
values.

Yellow lethal lb

Yellow white

744

2,787
846

23
0

0.81
0.82
0.0

Lethal \b white

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

NEW DATA.

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 diff"ers some-
what 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 9 9 (2), 112; not-facet d^ cT
(i), 57; facet cf cf (i), 51-

Of the three characters which were shown by the F2 males, one, facet,
is sex-linked, another, speck, is in the second chromosome, and maroon
is in the third chromosome. All eight F2 classes are therefore expected
to be equal in size, and each pair of characters should show free assort-
ment, 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
F2 males of table 58.

Table 58.— Pi speck maroon d" X zvild 9 9. B.C. F^ wild-type 9 X speck

maroon cf .

Refer-
ence.

F2 females. F2 males.

Speck
maroon.

Wild- „ ,

Ma-
roon.

Facet.

Speck
maroon.

Facet

speck

maroon.

Wild-
type.

1 1

Facet ,, , Fecct Ma-
maroon. '"^'" ■ speck.; roon.

66....

30 i 26

1
i

14 10

1
11 17 >- 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 Hnkage of all three gens, facet, vermilion, and
sable. The oflFspring of these three females are classified in table 59.

The cross-over fraction for vermilion sable as calculated from the
females is to- The cross-over value corresponding to this traction is
10 units, which v/as 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 arc
markedly difl'erent from those of the other two pairs. W hile the sable
males are fully represented, their opposite classes, the gray males, are

SEX-LINKED INHERITANCE IN DROSOPHILA.

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 vermiUon and
toward yellow. The cross-over values are: facet vermilion 40; vermil-
ion 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 ver-
milion shows that facet must lie verv near to yellow.

T.\BLE 59. — Pi facet & X verviilion sable 9 9

X vermilion sable cf d^.

B. C. Fi wild-type 9

Fj females.

F2 males.

Non-cross- ^

Cross-
overs. 1

overs.

V S

fa 1 V S 1 fa

■+5

Reference.

Ver-
milion
sable.

Wild- Ver-
type.milion.

Sable.

Facet.

Ver-
milion
sable.

Facet
ver-
milion
sable.

Wild-
type.

Facet
sable.

Ver-
milion.

Facet
ver-
milion

Sable.

149

29 3

ISO

17 1 2

. .

151

Total .

63 7

1
109 1 12

29 i 38 5

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 Fi females
were mated singly to wild males and they gave the results shown in
table 60. The F2 females were not counted, since they do not furnish
any information. The evidence of table 60 places facet at i.i units to
the right of eosin, or at 2.2.

Table 60. — Pi eosin vermilion 9 X facet c^. fi tcild-type 9 X "wild cf-

Refer-
ence.

w^-

\\\ fa

w«

". fa .

Total.

fa V

L.10SS-0VCI vaiuci.

1 Eosin
ver-
milion.

Facet.

Eosin
facet.

Ver-
milion.

„ Eosin

^ . ^^"' facet
Eosin. ver- ^^^_

""••'^"•milion.

Wild-
type.

Eosin
facet.

Facet
ver-
milion.

Eosin
ver-
milion.

512..

5I3--
514..
5I5--
S16..
S17..
518..

• 43
. 28

18
18

■ 24

• 44

43
35
31
60

34
38

I
2

13
19
17
20

7
10

5
II

15
12
12

•
I

116
89
78

"3
60
80

130

....

Total

■ 185

1 i 1 !

272 2 j 4 109 93 1 . . 1 I 666 1.05 30.5

1
31-3

NEW DATA. 70

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
m 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 experi-
mental 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, '^^^ ^7-^ may be multiplied by 1,641. The sum
of these two numbers divided by the sum of 1,734 ''^^id 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 191 4) in the inbred
stocks of Drosophila has not been located by means of linkage experi-
ments. It is interesting in that the males which receive the lethal
factor sometimes live long enough to hatch. These males are ex-
tremely feeble and never live more than two days. I here is, as far
as can be seen, no anatomical defect to which their extreme feebleness
and early death can be attributed.

SEX-LINKED INHERITANCE IN DROSOPHILA.

FURROWED.

In studying the effect of hybridization upon the production of
mutations in Drosophila, F. N. Duncan found a sex-Unked mutation
which he called "furrowed eye" (Duncan 191 5). 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 lin

kage of fu

rrowed, from Duncan,

1915-

Gens.

F2 males.

Total.

Cross-over values.

Eosin, miniature,
furrowed

Furrowed, sable,
forked

Vermilion, fur-
rowed, bar. . .

w", f w w^ m . f w

"^1 1

„ Minia- „ .
h-osm hosm

1
m

m tw

mmia-
ture.

fur- *"^-
rowed. '''^'^■

142

208

29.8

304

30.3

fw.S f

fw .f

fw s

Fur-
rowed
sable.

Sable
forked.

Fur-
rowed
forked.

S f

' ' f

166

209 I 5.7

16.3

19. 1

V fw V

V fw.B'

Ver-
milion
fur-
rowed.

Fur-
rowed
bar.

Ver-
milion
bar.

B' fw B'

188

9 43

240

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 pubHshed. We have included in the
white miniature data those published by P. W. Whiting (Whiting 1913).

NEW DATA.

Table 62. — Data upon the linkage of yellow, white, vermilion, and miniatu

{contributed by students).

Gens.

Non-cross

-overs.

Cross-overs.

Total.

Cross-over
values.

White miniature

^ I

6,2I9»

7,378

3,754

3,337

20,688

34 2

Yellow miniature

Vermilion miniature

Yellow white

5!^-+-

1,651

1,116

671

1,047

4,485

38.3

J— f-

761

923

421

653

2,758

51— H

1,685

1,460

3,213

2.1

y -H

1,600

1,807

3,424

o-S

Yellow vermilion

White bar

^ 1

509

587

328

284

1,708

35-8

'" +-

198

272

168

166

804

Bifid rudimentary

Rudimentary forked

5l_h

142

116

285

r— H

211

288

I 4

^The figures to the left in each double column correspond to the .symbols above the hc.ivy line.
as, in the first example 6,219 white miniature. The similar figure to the riRht corrcsi>ond» 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.

SEX-LINKED INHERITANCE IN DROSOPHILA.

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. Stiirievant and H. J. Muller.

jens.

Yellow white X bifid

Yellow X vermilion
bar

White bifid X forked

Vermilion miniature
X sable

Sable rudimentary X
forked

Classes.

y w

233 254

vB'

99 lOI

w bi

84 77

V m

152 III

143 195

y+Y_21

60 ss

y w bi

y ,B-

vv bi f

9 6 65

V S
-+

4 2

i*— f

26 27

V m s

s__^

w bi

y V

=H — I

-H h-

m s

+ — ^

r f

Total.

506

435 32

Cross-over values.

Yellow
white.

0.6

Yellow

ver-
milion

306

286

398

White
bifid

Ver-
milion
minia-
ture

2.1

Sable
rudi-
men-
tary.

13-3

White
bifid.

3-2

Ver-
milion
bar

Bifid
forked

Minia-
ture
sable.

Rudi-
men-
tary
forked

Yellow
bifid.

3.8

Yellow
bar.

White
forked.

Ver-
milion
sable.

;.i

Sable
forked.

White Bifid X Rudimentary.

F2 females.

w bi

F2 males.

wbi

w r

wbi r

+7— f

bi'r

Total

Cross-over values.

White
bifid.

Bifid
rudi-
men-
tary.

White
rudi-
men-
tary.

228 335

15 II

150 66

2 10

29 135

395

3-8

42.3

445

White BifidX Miniature Rudimentary.

w bi

-H H-

+-I-+

344

109

NEW DATA.

SUMMARY OF THE PREVIOUSLY DETERMINED CROSS-OVER VALUES.

The data of the earHer papers, namely, Dexter, 1912; Morgan, iQior,
1911^, 1911/, 1912/, 1912^; 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, 191 5) and are given here in table 64. Our summary com-
bines three summaries of Sturtevant, viz, that of single crossing-over
and two of double crossing-over.

Table 64. — Previously ptiblished data summarized from Sturtevant, IQIS-

Factors.

Total.

Cross-overs.

Cross-over
values.

Yellow white

46,564
10,603

18,797

2,563

191

15,257

41,034

5,847

5,151

5,329

1,554

7,514

12,567

3,112

159

498

3,644
6,440

1,100

4,910

13,513

2,461

2,267

212

376

1,895

2,236

636

1.07
33-4
34-3
42.9
46.1
32.1
32.8
42.1
44.0

4.0
24.1
25.2
17.8
20.4

4 4

Yellow vermilion

Yellow miniature

Yellow rudimentary

Yellow bar

White vermilion

White miniature

White rudimentary

White bar

Vermilion miniature

Vermilion rudimentary. . .
Vermilion bar

Miniature rudimentary. . .
Miniature bar

Rudimentary bar

SEX-LINKED INHERITANCE IN DROSOPHILA.

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 pub-
lished 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.

Yellow lethal 1

Yellow lethalli....

Yellow white

Yellow abnormal

Yellow bifid

Yellow club

Yellow vermilion. . . .
Yellow miniature. . .

Yellow sable

Yellow rudimentary.

Yellow bar

Lethal 1 white

Lethal 1 miniature. .

Lethal \b white

White facet

White abnormal. . . .

White bifid

White lethal 2

White club

White lethal sb

White lemon

White depressed . . . .

White lethal sa

White vermilion. . . .
White reduplicated. ,
White miniature.. . .

White furrowed

White sable

White rudimentary.

White forked

White bar

White fused

White lethal sc

Facet vermilion

Facet sable

Bifid vermilion

Bifid miniature. . . . ,
Bifid rudimentary..,

Bifid forked

Lethal 2 vermilion. .
Lethal 2 miniature..

Club lethal 3

Club vermilion. . . . ,
Lethal sh miniature.
Lemon vermilion

Total.

131

744
81,299

15,314

3,681

525

13,271

21,686

1,600

2,563
626

1,763

814

846

666

16,300

23,595

8,011

2,251

3,678

241

1,150

27,962

418

[10,701

208

2,511

6,461
3,664

5,955
430

3,053
852
186

2,724
219
899
306

1,400

6,752
222

5,558

3,678

241

Cross-overs.

87s
299
201

93
4,581

7,559

686

1,100

300

323

7
277
1,260
767
321
572

256

8,532

121

31,071

63
1,032

2,739
1,676
2,601

186
1,406

278
80

849
67

384
130
248

1,054
29

1,047

733

Cross-over
values.

0.8

I.I

2.0

5-5

17-7

34-5

34-3

42.9

47-9
0.4

39-7
0.0

X.I

1-7
5-3
9.6

143

IS. 6

145

20.3

22.2

30.5
28.9

33-2

30.3
41.2
42.4

45-7
43.6

43-3
46.0
32.6
43-0

311

30.6
42.7
42.5
17.7

15-4
13.0
18.8
19.9
12.0

NEW DATA. 85

Table 65. — A summary of all linkage data upon chromosome I — Continued.

jens.

Shifted vermilion

Shifted bar

Depressed vermilion

Depressed bar

Lethal 3 vermilion

Lethal 3 miniature

Vermilion dot

Vermilion reduplicated. .

Vermilion miniature

Vermilion furrowed

Vermilion sable

Vermilion rudimentary. .

Vermilion forked

Vermilion bar

Vermilion fused

Reduplicated bar

Miniature furrowed

Miniature sable

Miniature rudimentary. .

Miniature bar

Furrowed sable

Furrowed forked

Furrowed bar

Sable rudimentary

Sable forked

Sable bar

Sable lethal sc

Rudimentary forked

Rudimentary bar

Forked bar

Forked fused

Bar fused

Bar lethal sc

Total.

Cross-overs.

1,007

155

242

464

176

1,549

105

1,481

138

667

10,155

317

240

9,209

929

1,554

376

66s

163

23,522

5, 612

9,252

2,390

583

120

208

1,855

125

12,786

2,284

3,112

636

209

240

663

872

140

7,524

1,036

1,641

387

1,456

664

1,706

1,201

8,768

222

1,734

144

Cross-over
values.

ISS

31-4

17.0

38.0

6.8

9-3

0.0

3.8

10. 1

24.1

24s
23.9
25-8
20.6

17-9

20.5

5-7
19. 1

17-9

14 3

16.0

13.8

23.6

2-3

0.5

2.5

8.3

BIBLIOGRAPHY.

Bridges, Calvin B.

1913. Non-disjunction of the sex-chromosomes of Drosophtla. 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.

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BIBLIOGRAPHY. 87

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DESCRIPTIONS OF PLATES.
Plate I.

Fig. I. Normal 9 •

Fig. 2. Sable 9 •

Fig. 3. Lemon cf.

Fig. 4. Abnormal abdomen 9 .

Fig. 5. Abnormal abdomen 9 .

Fig. 6. Yellow 9

Plate II.

Fig. 7. Eosin, miniature, black cT.

Fig. 8. Eosin, miniature, black 9 •

Fig. 9. Cherry.

Fig. 10. Vermilion.

Fig. II. White.

Fig. 12. Bar (from above).

Fig. 13. Bar (from side).

Fig. 14. Spot 9 (abdomen from above).

Fig. 15. Spot 9 (abdomen from side).

Fig. 16. Spot d^ (abdomen from above).

Fig. 17. Spot c? (abdomen from side).

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