Sex-linked Inheritance In Drosophila Part 5

The calculation of the locus for sable gives 43.0.

DOT.

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

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

LINKAGE OF VERMILION AND DOT.

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

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

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

{45}

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

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

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

A = Wild-type [female].

B = Vermilion [male].

C = Wild-type [male].

D = Wild-type [female].

E = Vermilion [male].

F = Vermilion [female].

G = Vermilion dot [male].

H = Vermilion dot [female].

I = Dot [male].

J = Dot [female].

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

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

The evidence from the history of the stock, together with these experiments, shows that the character resembles club (wing) in that it is not expressed somatically in all the flies which are h.o.m.ozygous for it. In the case of club we were fortunate enough to find a constant feature {46} which we could use as an index, but, so far as we have been able to see, there is no such constant accessory character in the case of the dot.

Unlike club, dot is markedly s.e.x-limited in its effect; that is, there is a difference of expression of the gen in the male and female. This difference recalls the s.e.xual dimorphism of the eosin eye.

BOW.

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

[Ill.u.s.tration: FIG. C.--Bow wing.]

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

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

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

{47}

The F_2 ratio in table 21 is evidently the 2:1:1 ratio typical of s.e.x-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 cla.s.sified among the not-bow or straight wings. In favor of the view that the cla.s.sification was not strict is the fact that the sum of the two male cla.s.ses about equals the number of the females.

BOW BY ARC.

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

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

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

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

Bow and arc are so much alike that they give a single rather variable phenotypic cla.s.s in F_2. Therefore the F_2 generation is made up of only two separable cla.s.ses--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 s.e.x is ignored, the case is similar to the case of the two white races of sweet peas, which bred together gave wild-type or purple peas in F_1 and in F_2 gave 9 colored to 7 white. If s.e.x is taken into account, the theoretical expectation for the F_2 females is 6 straight to 2 arc, and for the F_2 males 3 straight to 1 arc to 3 bow to 1 bow-arc.

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

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

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

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

In this case the F_2 expectation is 6 straight to 10 not-straight. Since the s.e.x-linked gen bow entered from the female, half the F_2 males and females are bow. The half that are not-bow consist of 3 straight to 1 arc, so that both in the female cla.s.ses and in the male cla.s.ses 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 cla.s.sification 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 ma.s.s from those cultures that gave the greatest percentage of lemon flies. In a few cases lemon males mated with their gray sisters left offspring, but the stock obtained in this way had still to be maintained by breeding heterozygotes, as stated above. But from the gray sisters heterozygous for lemon (bred to lemon males) some lemon females were also produced.

LINKAGE OF CHERRY, LEMON, AND VERMILION.

In order to study the linkage of lemon, the following experiment was carried out. Since it was impracticable to breed directly from the lemon flies, virgin females were taken from stock throwing lemon, and were mated singly to cherry vermilion males. Only a few of the females showed themselves heterozygous for lemon by producing lemon as well as gray sons.

Half the daughters of such a pair are expected to be heterozygous for lemon and also for cherry and vermilion, which went in from the father. These daughters were mated singly to cherry vermilion males, and those that gave some lemon sons were continued, {49} and are recorded in table 24. The four cla.s.ses of females were not separated from each other, but the total of females is given in the table.

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

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

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