Sex-linked Inheritance In Drosophila Part 9

the two lethals being too close together to be differentiated by the small number of determinations that were made. It seems probable that lethal _sa_ and lethal _sb_ are about 5 units apart.

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

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

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

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

{65}

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

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

BAR.

(Plate II, figures 12 and 13.)

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

Bar is the most useful s.e.x-linked character so far discovered, on account of its dominance, the certainty of its cla.s.sification, 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 s.e.x-linked dominant factor that brings about a notch at the ends of the wings appeared in March 1913, and has been described and figured by Dexter (1914, p. 753, and fig. 13, p. 730). The factor acts as a lethal for the male. Consequently a female heterozygous for notch bred to a wild male gives a 2:1 s.e.x-ratio; half of her daughters are notch and half normal; the sons are only normal. The actual figures obtained by Dexter were 235 notch females, 270 normal females, and 235 normal males.

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

DEPRESSED.

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

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

LINKAGE OF DEPRESSED AND BAR.

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

TABLE 49.--_P_1 depressed_ [female] [female] _bar_ [female] [female].

_B.C. F_1 bar_ [female] _depressed_ [male] [male].

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

{67}

[Ill.u.s.tration: FIG. G.--Depressed wing.]

LINKAGE OF CHERRY, DEPRESSED, AND VERMILION.

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

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

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

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

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

{68}

CLUB.

In May 1913 there were observed in a certain stock some flies which, although mature, did not unfold their wings (text-fig. H_a_). This condition was at first found only in males and suspicion was aroused that the character might be s.e.x-linked. When these males were bred to wild females the club-shaped wings reappeared only in the F_2 males, but in smaller number than expected for a recessive s.e.x-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 ill.u.s.trated and proven by the following history of the stock:

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

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

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

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

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

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

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

GENOTYPIC CLUB.

Accurate work with the club character was made possible by the discovery of a character that is a constant index of the presence of h.o.m.ozygous club.

This character is the absence of the two large bristles (text-fig. H_c_) that are present on each side of the thorax of the wild fly as shown in figure Hb. All club flies are now cla.s.sified 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 cla.s.ses by the cross-over fraction 276/1463 or 19 per cent.

LINKAGE OF YELLOW, CLUB, AND VERMILION.

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

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

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

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