Sex-linked Inheritance In Drosophila Part 2

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

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

[Ill.u.s.tration: Fig. A. _a._ rudimentary wing; _b._ the wild fly for comparison.]

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

MINIATURE.

(Plate II. figures 7 and 8.)

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

VERMILION.

(Plate II. figure 10.)

The recessive s.e.x-linked mutant vermilion eye-color (Morgan, 1911_c_ and 1912_a_) appeared in November 1910, and has appeared at least twice since then (Morgan and Plough, 1915). This is one of the best of the s.e.x-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 s.e.x-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 s.e.x-linked mutant yellow body and wing-color appeared in January 1911 (Morgan, 1911_c_ and 1912_a_). Its first appearance was in black stock; hence the fly was a double recessive, then called brown. Later the same mutation has appeared independently from gray stock. Yellow was found to be at the end of the X chromosome, and this end was arbitrarily chosen as the zero or the "left end," while the other gens are spoken of as lying at various distances to the right of yellow. Recently a lethal gen has been located less than one-tenth of a unit (-0.04) to the left of yellow, but yellow is still retained as the zero-point.

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

ABNORMAL ABDOMEN.

(Plate I. figure 4.)

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

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

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

EOSIN.

(Plate II, figures 7 and 8.)

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

BIFID.

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

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

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

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

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

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

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

{30}

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

LINKAGE OF CHERRY, BIFID, AND VERMILION.

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

TABLE 3.--_P_1 cherry vermilion [female] [female] bifid [male] [male]. B.

C.[2] F_1 wild-type [female] white bifid [male] [male]._

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

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

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

REDUPLICATED LEGS.

In November 1912 Miss Mildred Hoge found that a certain stock was giving some males whose legs were reduplicated, either completely or only with respect to the terminal segments (described and figured, Hoge, 1915).

Subsequent work by Miss Hoge showed that the condition was due to a s.e.x-linked gen, but that at room temperature not all the flies that were genetically reduplicated showed reduplication. However, if the flies were raised through the pupa stage in the ice-box at a temperature of about 10 to 12 a majority of the flies which were expected to show reduplication did so. The most extremely reduplicated individual showed parts of 14 legs.

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

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

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

The most accurate data, those upon the value for reduplicated and vermilion, give for reduplicated a distance of 1.7 from vermilion, either to the right or to the left. The distance from white is 29, which would place the locus for reduplication to the left of vermilion, which is at 33.

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

LETHAL 1.

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

The explanation of this mode of transmission became clear when it was found that the cause of the death of half of the males was a particular factor that had as definite a locus in the X chromosome as have other s.e.x-linked factors (Morgan, 1912_e_). Morgan mated females (from the stock sent to him by Miss Rawls) to white-eyed males. Half of the females, as expected, gave 2 : 1 s.e.x-ratios, and daughters from these were again mated to white males.

Here once more half of the daughters gave 2 : 1 s.e.x-ratios, but in such cases the sons were nearly all white-eyed and only rarely a red-eyed son appeared, when under ordinary circ.u.mstances there should be just as many red sons as white sons. The total output for 11 such females was as follows (Morgan, 1914_b_): white [female], 457; red [female], 433; white [male], 370; red [male], 2. It is evident from these data that there must be present in the s.e.x-chromosome a gen that causes the death of every male that receives this chromosome, and that this lethal factor lies very close to the factor for white eyes. The linkage of this lethal (now called lethal 1) to various other s.e.x-linked gens was determined (Morgan 1914_b_), and is summarized in table 5. On the basis of these data it is found that the gen lethal 1 lies 0.4 unit to the left of white, or at 0.7.

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