Sex-linked Inheritance In Drosophila Part 6

LETHAL 2.

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

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

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

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

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

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

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

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

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

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

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

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

CHERRY.

(Plate II, figure 9.)

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

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

A SYSTEM OF QUADRUPLE ALLELOMORPHS.

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

LINKAGE OF CHERRY AND VERMILION.

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

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

TABLE 28.--_P_{1} cherry vermilion [male] [male] wild [female] [female].

F_{1} wild-type [female] [female] F_{1} wild-type [male] [male]._

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

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

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

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

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

Eosin-cherry compound was also made. An eosin female was mated to a cherry male. The eosin-cherry daughters were darker than their eosin brothers.

Inbred they gave the results shown in table 31.

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

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

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

Although in the F_2 results there are two genotypic cla.s.ses 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 cla.s.ses of males can be readily distinguished; of these, one cla.s.s, cherry, has the same color as the females, while the other cla.s.s, eosin, is much lighter. Such an F_2 group will perpetuate itself, giving one type of female (of three possible genotypic compositions, but somatically practically h.o.m.ogeneous) and two types of males, only one of which is like the females.

FUSED.

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

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

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

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

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

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

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

LINKAGE OF EOSIN AND FUSED.

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

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

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

{54}

The data give 43 per cent of crossing-over, which places fused far to the right or to the left of eosin. The latter position is improbable, since eosin already lies very near the extreme left end of the known series.

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

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

{55}