Sex-linked Inheritance In Drosophila Part 3

81-92._

+------------------------+---------+--------+-------------+ Gens. Total. Cross- Cross-over overs. values. +------------------------+---------+--------+-------------+ Yellow lethal 1 131 1 0.8 Yellow miniature 131 45 34.4 Lethal 1 white 1,763 7 0.4 Lethal 1 miniature 814 323 39.7 White miniature 994 397 39.9 +------------------------+---------+--------+-------------+

LETHAL 1a.

In the second generation of the flies bred by Miss Rawls, one female gave (March 1912) only 3 sons, although she gave 312 daughters. It was not known for some time (see lethals 3 and 3_a_) what was the cause of this extreme rarity of sons. It is now apparent, however, that this mother carried lethal 1 in one X and in the other X a new lethal which had arisen by mutation. The new lethal was very close to lethal 1, as shown by the rarity of the surviving sons, which are cross-overs between lethal 1 and the new lethal that we may call lethal 1a. There is another cla.s.s of cross-overs, namely, those which have lethal 1 and get lethal 1_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 (3 2) 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, {33} we may take 300 as the number of the males which died.

There must have been, then, about 2 per cent of crossing-over, which makes lethal 1_a_ lie about 2 units from lethal 1. This location of lethal 1_a_ is confirmed by a test that Miss Rawls made of the daughters of the high-ratio female. Out of 98 of these daughters none repeated the high s.e.x-ratio and only 2 gave 1 [female] : 1 [male] ratios. The two daughters which gave 1 : 1 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 (2 2) and taking 98 - 2 as the number of non-cross-overs. The cross-over fraction {6 + 4}/{300 + 96} gives 2.6 as the distance between the two lethals. Lethal 1_a_ is probably to the right of lethal 1 at 0.7 + 2.6 = 3.3.

SPOT.

(Plate II, figures 14 to 17.)

In April 1912 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_a_). 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, _i. 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 F_2 have, in addition to the spot on the abdomen, 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. The 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 ill.u.s.tration of the fact that mutant gens ordinarily affect many parts of the body, though these secondary effects often pa.s.s unnoticed.

In the F_2 of the cross of spot by black one yellow black fly appeared, although none are expected, on the a.s.sumption that spot and yellow {34} 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[3] a fly appeared (July 19, 1912) whose body-color differed 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 heterozygous for ordinary recessive black, but at the same time that his dark color must be due to another kind of black. The gray F_1 flies when mated together gave a series of gray and dark flies in F_2 about as follows: In the females 3 grays to 1 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 s.e.x-linked factor. It was difficult to discover which of the heterogeneous F_2 males were the new blacks. Suspected males were bred (singly) to wild females, and the F_2 dark males, from those cultures that gave the closest approach to a 2 gray [female] : 1 gray [male] : 1 dark [male], were bred to their sisters in pairs in order to obtain sable females and males. Thus stock h.o.m.ozygous 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 effects.

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 F_2, as seen in table 6. The reciprocal cross gave the results shown in table 7.

{35}

The F_1 males were sable like their mother. The evidence thus shows that sable is a s.e.x-linked recessive character. Our next step was to determine the linkage relations of sable to certain other s.e.x-linked gens, namely, yellow, eosin, cherry, vermilion, miniature, and bar.

TABLE 6.--_P_1 wild [female] [female] sable [male]. F_1 wild-type [female] [female] F_1 wild-type [male] [male]._

+---------------+-------------------+-------------------+---------------+ Reference.[4] Wild-type [female]. Wild-type [male]. Sable [male]. +---------------+-------------------+-------------------+---------------+ 88 C 218 100 70 143 C 245 108 72 146 C 200 115 82 +-------------------+-------------------+---------------+ Total 663 323 224 +---------------+-------------------+-------------------+---------------+

TABLE 7.--_P_1 sable [female] wild [male] [male]. F_1 wild-type [female]

F_1 sable [male]._

+--------------+-------------+-------------+-------------+-------------+ Reference. Wild-type Wild-type Sable Sable [female]. [male]. [female]. [male]. +--------------+-------------+-------------+-------------+-------------+ 4 I 10 10 6 10 +--------------+-------------+-------------+-------------+-------------+

LINKAGE OF YELLOW AND SABLE.

The factor for yellow body-color lies at one end of the known series of s.e.x-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 F_2 two cla.s.ses of females, namely, yellow and gray, and four cla.s.ses of males, namely, yellow and sable (non-cross-overs), wild type and the double recessive yellow sable (cross-overs). From off-spring (F_3) of the F_2 yellow sable males by F_2 yellow females, pure stock of the double recessive yellow sable was made up and used in the crosses to test linkage.

In color the yellow 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 F_1 consisted of wild-type males and females. These inbred gave the results shown in table 8. {36}

TABLE 8.--_P_1 wild [female] [female] yellow sable [male] [male]. F_1 wild-type [female] [female] F_1 wild-type [male] [male]._

+-----------+---------+--------------+--------------+-------+-----------+ Non-cross-over Cross-over Wild- [male]. [male]. Reference. type +-------+------+-------+------+ Total Cross-over [female]. Yellow Wild- males. value. sable. type. Yellow. Sable. +-----------+---------+-------+------+-------+------+-------+-----------+ 44 I 292 110 43 75 36 264 42 45 I 384 104 58 71 60 293 45 +---------+--------------+-------+------+-------+-----------+ Total 676 214 101 146 96 557 43 +-----------+---------+-------+------+-------+------+-------+-----------+

Some of the F_1 females were back-crossed to yellow sable males and gave the data for table 9.

TABLE 9.--_P_1 wild-type [female] [female] yellow sable [male] [male]. B.

C. F_1 wild-type [female] yellow sable [male] [male]._

+----------+-------------------------+---------------+-------+----------+ Non-cross-overs. Cross-overs. Reference.+-----------+-------------+-------+-------+ Total. Cross-over value. Wild-type. Yellow sable. Yellow. Sable. +----------+-----------+-------------+-------+-------+-------+----------+ 31 I 108 51 58 56 273 42 49 I 265 175 161 169 770 43 +-----------+-------------+-------+-------+-------+----------+ Total 373 226 219 225 1,043 43 +----------+-----------+-------------+-------+-------+-------+----------+

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 a.s.sortment would give 50 per cent of cross-overs and absolute linkage 0 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 1,600 individuals in which crossing-over might occur. The females of table 8 are all of one cla.s.s (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 relations to the other s.e.x-linked factors. {37}

LINKAGE OF CHERRY AND SABLE.

The origin of cherry eye-color (Plate II, fig. 9) has been given by Safir (Biol. Bull., 1913). From considerations which will be discussed 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.--_P_1 cherry [female][female] sable [male][male]. F_1 wild-type [female] F_1 cherry [male] [male]._

+---------+---------+---------+--------------+------------+------+------+ Non-cross- Cross-over Wild- Cherry over [male]. [male]. Cross- Refer- type [female].+-------+------+------+-----+Total over ence. [female]. Cherry. Sable. Cherry Wild- males. value. sable. type. +---------+---------+---------+-------+------+------+-----+------+------+ 24 I 94 105 51 42 20 43 156 40 55 I 101 131 63 52 38 48 201 43 55' I 96 94 52 31 29 30 142 42 +---------+---------+-------+------+------+-----+------+------+ Total 291 330 166 125 87 121 499 42 +---------+---------+---------+-------+------+------+-----+------+------+

The percentage of crossing-over between cherry and sable is 42. Since cherry is one point from yellow, 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 (_i. e._, to the right). Sable will lie 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) vermilion (gray) males, the daughters were wild type and the males were eosin sable. Inbred these gave the cla.s.ses shown in table 11. {38}

TABLE 11.--_P_1 eosin sable [female] vermilion [male][male]. F_1 wild-type [female][female] F_1 eosin sable [male][male]._

+------+--------------------------+ F_2 females. +------------+-------------+ w^e s w^e Refer- ---------- -----+----- s ence. +------+-----+------+------+ Eosin Wild- Eosin. Sable.~ sable. type. ~ +------+------+-----+------+------+ 26 I 132 171 113 109 26'I 96 146 86 78 +------+-----+------+------+ Total. 228 317 199 187 +------+------+-----+------+------+

+------+---------------------------------------------------------+ F_2 males. +---------------+--------------+-------------+------------+ w^e s w^e v w^e w^e v s Refer- ----------- -----+----- --------+-- ---+---+-- v s v s ence. +-------+-------+-------+------+------+------+------+-----+ Eosin Ver- Eosin ~ Eosin Ver- ver- Sable. Eosin. milion ver- Wild- ~ sable. milion. milion. sable. milion type. sable. +------+-------+-------+-------+------+------+------+------+-----+ 26 I 127 163 75 76 37 14 2 5 26'I 74 128 76 59 18 21 4 3 +-------+-------+-------+------+------+------+------+-----+ Total. 201 291 151 135 55 35 6 8 +------+-------+-------+-------+------+------+------+------+-----+

If we consider the male cla.s.ses of table 11, we find that the smallest cla.s.ses are eosin vermilion sable and wild type, which are the expected double cross-over cla.s.ses if sable lies to the right of vermilion, as indicated by the crosses with eosin and with yellow. The cla.s.ses 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.