The first lethal found by Miss Rawls was in a stock that had been bred for about 3 years. While there was no a priori reason that could be given to support the view that lethal mutations would occur more frequently among flies inbred in confinement, nevertheless a hundred females from each of several newly caught and from each of several confined stocks were examined for lethals (Stark, 1915). No lethals were found among the wild stocks, but 4 were found among the confined stocks. Whether this difference is significant is perhaps open to question. The first lethal was found in January 1913, in a stock that had been caught at Falmouth, Massachusetts, in 1911, and had been inbred for 18 months, i.e., for about 50 generations. This lethal, lethal sa, was recessive and behaved like the former lethals, being transmitted by half the females and causing the death of half the sons. The position of this lethal to the X chromosome was found as follows, by means of the cross-over value white lethal sa. Lethal-bearing females were mated to white males and the lethal-bearing daughters were again mated to white males. The white sons (894) were non-cross-overs and the red sons (256) were cross-overs. The percentage of crossing-over
is 22.2. A correction of 0.4 unit should be added for double crossing-over, indicating that the locus is 22.6 units from white, or at 23.7.
When the work on lethal sa had been continued for 3 months, the second lethal, lethal sb, was found (April 1913) to be present in a female which was already heterozygous for lethal sa. It is probable that this second lethal arose as a mutation in the father, and that a sperm whose X carried lethal sb fertilized an egg whose X carried lethal sa. As in the cases of lethals 1 and 1a and lethals 3 and 3a, this lethal, lethal sb, was discovered from the fact that only a very few sons were produced, there being 82 daughters and only 3 sons. If, as in the other cases, the number of daughters is taken as the number of non-cross-overs and twice the number of sons as the cross-overs, it is found that the two lethals are about 7 units apart. Since the two lethals were in different X chromosomes, all the daughters should receive one or the other lethal, except in those few cases in which crossing over had taken place. Of the daughters 19 were tested and every one was found to carry a lethal. Again, if the cross-over values of the lethals with some other character, such as white eyes, be found and plotted, the curve should show two modes corresponding to the two lethals. This test was applied, but the curve failed to show two modes clearly,[[7]] 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 homologous 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.
|
|
|
|
| Total. | Cross-over values. | ||
| Eosin miniature. | White miniature. | Eosin. | White. | White lethal sb. | Lethal sb. miniature. | White miniature. | |
| 2,421 | 524 | 685 | 48 | 3,678 | 15.6 | 19.9 | 32.9 |
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 sex-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.