([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 F1 flies when mated together gave a series of gray and dark flies in F2 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 sex-linked factor. It was difficult to discover which of the heterogeneous F2 males were the new blacks. Suspected males were bred (singly) to wild females, and the F2 dark males, from those cultures that gave the closest approach to a 2 gray ♀ : 1 gray ♂ : 1 dark ♂, were bred to their sisters in pairs in order to obtain sable females and males. Thus stock homozygous 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 F2, as seen in table 6. The reciprocal cross gave the results shown in table 7.
The F1 males were sable like their mother. The evidence thus shows that sable is a sex-linked recessive character. Our next step was to determine the linkage relations of sable to certain other sex-linked gens, namely, yellow, eosin, cherry, vermilion, miniature, and bar.
Table 6.—P1 wild ♀ ♀ × sable ♂. F1 wild-type ♀ ♀ × F1 wild-type ♂ ♂.
| Reference.[[4]] | Wild-type ♀. | Wild-type ♂. | Sable ♂. |
| 88 C | 218 | 100 | 70 |
| 143 C | 245 | 108 | 72 |
| 146 C | 200 | 115 | 82 |
| Total | 663 | 323 | 224 |
Table 7.—P1 sable ♀ × wild ♂ ♂. F1 wild-type ♀ × F1 sable ♂.