In the F2, from a cross of a double recessive (white vermilion) female by a triple recessive (eosin vermilion pink) male, there appeared, July 21, 1912, three white-eyed females which had two small, symmetrically placed, black, granular masses upon the thorax. These "dots" appeared to be dried exudations from pores. It did not seem possible that such an effect could be inherited, but as this condition had never been observed before, it seemed worth while to mate the three females to their brothers. In the next generation about 1 per cent of the males were dotted. From these females and males a stock was made up which in subsequent generations showed from 10 to 50 per cent of dot. Selection seemed to have no effect upon the percentage of dot. Although the stock never showed more than 50 per cent of dot, yet it was found that the normal individuals from the stock threw about the same per cent as did those that were dotted, so that the stock was probably genetically pure. The number of males which showed the character was always much smaller than the number of dotted females; in the hatches which produced nearly 50 per cent of dot, nearly all the females but very few of the males were dotted. Quite often the character showed on only one side of the thorax.
Since this character arose in an experiment involving several eye-colors an effort was made by crossing to wild and extracting to transfer the dot to flies normal in all other respects. This effort succeeded only partly, for a stock was obtained which differed from the wild type only in that it bore dot (about 30 per cent) and in that the eyes were vermilion. Several attempts to get the dot separated from vermilion failed. Since this was only part of the preliminary routine work necessary to get a mutant stock in shape for exact experimentation, no extensive records were kept.
LINKAGE OF VERMILION AND DOT.
When a dot male with vermilion eyes was bred to a wild female the offspring were wild-type males and females. These inbred gave the data shown in table 19.
Table 19.—P1 vermilion dot ♂ × wild ♀ ♀. F1 wild-type ♀ ♀ × F1 wild-type ♂ ♂.
| Reference. | F2 females. | Wild-type ♂. | Vermilion ♂. | Vermilion dot ♂. | Dot ♂. |
| 7 | 345 | 151 | 130 | 0 | 0 |
| 8 | 524 | 245 | 220 | 3 | 0 |
| Total. | 869 | 396 | 350 | 3 | 0 |
Only three dot individuals appeared in F2, but since these were males the result indicates that the dot character is due to a sex-linked gen. These three males had also vermilion eyes, indicating linkage of dot and vermilion. The males show no deficiency in numbers, therefore the non-appearance of the dot can not be due to its being semi-lethal. It appears, therefore, that the expression of the character must depend on the presence of an intensifying factor in one of the autosomes, or more probably, like club, it appears only in a small percentage of flies that are genetically pure for the character.
The reciprocal cross (dot female with vermilion eyes by wild male) was made (table 20). The daughters were wild type and the sons vermilion. Not one of the 272 sons showed dot. If the gen is sex-linked the non-appearance of dot in the F1 males can be explained on the ground that males that are genetically dot show dot very rarely, or that its appearance is dependent upon the intensification by an autosomal factor of the effect produced by the sex-linked factor for dot.
Table 20.—P1 vermilion dot ♀ × wild ♂.