Only three dot individuals appeared in F_2, 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 F_1 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.--_P_1 vermilion dot [female] x wild [male]._ A = Wild-type [female]. B = Vermilion [male]. C = Wild-type [male]. D = Wild-type [female]. E = Vermilion [male]. F = Vermilion [female]. G = Vermilion dot [male]. H = Vermilion dot [female]. I = Dot [male]. J = Dot [female]. +--------------------++-----------------------------------------------+ | First generation. || Second generation. | +----------+----+----++----------+----+----+----+----+----+---+---+---+ |Reference.| A | B ||Reference.| C | D | E | F | G | H | I | J | +--------------------++----------+----+----+----+----+----+---+---+---+ | 137 C. | 44 | 45 || 19 |211 |198 |228 |206 | 20 | 3 | 0 | 0 | | 138 C. | 77 | 62 || 22 |266 |220 |227 |227 | 16 | 0 | 0 | 0 | | |124 |124 || 28 |143 |149 |125 |124 | 14 | 1 | 0 | 0 | | | 57 | 41 || +----+----+----+----+----+---+---+---+ | |----|----|| Total.|620 |567 |570 |557 | 50 | 4 | 0 | 0 | | Total.|291 |272 || | | | | | | | | | +--------------------++----------+----+----+----+----+----+---+---+---+ The F_2 generation is given in table 20. The dot reappeared in F_2 both in females and in males, but instead of appearing in 50 per cent of both sexes, as expected if it is simply sex-linked, it appeared in 4.0 per cent in the females and in only 0.4 per cent in the males. The failure of the character to be fully realized is again apparent, but here, where it is possible for it to be realized equally in males and females, we find that there are 50 females with dot to only 4 dot males. This would indicate that the character is partially "_sex-limited_" (Morgan, 1914_d_) in its realization. The dot appeared only in flies with vermilion eyes, indicating extremely strong linkage between vermilion and dot.