NEW DATA. 



45 



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 Fi 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. — Pi vermilion dot 9 X '^vild cf. 



The F2 generation is given in table 20. The dot reappeared in F2 

 both in females and in males, but instead of appearing in 50 per cent of 

 both sexes, as expected if it is simply sex-Hnked, 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, 1914J) 

 in its reahzation. The dot appeared only in flies with vermilion eyes, 

 indicating extremely strong linkage between vermilion and dot. 



The evidence from the history of the stock, together with these 

 experiments, shows that the character resembles club (wing) in that it 

 is not expressed somatically in all the flies which are homozygous for it. 

 In the case of club we were fortunate enough to find a constant feature 



