154 HEREDITY AND SEX 



To make this clearer, I have written out the case 

 more fully : 



X A BC S X AB CS 9 

 X ABC S A BCs 6 



In this, as in all such Mendelian formulae, the result 

 (or character) that a factor produces depends on its 

 relations to other things in the cell (here ABC). We 

 are dealing, then, not with the relation of X to aS alone, 

 but this relation in turn depends on the proportion of 

 both X and S to A B C. It is clear, if this is admitted, 

 that the two formulae above — the one for the male 

 and the other for the female — are neither identical 

 nor multiples. 



It will be noted that in only one of these attempts to 

 explain in insects the heredity of the secondary sexual 

 characters have the factors for the characters been 

 assumed to be caused by the sex chromosomes. If 

 one accepts the chromosome basis for heredity, these 

 results may be explained on the assumption that the 

 factors lie in other chromosomes than the sex chromo- 

 somes. 



In the next case, however, that I shall bring forward 

 the factors must be assumed to be in the sex 

 chromosomes themselves. 



The mutant of drosophila with eosin eyes that arose 

 in my cultures is the case in question. The female 

 has darker eyes than the male. The experimental 

 evidence shows that the factor for eosin is carried 

 by the sex chromosomes. In the female it is present, 

 therefore, in duplex, or, as we say, in two doses; in 

 the male in one dose. 



