THE MECHANISM OF SEX-DETERMINATION 69 



planation of these facts. For the present it will suffice 

 to point out that the cases just described, and all like 

 them, may be accounted for by means of a very simple 

 hypothesis. We have traced the history of the X 

 chromosomes. If the factors that produce white eyes, 

 short (miniature) wings, and yellow body color are 

 carried by the X chromosomes, we can account for 

 these results that seem at first sight so extraordinary. 

 The history of the sex chromosomes is accurately known. 

 Their distribution in the two sexes is not a matter of 

 conjecture but a fact. Our hypothesis rests therefore 

 on a stable foundation. 



At the risk of confusion I feel bound to present here 

 another type of sex-linked inheritance. In principle 

 it is like the last, but the actual mechanism, as we shall 

 see, is somewhat different. Again I shall make use 

 of an illustration. If a black Langshan hen is mated 

 to a barred Plymouth Rock cock, all the offspring will 

 be barred (Fig. 40). If these are inbred, there are pro- 

 duced barred females and males, and black females. 

 The numerical proportion is 50 per cent barred males, 25 

 per cent barred females, and 25 per cent black females. 



The black hen has transmitted her character to half 

 of her granddaughters and to none of her grandsons. 

 The resemblance to the case of the red-eyed, white- 

 eyed flies is obvious, but here black appears as a sex- 

 linked character in the females. 



The converse cross is also suggestive. When a 

 barred hen is mated to a black cock, all the daughters 

 are black and all the sons are barred (Fig. 41). When 

 these are inbred, there are produced black males and 

 females and barred males and females in equal num- 



