•^^^ 



X SEX 109 



the 2 X chromosomes are present, the presence of a 

 y chromosome appears to have no influence upon 

 either sex or fertihty. The males, however, all 

 proved to be sterile. A single X chromosome is 

 sufficient to lead to the production of a male, but it 

 would appear that the presence of a K chromosome 

 is essential if the male is to be fertile. Though this 

 chromosome acts as a dummy in so far as the trans- 

 mission of sex - limited characters is concerned, it 

 nevertheless fulfils some important 

 function in the male economy. 



Though the analysis of sex has 

 in no case been pushed so far as in ~^ ? ^-^ 

 Drosophila, we have evidence that 

 sex-limited inheritance of the same ^'^" 33- 



type occurs in vertebrates. One of '"'the^cToTaTn.aV: 

 the best known instances is that of "^Ts.i^lJn\Mhlon 

 the tortoiseshell cat, which, as Don- mosomes^ lirmairy 

 caster has shown, can be explained onhe female' The 

 on similar lines. It is well known t^hehcok'hapTJone 

 that tortoiseshell cats are almost J,"rt'^of IhTlguf^'. 

 always females, and experiment has SteX^dg^!')^'^'''^ 

 shown that they are the heterozygous 

 form of black x yellow. When a yellow female 

 is mated with a black torn, the male kittens are 

 yellow and the female are tortoiseshell. We may 

 express this by saying that yellow is dominant 

 in the male, but incompletely so in the female. 

 Bred together, such animals give a generation 

 consisting of yellow and of tortoiseshell females, 

 together with yellow and black males in approxi- 

 mately equal numbers. The recessive black re- 

 appears in F and, as in the white-eyed DrosopJiila^ 



