X SEX 113 



the male, may cany the factors for sex-limited 

 characters, the other may be merely a female 

 determiner incapable of carrying sex-limited factors. 

 On this hypothesis the silver hen must always 

 be heterozygous for silver. Further, she must 

 transmit her- normal sex -chromosome containing 

 the silver factor to a son, and the female deter- 

 mining chromosome which cannot carry silver to 

 her daughter. As indicated in Fig. 37, she would 

 produce, when mated with a gold cock, silver sons 

 and gold daughters. And this, as we have already 

 seen, is in accordance with 



the experimental results. An Silver hen Gold cock 

 objection to such a view is, 

 of course, that we cannot 

 bring it fully into line with 

 the Drosophila case. We can- 

 not say here that 2 ^chromo- 

 somes spell a female, and i ^^^^ "^^^^ ^^^ "^^^ 

 X chromosome a male ; a ^^^ 



generalisation which in the 



case of the pomace fly seems well substantiated. 

 At present we must accept the curious position 

 that in one group of insects and one group of 

 vertebrates it is the female sex which is evidently 

 heterozygous, while in another group of insects 

 and another group of vertebrates it is the male 

 sex of which this appears to be true. It is possible 

 that these strange seeming anomalies may be brought 

 ultimately into a general theory of sex when the 

 nature and behaviour of the chromosomes in the 

 other groups have been as fully and as accurately 

 determined as in Drosophila. 



I 



