INHERITANCE OF SEX 249 



However, transmit it to their sons. In a word, men 

 cannot transmit the peculiarity without having it, 

 whilst women can. The mode of transmission and 

 the interference of sex would appear to be exactly 

 the same as in the case of the horned character in 

 sheep. A woman is only colour-blind when she is 

 homozygous for the character ; a man is colour-blind 

 both when he is homozygous and when heterozygous 

 for it. The normal women who transmit it are 

 heterozygous for it. If this theory is true all the 

 sons of colour-blind women should be affected, because 

 even if she is mated to a normal male all her offspring 

 will be heterozygous for the peculiarity, and that 

 is sufficient to make her male offspring develop it. 

 Kecords in the possession of Mr. Bateson show that 

 the seventeen sons (who lived to be tested) of seven 

 colour-blind women were all colour-blind. The bear- 

 ing of these facts of sex-limited inheritance on the 

 Mendelian theory of sex will shortly appear. 



Another familiar instance of the results of sex- 

 limited inheritance is afforded by the difference 

 between the male and female offspring resulting from 

 a cross between a black and an orange cat. The 

 general rule would appear to be that the male offspring 

 are orange, whilst the female are tortoiseshell. This 

 is why male tortoiseshells are practically unknown, 

 and why the females occur sporadically, inasmuch 

 as it is not possible to raise a race of tortoise- 

 shells because there are no toms to mate the cats 

 with. 



The idea that sex was a Mendelian character 



