248 BREEDING 



horns, whilst the female remained hornless. The result 

 of reciprocal crosses was identical. It looks as if the 

 horned character were dominant in males, but recessive 

 in females. But this view is shown to express the 

 truth imperfectly by the appearance in the second 

 hybrid generation of horned ewes and hornless rams. 

 The explanation of these phenomena, so far as 

 it is an explanation, is found to be that a ram is 

 horned both when he is heterozygous (DR) and 

 homozygous (DD) for the horned character, whilst 

 a ewe is horned only when she is homozygous for it. 

 That is why all the ewes in the first hybrid generation 

 were hornless. In other words, a ram will develop 

 horns with only one dose of the horned character, 

 which is present in the heterozygous condition of 

 that character ; but a ewe needs the two doses which 

 are only present in the homozygous condition of 

 that character. There appears to be something in 

 the constitution of the female which inhibits the 

 development, in this case, of the horned character 

 when this is only present in the diluted heterozygous 

 condition. This phenomenon is known as sex-limited 

 inheritance, because the course of descent of a 

 character is affected by the sex of the individuals 

 which bear it. 



Another instance of it is afforded by the pheno- 

 mena of colour-blindness in man. This peculiarity 

 occurs both in men and women, but is very rare 

 amongst the latter. Colour-blind men may transmit 

 the peculiarity to their sons, but very rarely do so 

 to their daughters. These unaffected daughters may. 



