84 HEREDITY [gel vi 



be explicable on the assumption that what appears 

 to be a simple character is really dependent on two 

 or more distinct factors, which become separated on 

 crossing with a different form. 



In conclusion, it must be mentioned that a number 

 of cases are now known in which a pair of Mendelian 

 characters are closely associated with Sex. In some 

 cases the sex of the individual determines whether 

 a character is dominant or recessive ; for example, if 

 a horned race of sheep is crossed with a hornless, the 

 male offspring are horned and the females hornless ; 

 and in cats, it appears that yellow crossed with black 

 gives yellow males but tortoiseshell females. In other 

 cases certain Mendelian characters can be borne only 

 by germ-cells which will give rise to one or the other 

 sex. These two apparently different relations between 

 sex and a body character may sometimes be combined ; 

 for example in man, colour-blindness is dominant in 

 males but recessive in females, but at the same time 

 an affected man transmits the 'factor' for colour- 

 blindness only to his daughters, so that while his 

 sons and their descendants are free, his grandsons 

 through his daughters may be affected. Lack of 

 space forbids a full discussion of these phenomena, 

 but they suggest that maleness and femaleness are 

 possibly in reality a pair of Mendelian characters, 

 inherited in the manner described in the last two 

 chapters. 



