84 HEREDITY [ch. 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. This aspect of the subject, which includes some 

 of the most interesting recent advances in our know- 

 ledge of Heredity, will be considered in a subsequent 

 chapter (ix), after some other questions in which the 

 sex-factor is not directly concerned have been dis- 

 cussed. 



