120 HEREDITY [ch. 



entering in greater detail into the discussion of such 

 cases it will be simpler to consider some other instances 

 of 'sex-limited' or sex-modified inheritance which 

 have been worked out in recent years, and which 

 will probably throw light on sexual dimorphism in 

 general. 



It has been seen that in Mendelian inheritance 

 one allelomorphic character is commonly dominant 

 over the other, but that many cases are known in 

 which dominance is partial, and it is probable that in 

 some instances a positive or 'present' character may 

 be recessive in the sense that it cannot appear unless 

 introduced into the individual (zygote) from both 

 parents. Now recent work has shown that some 

 characters are dominant in one sex but recessive or 

 only partially visible in the other (when heterozygous). 

 One of the first cases of this to be discovered was the 

 Tortoiseshell and Yellow Cat, and although there 

 are probably complications in this instance which are 

 not fully worked out, yet it is fairly certain that in 

 cats which are in constitution heterozygotes between 

 yellow and black, in males the yellow is completely 

 dominant, but in females the dominance is partial, 

 giving tortoiseshell. That is to say, in yellow-black 

 heterozygotes, in males the yellow colour completely 

 suppresses the black, but in females the black appears 

 with the yellow. Thus tortoiseshell cats are almost 

 invariably females, the corresponding males being 



