36 Mr Doncaster, On the Inheritance of Tortoiseshell 



been paired with a tortoiseshell </, orange and black kittens have 

 been produced in addition to tortoiseshells. The question then at 

 once arises, why are tortoiseshells almost exclusively females, the 

 number of certainly known males of this colour being very small ? 

 The answer appears to be that in the male orange is completely 

 dominant over black, while in the female the dominance is in- 

 complete, and tortoiseshell results. On this hypothesis all the facts 

 given above are explicable : for example, in mating (e) orange $ 

 x black </ gives only tortoiseshell and orange kittens, since both 

 the parents are homozygous, and of the offspring (heterozygous) 

 the females are tortoiseshell, the males orange. In the reverse 

 mating (f) black $ x orange <f , the male may be heterozygous, 

 and hence blacks appear in the offspring. If black be represented 

 by B, orange by 0, we get 



(e) BxO; (/) B x B O 



BO? B <r BBc?,? BO? BOc? 



Tortoiseshell Orange Black Tortoiseshell Orange 



In the same way in mating (b) tortoiseshell $ x black </, orange 

 appears in the offspring, since the male heterozygotes are orange, 

 the female tortoiseshell. 



This accounts also for the fact that orange females are very 

 rare, although males are common, for in all matings in which one 

 of the parents is black, orange can appear only in the male 

 offspring. If therefore the great majority of orange males contain 

 recessive black, when they are paired with tortoiseshells, only 

 a quarter of the kittens will be pure orange, and only half of these 

 females. 



In addition to the evidence concerning the relations between 

 orange and black, I have obtained a quantity of information 

 respecting cream and blue. These two colours appear to be 

 dilute forms of orange and black, and have exactly similar re- 

 lations to one another. Two blues, from yellow ancestors, when 

 paired together give only blue offspring, and are therefore homo- 

 zygous ; a cream $ x blue </ gives blue tortoiseshell (blue and 

 cream) females, cream males, but no blues ; a blue $ x cream 

 cf on the other hand gives blue tortoiseshell females, blues of 

 both sexes (and possibly cream males, but of this I have no 

 certain evidence). Blue tortoiseshell $ x blue £ gives blue 

 tortoiseshell females, blues of both sexes, and creams (mostly, 

 possibly always males); blue tortoiseshell % x cream g gives blue 

 tortoiseshell females, blues and creams of both sexes. These 

 results are exactly like those obtained with orange and black, and 

 can be accounted for completely on the hypothesis that cream is 



