172 Tortoise-Shell Cats [CH. 



those of the two parent species. Unfortunately the hybrids 

 were completely sterile and the experiment went no further. 

 Here we see that the one "dose" of wingedness as we 

 may call it sufficed only to bring the wings to half the full 

 size, and two " doses " are needed to develop them properly. 



Doncaster has collected evidence about the inheritance 

 of tortoise-shell colour in cats which illustrates the same 

 phenomenon. It has long been known that tortoise-shells 

 are almost always females. The suggestion which Don- 

 caster made (109) is that this is the female form of the 

 heterozygote between the colours orange and black. The 

 facts, as he pointed out, show fairly clearly that the corre- 

 sponding heterozygote in the male is orange. Orange 

 colour is thus dominant in males, but in females the domi- 

 nance is imperfect and the patch-work form, tortoise-shell, 

 results. The same is true for cream and blue, which are 

 only the dilute forms of the colours orange and black re- 

 spectively*. It is true that the exceptional tortoise-shell 

 males do occasionally exist, but they are exceedingly rare 

 and nothing as yet is known respecting their breeding. 



In man the ordinary form of colour-blindness, which may 

 roughly be described as the inability to distinguish red 

 from green, follows a system of descent comparable with 

 that of the horns in sheep. Colour-blind persons are 

 commonly male, and in European countries it appears that 

 at least 4 per cent, of the male population are colour-blind 

 (of females less than o - 5 per cent). They may transmit 

 their peculiarity to their children, especially to their sons. 

 Their daughters and sisters however are with rarest excep- 

 tions normal. These women, on the contrary, frequently 

 transmit colour-blindness to their sons again. The unaffected 

 males in these families do not transmit the condition, and 

 their posterity are exempt unless the colour-blindness be 

 introduced afresh. Until the facts were examined in the 

 light of Mendelian discoveries nothing could be more 

 puzzling. The statement however that colour-blindness is 

 a condition dominant in males but recessive in females will 

 express all the facts with which I am so far acquainted 



* The idea is sometimes met with that it is only tortoise-shells without 

 white which are almost exclusively female; but there is no truth in this 

 supposition. 



