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. 



More complex systems of sex-limited descent are fol- 

 lowed by certain abnormal conditions met with in Man. 

 Of these the most familiar is colour-blindness. 



The ordinary form of that affection may roughly be 

 described as the inability to distinguish red from green. 

 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 0*5 per 

 cent). The children of colour-blind fathers are usually not 

 colour-blind, whether they be sons or daughters. The 

 datightei's however 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 dis- 

 coveries nothing could be more puzzling. The statement 

 however that colour-blindness is a condition dominant in 

 males but recessive in females will express a great part of 



* 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. 



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