X SEX I I I 



certain defects, colour-blindness for example, are 

 much more common in men than in women. Many 

 pedigrees have now been collected of families in 

 which colour-blinds occur, and the data, with few 

 exceptions, are explicable on the assumptions : (cz) 

 that the colour-blind state is recessive to the normal, 

 (d) that it is a character showing sex-limited inherit- 

 ance, and (c) that the female is the homozygous, 

 the male the heterozygous sex, as in Drosophila. 

 If in Figs. 27 and 28 we substitute normal for red 



eye and colour-blind , ^ — 



for white eye, the ?c? ^ 9 ? 



schemes given for ^ i % > > > A 



Drosophila will serve 



to explain the inherit- ^ 9 Hi ^i 

 ance of colour-blind- Fig. 36. 



neSS in man. The Part ofacolour-blind pedigree from Neuleship. 



It illustrates the fact that the sonsof colour- 



raritV of colour-blind blind women are all colour-blind, while the 



^ daughters, though carriers, are normal. 



women is due to the Nothing is known of the father of the two 



colour-blind women. On theory' he should 



fact that they can only have been affected That the mother was 



^ "'a earner is strengthened by the fact that a 



appear from the union sister, mated with a normal, produced a 



^ ^ colour-blind son among her children. 



between a colour-blind 



man and a heterozygous woman, just as the white- 

 eyed female Drosophila is only produced from the 

 mating of a heterozygous female and a white-eyed male. 

 Since neither colour-blind men nor heterozygous 

 women (or " carriers" as they are sometimes termed) 

 are common, the chances of a marriage between 

 them are very rare. On the other hand, any female 

 "carrier" mated with a normal male will produce 

 normals and colour-blinds in the ratio 3:1, and the 

 colour-blinds in such cases will all be sons. Since 

 the female "carrier" is very much more likely 



