SEX 



1 1 1 



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 : (a) 

 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 PC? '5? 9 



schemes given for ^ ^ ^A ^ k k O 



Drosophila will serve ^ f " # ¥ 



to explain the inherit- ^ 9 did 9i 



ance of colour-blind- fig. 36. 



neSS in man. The Part of a colour-blind pedigree from Nettleship. 



It illustrates the fact that the sons of colour- 



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



^ daughters, though carriers, are normal. 



^Omen 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 carrier is strengthened by the fact that a 



appear from the union lister, 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 



