XII] 



Colour-Blindness 



223 



(3) They are nevertheless transmitted by the unaffected 

 females. Apparently normal women, daughters or sisters 

 of the affected males, thus may transmit the condition to 

 some of their sons (Fig. 32). 



Such a system of heredity has long been a physiological 

 paradox, and one of the most curious and interesting de- 

 ductions from Mendelian research is the clue which it has 

 provided to the solution of this problem of sex-limited 

 descents (cp. p. 173). 



The experiment dealing with the inheritance of horns 

 in sheep showed very clearly the probable lines on which 

 an explanation was to be found. Just as in the sheep the 

 horned character is dominant in males and recessive in 

 females, so with these sex-limited conditions. If the males 



. \ 



k 





It 



Hm 





Fig. 32. Pedigree illustrating descent of colour-blindness. This family 

 was found by Dr W. H. R. Rivers among the Todas, a hill-tribe of 

 Southern India. 



contain the factor for the condition, they exhibit it ; conse- 

 quently the affected males can transmit, while the unaffected 

 males cannot. In the females, on the contrary, something — 

 almost certainly the presence of some other factor — prevents 

 or inhibits the development of the condition, and then they 

 may possess the factor without its making itself apparent. 

 Such females may then transmit it to their offspring, but 

 it will only be visible in the males, except in the rare case 

 of a union between a heterozygous female and an affected 

 male. Then the female children also may be affected, 

 because they may be homozygous to the factor, receiving 

 two ** doses " of it, one from their father and one from 

 their mother. 



