244 



HEREDITY AND SEX 



two such individuals are normal, but the color blindness 

 reappears in one-fourth of the grandchildren, and in 

 these only in the males. The reverse mating is shown 

 in the next diagram in which the female is color-blind. 

 She will have color-blind sons and normal daughters 

 (criss-cross inheritance), and all four kinds of grand- 

 children. 



Other cases in man that are said to show sex-linked 

 inheritance are atrophy of the optic nerve, multiple 





■|0 



lO 



ik 



o 





ciifflfflcniii ii 1 





Fig. lis. — Pedigree of night blindness in a negro family from Bordley. 



(After Davenport.) 



sclerosis, myopia, ichthyosis (Fig. 116), muscular 

 atrophy (Fig. 117), and night-blindness (Fig. 118). 

 There are also other cases in man that appear to come 

 under the same category, but for which the evidence 

 is not so clear. 



All these cases of sex-linked -inheritance in man 

 are explained by the assmnption that the factor that 

 produces these characters is carried by the sex chromo- 

 some, which is duplex (XX) in the female and simplex 

 (X) in the male. A simpler assumption has not yet 

 been found. If one is fastidious and objects to the 



