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 



f , 



Up ■ ■ B ■ 



■■ 



Fig. 118. — Pedigree of night blindness in a negro family, from Bordley. 



(After Davenport.) 



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

 atrophy (Fig. 117). Night blindness is described in 

 certain cases as sex-linked ; in other cases, however, a 

 disease by the same name appears to be a simple domi- 

 nant and not sex-linked (Fig. 118). 



All these cases of sex-linked inheritance in man 

 are explained by the assumption 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 



