HEREDITY 



279 



eye color. In the diagrammatic explanation of the results of 

 crossing red-eye and white condition shown in Figs. 165 and 166, 

 only the distribution of the X and Y chromosomes is considered, 

 since the autosomes are not involved in the explanation and are 

 common to both males and females. 



Color blindness in man is inherited in a similar way. That 

 color blindness is sex-linked accounts for the well-known fact 

 that color blindness is more common in men than in women; 

 since, according to the explanation, color blindness occurs in 



Eyes 



Chromosomes 

 XO IK Parents 



•| Xi I Gametes 



X i 9 J 



9 



d 



K ? 



IX i 





Fi 



Gametes 



<Q> <Q> <D> 

 9 9 * 



m m m xo F 2 



9 ? 6 6 



Fig. 167. — Diagram of the inheritance of color blindness through the male. 

 A color-blind male (here black) transmits his defect to his grandsons only. 

 The corresponding distribution of the sex chromosomes is shown on the right, 

 the one carrying the factor for color blindness being black. The Y chromosome is 

 shown as an O. (From Conklin, Heredity and Environment, Princeton University 

 Press, after Morgan. By permission.) 



women only when both X chromosomes carry the gene for color 

 blindness. Heterozygous women are not color-blind, because 

 color blindness is recessive to normal vision. In the male, on 

 the other hand, a single gene in the X chromosome produces 

 color blindness. The Y chromosome is not concerned in color 

 blindness. As shown in the diagram (Fig. 167), the children of 

 a color-blind father and normal mother are not color-blind. In 

 the next generation, one-half of the sons of Fi daughters may be 

 color-blind, if mating is with a normal male. If such heterozy- 

 gous F\ daughters were mated with color-blind males, one-half of 

 the daughters would be color-blind as well as one-half of the 



