HEREDITY 



585 



for many other traits. For example, in cases 

 of color blindness in which the affected 

 person cannot distinguish red from green, a 

 color-blind father mated to a normal mother, 

 as shown in Fig. 413, has no color-blind 

 children, since the XY zygotes develop into 

 normal males possessing one normal X 

 chromosome and one Y chromosome which 

 carries no gene for color blindness. Similarly 



in this case, the X zygotes develop into 

 normal females since only one X chromo- 

 some bears the gene for color blindness, 

 which is recessive to the normal condition. 

 In an F.> generation, however, half of the 

 grandsons and half of the granddaughters 

 are free from this defect; and the other half 

 of the granddaughters carr)' the gene for 

 color blindness as a recessive, whereas the 



Color-blind male x normal female 



Parents 



Gametes ^.. .. 

 of parents tiv^t- 



Carrier females 

 (50%) 



Normal males 

 (50%) 



Normal male x carrier female 



of Fi m 



parents 



Carrier 

 female 

 (25%) 



Normal 



female 



(25%) 



Color- 

 blind 

 mole 

 (25%) 



Normal 



mole 



(25%) 



Normal male x color-blind female 



Parents 



Gametes f. 

 of parents 



Carrier females 

 (50%) 



Color-blind 

 males (50%) 



Color-blind male x carrier female 



F, Parents 



Gametes 

 of Fi 

 parents 



X N 



orma 



X Gene for color blindness 



Color- Carrier Color- Normal 



blind female blind male 



female (25%) male (25%) 



(25%) (25%) 



Y Carries no gene for color blindness 



Figure -413. Inheritance of red-green color blindness in man. Diagram showing the 4 possible 

 combinations of color blindness, excluding homozygous crosses, and the ways in which the defect 

 is inherited from these combinations. The female, designated as a carrier, is one heterozygous for 

 the gene for color blindness, but is normal in color vision. Marriages of brothers and sisters in 

 the human species are so very rare that usually no Fa is shown in texts; however, it is designated 

 here to illustrate what commonly occurs in other species. 



