HEREDITY IN MAN 



365 



there is so little chance for a complete expression of Mcndelian 

 ratios that this may easily occur, while a high rate of reproduction 

 would be almost certain to bring out all possibilities within one or 



Color-blind male 

 XY 

 Gametes X Y 



Normal female 

 XX 

 X 



generation XX XX 



Carrier females 



Normal male 

 XY 

 Gametes X 



generation 



XY XY 



Normal males 



Color-blind female 

 XX 

 X X 



XX XX 



Carrier females 



B 



Color-blind male 

 XY 

 Gametes X Y 



XY XY 



Color-blind males 



Carrier female 

 XX 

 X 



generation XX XX XY XY 



50% Color- 50% carrier 50% Color- 50% normal 



blind females females blind males males 



C 



"Normal male Carrier female 



XY XX 



Gametes X Y X X 



^1 

 generation XX XX XY XY 



50% carrier 50% normal 50% Color- 50% normal 

 females females blind males males 

 D 

 Fig. 201. Diagram showing the four possible combinations of color blind- 

 ness and the way in which the condition is inherited from these crosses. 

 X represents the x chromosome l)earing the determiner for color blindness 

 and X that bearing the determiner for normal vision which is dominant 

 over X. 



a few generations. The small size of human families also makes 

 possible the elimination of the defect from a line of descent in 

 some cases. 



