702 General and Applied Biology 



When a white-eyed female is crossed with a red-eye male, the Fi 

 males are white eyed and the females are red eyed. In the F2 generation 

 one-half of the individuals of each sex are white eyed and one-half are 

 red eyed (Fig. 345). 



In the reciprocal cross in which a red-eyed female is crossed with a 

 white-eyed male, all the offspring (both male and female) of the Fi 

 generation are red eyed. All the females of the Fo generation are also 

 red eyed. One-half of the males of the F2 generation are red eyed, while 

 the other half are white eyed. (Fig. 346.) 



Color blind 



Normal 



c? 



^y 



Normal 



Color blind 



9 



XX 



XY 



Normal 



Normal Color blind 



<^<®><®><e> <^ 



9 9 c? d" 9 



XX x^ xy 2sy X2? 



Fig. 347. — Inheritance of color blindness in man. On the left a color-blind 

 man marries a normal woman. None of the children are color blind, but the 

 defect is transmitted through the daughters to half of their sons. Color-blind per- 

 sons and the chromosomes carrying the gene for color blindness are shown in white. 

 On the right a normal man marries a color-blind woman, and all the sons are 

 color blind, while the daughters are normal but carry a color-blind gene. In the 

 second generation (on the right) from the type of mating shown in the diagram, 

 half of the daughters and half of the sons will be color blind. Contrast the 

 results of the two pedigrees and observe the effects of a sex-linked trait. (From 

 Turner: Personal and Community Health, The C. V. Mosby Co.) 



Human color blindness, or the inability to distinguish red from green, 

 is usually transmitted from a color-blind mother to all of her sons but 

 to none of her daus^hters. From a color-blind father it is transmitted 

 through his daughters (who are normal as far as color blindness is con- 

 cerned) to one-half of his grandsons (Fig. 347). When both parents 

 are color blind, all the offspring are color blind. Another human sex- 

 linked trait is hemophilia, a condition characterized by inability to form 

 blood clots. 



