i88 Heredity and Environment 



linked with sex, such as the color of the eyes and of the body, 

 the length of the wings, etc. A typical case is shown in Figs. 61 

 and 62. The eye color of this fly is normally red, but mutations 

 have arisen in which the eye is white. Such a mutation first ap- 

 pears in males, though it may later be transferred to females, as 

 we shall see. If now a white-eyed male and a red-eyed female are 

 crossed all the F^s are red-eyed, but if these F/s are interbred all 

 the females of F 2 have red eyes while half of the males have red 

 eyes and the other half have white eyes (Fig. 61). On the other 

 hand if one of the F females of this cross is bred with a white- 

 eyed male (Fig. 62, F x ), half of the females of F 2 are red-eyed 

 and half are white-eyed, and half of the males are red- eyed and 

 half are white-eyed. 



If now one of these white-eyed females is bred with a red-eyed 



FIG. 63. DIAGRAM OF INHERITANCE OF COLOR BLINDNESS THROUGH THE 

 MALE. A color blind male (here black) transmits his defect to his grand- 

 sons only. The corresponding distribution of the sex chromosomes is 

 shown on the right, the one carrying the factor for color blindness being 

 black. Recent work shows that the six chrosomes of the human male 

 are XY and not XO, as shown in this diagram. 



