222 



HEREDITY AND SEX 



It is evident that one class of males has failed to ap- 

 pear the red males. If we trace their history through 

 these two generations, we find that the single sex chro- 



FIG. 106. Diagram to show the heredity of the lethal factor (carried 

 by black X). A, red-eyed female, carrying the factor in one -X", is bred to 

 normal white-eyed male. B, her red-eyed daughter, is bred again to a normal 

 white-eyed male, giving theoretically the four classes shown in C, but one of 

 the classes fails to appear, viz. the red-eyed male (colored black in the dia- 

 gram). The analysis (to right) shows that this male has the fatal X. One 

 of his sisters has it also, but is saved by the other X. She is the red-eyed 

 female. If she is bred to a white-eyed male, she gives the results shown in 

 D, in which one class of males is again absent, viz. the red-eyed male. In 

 this diagram the black X represents red eyes and lethal (as though completely 

 linked). 



