SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 20 



such maggots always die. They are always males. Conse- 

 quently there are twice as many daughters as sons in such a 

 strain. The gene is carried by the X-chromosome and its in- 



Fig. 14. Diagram showing inheritance of a sex-linked recessive lethal 

 ("tumor") factor in Drosophila melanogaster. Here, in the center of the 

 diagram, the sex-chromosome that carries the lethal factor is represented by 

 the black rod. A female with the tumor-factor, normal wings and red eyes, 

 in one of her sex-chromosomes and with the factors for yellow wings and eosin 

 eyes in the other is bred in each generation to a male with yellow wings and 

 eosin eyes. In the next generation there are twice as many daughters as sons, 

 since all the sons that carry the black chromosome die. The half of the 

 daughters (i.e., those not yellow eosin) that carry the black chromosome re- 

 peat the same history. The linkage of yellow and eosin enables one to pick 

 out in each generation those daughters that carry the tumor-factor. 



