252 



The Induction oj Gene Mutations 



mutation will appear in the next generation and will be mani- 

 fested as a deficiency in the male population. The treated X 

 chromosome of the male passes into a female and subsequently 

 again into males. These males have no X chromosome other 

 than the one from the treated male; therefore, a lethal mutation 

 in that X chromosome results in the early death of these males. 



First Cross 



Offspring 



C / B 



Marker chromosome 

 (Female) 



X 



Treated ^ 

 (Male) 



C I B 

 Type used ; others lethal or discarded 



Second Cross 



C / B 



X 



Offspring < 



Wild-type female 



C / B 



Bar- eye female 



Male ; present/^ 

 if no induced lethal 



C / B 



Male; lethaL 



Fig. 76. The Clb method of detecting X-ray induced mutations in the 

 X chromosome of Drosophila melanogaster. (Based upon the work of Dr. 



H. J. Muller.) 



• 



In actual practice, the female first mated to the irradiated 

 male is of a special stock. It contains one X chromosome which 

 has a lethal gene, I, the dominant gene, B, for bar eye, and an 

 inversion of a segment of the chromosome w^hich is usually 

 designated by the symbol C. When these flies are mated, a 

 fourth of the offspring w^ill have one treated X chromosome and 

 one CIB X chromosome and will be females. One fourth will 

 be male but will have a CIS X chromosome and will die because 

 of the lethal. The other half of the offspring will not contain 

 the CIB chromosome and will not be used for further study. The 

 CIB females with the treated X chromosome will then be mated 

 with a normal male. In a normal female, crossing over takes 



