256 



The Induction of Gene Mutations 



When the treated flies are crossed with the marker stock, six- 

 teen possible types of offspring are found (combining four treated 

 with four marker chromosomes). Twelve are discarded that 

 have S or H or both and therefore have no crossover suppressor. 



First Cross 



Offspring 



X 



C Cy C SbjorP) 

 S H 



II III 



Treated chromosomes 



C Ql 



II in 



Marker chromosomes 

 'Sb 



Type used; others discarded 



Second Cross 



C Cy 



C Sb 



X 



Cy 



C Sb 

 H 



Offspring 



C Cy 



C Sb 



Type used; others lethal or discarded 



Third Cross 



Offspring 



Sb 



3 



C Sb 



Present if no lethal in III 



X 



C Cv 



C Sb 



Present if no lethal in II 



Present if no lethal in II and III 



Fig. 77. A method for the detection of X-ray induced lethal mutations 

 in the second and third chromosomes of Drosophila melanogaster. (Based 

 upon the work of Child, Plough, and others.) 



To discard undesirable types is very simple because dominant 

 marker genes are used. The four remaining types are all Cy Sb, 

 but differ in the treated chromosomes. If it is desired to test 

 the number of lethal mutations in a certain member of chromo- 

 some II and one of chromosome III, one of the Cy Sb flies hetero- 

 zygous for treated wild-type chromosomes is then mated with 

 the original marker stock to produce a number of identical flies, 

 all with the same two treated chromosomes. This is the second 



