1220 



BIOLOGICAL EFFECTS OF RADIATION 



It has long been known, from the work on frequently mutating genes 

 in plants, that mutation occurred in only one member of a pair, since the 

 recessive mutants were only detected in the heterozygotes. Muller (98) 

 pointed out a similar possibility in Drosophila melanog aster from the fact 

 that new mosaic mutants occurred only in the case of sex-linked recessives 

 in the male, except for the cases of dominants. The reasoning was 

 essentially that used in the plant work — the male is haploid for the 

 X-chromosome, hence any recessive mutant shows immediately. In the 

 female, and in both sexes for the autosomes, two members of each allelo- 

 morphic pair are present, and only if mutation occurred simultaneously 

 in both could the mosaic be detected. In the X-ray work on Drosophila 

 it has been possible to test this experimentally by studies of induced 

 somatic mutations obtained by the irradiation of larvae. Patterson 

 (131, 133) has shown that somatic mutations to white are detected only 

 in the male, or in heterozygotes where white is semidominant (Table 6). 



Table 6. — Somatic Mutation to White in Drosophila melanogaster 



Only one of the two allelomorphs present in the female mutates, hence no mosaics are 



found in the females. They are, however, found in the males. (Patterson, 133) 



A. Wild-type larvae irradiated bj^ X-rays at various stages of development (up to 



84 hr.) 



B. Eosin and apricot larvae, treated as above. Here the mutation to white in one 

 of the allelomorphs of the female is detectable in the heterozygote as a "light 



area" 



Group 



Apricot 9 9 

 Apricot c'' <f 

 Eosin 9 9.. 



Eosin d" cf . 



Total 



271 



230 



1020 



925 



Light areas 



2 

 19 



White areas 



2 

 9 



A more direct method of test is afforded by the study of individuals 

 which have received both members of a pair of chromosomes from one 

 parent. This was done by Bridges [Morgan, Bridges, and Sturtevant (93)1 

 in his work on stocks which gave high nondisjunction of the X-chromo- 

 somes. Two cases were found in which a mutation had occurred in one 

 chromosome, but not in its partner. Thus only one of the genes of a 

 pair had mutated, and not the other. 



