142 Applied Biophysics 



Most researches on the genetic efifects of radiations have heen 

 confined to a few organisms that are technically favorable from 

 the point of view of ease in handling the large numbers of 

 individuals needed in controlled experiments. The principal ones 

 are the fly Drosophila mclanogastcr, maize, and some fungi such 

 as Ncurospora, together with the flowering plant, Tradcscantia, 

 for chromosome studies. 



Racliation-indueed Mutation in Drosophila 



When adult male flies are exposed to radiations and subse- 

 quently mated to untreated virgin females, a proportion of the 

 eggs laid fail to hatch although they have been fertilized. The 

 premature death of the individual is ascribed to the induction 

 of a dominant lethal mutation in the sperm. The existence of 

 such mutations was first proved by MuUer,-" who showed that 

 their "number was so great that thorough egg counts and efifects 

 on the sex-ratio evidence could be obtained from them r// masse." 

 At moderate doses, ^- ^ the graph relating the logarithm of the per- 

 centage of eggs reaching the larval or adult stages to the dose 

 is a linear one. Above 4,000 r the gradient becomes steeper, 

 suggesting that a mixture of "single-hit" and "multiple-hit" 

 efifects contributes to the total yield of dominant lethals. The 

 predomiiuant contribution, particularly in the lower dose-range, 

 is single-hit, and dominant lethals involving more than one hit. 

 and so increasing more rapidly than the first power of the dose, 

 become important only at higher doses ( figure 1 , A ) . 



The occurrence of dominant lethals is expressed also in the 

 sex ratio, i.e., the proportion of females relative to males hatch- 

 ing from a batch of eggs. As the X-ray dose increases, the sex 

 ratio declines (figure 1, B), owing to the extra probability of a 

 dominant lethal being induced in an X-chromosome-bearing 

 sperm as compared with a Y-chromosome-bearing sperm ex- 

 posed to the same dose. The female-producing X-chromosome 

 is a little larger than the male-producing Y-chromosome, and 

 so presents a larger target in which the dominant lethals may 

 be induced. 



