Genetic Effects of Radiations 145 



and the grouping together of such a heterogeneous group is 

 justified mainly by the convenience of their frequency. 



When viable recessive mutations are to be studied, the 

 attached-X method may be adopted. In this case, the treated 

 male is mated to an attached-X female, whose two X-chromo- 

 somes are joined together and so are segregated together at 

 gamete formation. Her eggs wnll be of two kinds, one with 

 two X-chromosomes, and therefore female-producing, and the 

 other without any X-chromosomes. The latter, with an X-bear- 

 ing sperm from the irradiated father, will produce a male in 

 which any visible mutation in the treated X-chromosome could 

 be detected. 



, These techniques, and others like them, are simple but 

 enormously laborious, since the mutation-rates involved are small 

 even for fairly large doses of X-rays. Nevertheless, many facts 

 about the mutation process are well established. In the first 

 place, the mutations induced by radiations do not dififer qualita- 

 tively from those occurring spontaneously. In both cases, too, 

 the mutation rate differs from one locus to another, and from 

 one allelomorph to another at the same locus. ^^' ^"^ It can be 

 concluded that the genes differ among themselves in stability, 

 the less stable ones undergoing the more frequent mutation. An 

 important point to note is that the radiation cannot determine 

 what particular mutation is produced. Which gene is activated 

 and what allelomorph is finally formed is a matter of chance. 

 The former depends upon the chance of the target, the gene, 

 being hit, and the latter upon the innate characteristics of the 

 individual locus ; in particular, apparently, upon the relative 

 stabilities of the difi:'erent allelomorphs.^^ 



Further, a given gene A may be changed to the allelomorph 

 a, and the latter on being irradiated changed back to A. Such 

 back-mutations, demonstrated first by TimofeeiT-Ressovsky,^^' ^^^ 

 are important in showing that whatever change is involved in the 

 conversion of ^ to a cannot be a loss that may not be restored 

 with relative ease. 



The quantitative relationship between the mutation rate and 

 the radiation dosage, intensity, wave length, etc., has be^n 



