454 RADIATION BIOLOGY 



Hymenoptera. In Habrobracon the addition of radiation-induced gene 

 mutations to the spontaneous ones allowed the genetic "marking" of all 

 the chromosomes. This provided a means of proving that only one of 

 the chromosomes of any set, and its homologue in other sets, was sex- 

 determining. More intensive study of this chromosome then established 

 the fact that it existed not in two but in multiple forms, any two of which 

 together resulted, by a complementary action, in the female sex, whereas 

 one kind by itself resulted in the male. Another case is that of the work- 

 ing out of the system of Mendelian, chromosomal heredity present in 

 E. coli, by Lederberg (1947) and Lederberg et al. (1952), through studies 

 of the linkage relations of mutant genes most of which were produced 

 by radiation. This in turn has served as the necessary basis for the 

 establishment in these bacteria of a number of important genetic princi- 

 ples, of a hitherto unique type. In other cases such work has facilitated 

 genetic comparisons between species, for the purpose of determining the 

 types of changes undergone by them in their evolutionary divergence 

 from common ancestors. 



The utility of radiation for making surveys of the distribution of gene 

 mutations in the germ plasm as a whole, and also for the intensive study 

 of the mutational potentialities of individual genes, is obvious. Work of 

 both these types has thus far been carried furthest in Drosophila and 

 maize, but results along both lines in Neurospora and mice are also becom- 

 ing impressive. It would require too much of a digression even to sum- 

 marize them here, though with regard to the first line of attack reference 

 may again be made to the studies on the relative freciuencies of different 

 phenotypic classes of mutations (e.g., lethals, detrimentals, visibles). 

 As for the intensive studies of individual loci, the most detailed work, 

 such as that on the scute, white, dumpy, lozenge, bithorax, and Stubble 

 loci or groups of loci in Drosophila and on the A locus in maize, have 

 demonstrated the high complexity of some of these genes, as evidenced by 

 the number of different alleles they could form and the diverse directions 

 the mutations of one gene could take. The same work also showed that 

 different parts or operational features may undergo alteration separately 

 from one another in some mutations, and together in other mutations of 

 the same locus or group of loci. This was true both of radiation-induced 

 and spontaneous mutations. The qualification, mentioned on p. 411, 

 should here be repeated, however, that ionizing radiation in maize 

 (unlike ultraviolet in maize and Drosophila, and unlike ionizing radiation 

 itself in Drosophila) seemed to give only complete deficiencies of one or 

 more loci, when a given locus (A) was chosen for observation (Stadler, 

 1941; Stadler and Roman, 1948). 



The term "group of loci" was used advisedly in the foregoing para- 

 graph since some of the induced mutations at first thought to be allelic, 

 in the sense of consisting of changes of the same gene, proved to be muta- 



