NATURE OF THE GENETIC EFFECTS 453 



as seems most likely, a mutational change involves only a single genetic 

 element at a time, then if the gene before its mutation is compounded of 

 several identical parts, one of which mutates, it should follow that suc- 

 cessive reproductions of the parts and their distribution at the consequent 

 mitoses will result in a mosaic of cells, some of which will contain elements 

 of that gene all of which are mutant and others elements all of which are 

 normal. In contrast to this, no such patchwork of mutant and normal 

 tissues could be found in the case of visibly expressed mutations, in 

 individuals derived from irradiated spermatozoa. Nor did there seem 

 to be any such long-delayed production of stable mutations (representing 

 genes which had come to receive elements all of which were mutant) as 

 this view would call for. It was accordingly concluded that the entire 

 gene constituted a unitary (though complex) rather than a compound 

 organization, and that the genetic material of the chromosome thread, in 

 the Drosophila spermatozoon at least, was probably single. 



Perhaps the most important finding in the work on the production of 

 gene mutations by radiation, as far as its bearing on theoretical genetics, 

 evolution study, and general biology is concerned, is that, as pointed out 

 on pp. 39-1-395, the mutations bear so much resemblance to those which 

 occur spontaneously, and have such a similar even though not always 

 identical distribution and relative incidence of the different phenotypic 

 effects. Since the production by radiation of one rather than another 

 mutation on any given occasion must have been determined by factors 

 involving the physical distribution of ionizations or excitations, and 

 since these events must have been accidental, in the sense of being 

 unregulated by the organism itself, it became reasonable to conclude that 

 the spontaneous mutations, inasmuch as they so resemble those produced 

 by radiation both in types and manner of incidence, must be similarly 

 accidental in their origination, rather than representing any sort of 

 adaptive biological response to given conditions. In this way the radia- 

 tion results provided significant support to the theory of fortuitous genetic 

 variation, which has as its corollary the conclusion that natural selection 

 constitutes the guiding factor in the genesis of adaptations, and hence in 

 biological evolution in general, as Darwin proposed (Muller, 1929, 1947). 



In many cases radiation has been used for the purpose of furnishing 

 mutant genes to serve as "markers" in the making of genetic maps, for 

 such maps, together with the stocks containing the genes shown in them, 

 then prove useful in further genetic studies of varied kinds. In several 

 such instances the investigator has thereby been put into a position in 

 which it was possible for him, in the given organism, to establish new 

 principles of genetics, which the other organisms used in genetic work 

 were not adapted to reveal. One case of this kind is the elucidation, by 

 P. W. Whiting (1940, 1943), of a hitherto unknown mechanism of sex 

 determination, that in Habrohracon and probably in various other 



