Muller: Production of Mutations 



X chromosome, including the locus of 

 the so-called "scute" effect, it was found 

 that there are in fact, within the normal 

 X chromosome, two genes of closely re- 

 lated effect ("achaete" and "scute") 

 very close or adjacent to one another, 

 it became evident that this was in all 

 probability an example of the above 

 postulated occurrence. This then showed 

 the way, and apparently the main if not 

 the only way (aside from the far rarer 

 phenomena of polyploidy and "tetra- 

 somy"), by which the number of genes 

 has become increased during the course 

 of evolution. By a curious coincidence. 

 Bridges was at the same time making 

 his studies of salivary chromosomes and 

 finding direct cytological evidence for 

 the existence of such "repeats," as he 

 called them, in the normal chromosome, 

 and he interpreted these in the same 

 manner. In the twelve years since that 

 time, various other clear cases of the 

 same kind have been demonstrated. 

 Thus, increase in gene number, brought 

 about by the duplication of small parts 

 of chromosomes, more usually in posi- 

 tions near their original ones, must 

 be set down as one of the major pro- 

 cesses in evolution, in addition to the 

 mutations in the individual genes. By 

 itself, this process would not be of great 

 importance, but it becomes important 

 because, by allowing gene mutations to 

 come afterwards that differentiate the 

 genes in one position from the originally 

 identical ones in the other position, the 

 number of different kinds of genes is in- 

 creased and so the germ plasm, and with 

 it the processes of development and the 

 organism as a whole, are eventually en- 

 abled to grow more complex. 



Rearrangements of chromosome parts 

 which do not lead to an increase in gene 

 number can of course also occur in evo- 

 lution, although it is unlikely that their 

 role is so fundamental. By producing 

 such changes in the laboratory it has 

 been possible to find out a good deal 

 more about what types can arise, and 

 what their properties are. Various in- 

 ferences can then be drawn concerning 

 the viability and fertility that the differ- 

 ent types would have, under varied ge- 



netic circumstances, and whether they 

 would tend to become eliminated or to 

 accumulate in a population of a given 

 type. Some of them can be shown to 

 have, under given conditions, an evolu- 

 tionary survival value, both by aiding 

 in the process of genetic isolation and in 

 other ways, as by affecting heterosis. In 

 this manner, evolutionary inferences 

 have been drawn which have later been 

 confirmed by comparison of the chromo- 

 some differences actually existing be- 

 tween related races, sub-species, and 

 species. 



Probably of greater ultimate interest 

 will be the results of studies of gene mu- 

 tations occurring at individual loci. Radi- 

 ation mutations are frequent enough to 

 lend themselves to comparisons of the 

 potentialities of different loci, although 

 not nearly enough has yet been done 

 along these lines. Similarly, a compari- 

 son of the different mutations which can 

 occur at the same locus can lead to very 

 important results, especially since it has 

 been shown that the different alleles may 

 have every complex relationships to one 

 another, so as even, in some cases, to 

 reconstitute the normal type when they 

 are crossed together. The way in which 

 genes may change as a result of succes- 

 sive mutations remains to be gone into 

 at much greater length. So, too, does 

 the question of changes in gene muta- 

 bility, brought about by gene mutation 

 itself. 



Somatic Radiation Effects 



The further the analysis of the genetic 

 effects of irradiation has gone, particu- 

 larly of the breakage "and rearrangement 

 of chromosome parts, the more does our 

 conviction grow that a large proportion 

 if not the great majority of the somatic 

 effects of irradiation that have been ob- 

 served by medical men and by students 

 of embryology, regeneration, and gen- 

 eral biology, arise secondarily as conse- 

 quences of genetic effects produced in 

 the somatic cells The usefulness of this 

 interpretation has been shown in recent 

 studies of Roller, dealing with improved 

 methods of irradiation of mammalian 

 carcinoma. This is too large a subject 



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