400 RADIATION BIOLOGY 



gene mutation proper. However, it seems reasonable to infer that prac- 

 tically any types of alterations in gene functioning that can be brought 

 about by a position effect could also, on occasion, be accomplished by 

 some kind of change within the gene itself. In fact, if neomorphs had 

 not been able to arise by gene mutation, genes could hardly have become 

 differentiated from one another by mutation, in the long course of evolu- 

 tion, in such wise as to give rise to ever more complicated organisms, 

 incorporating new types of biochemical reactions. 



It must not be supposed that the above classes are absolute. Some 

 mutant genes have a complicated series of effects, some of which belong 

 more nearly in one category and others in another. Moreover, although 

 a given gene may undergo different mutations, the effects of which 

 appear to differ only in degree, in not a few cases (both of spontaneous 

 and radiation origin) the effects of the different mutations are qualitatively 

 unlike. Usually a gene, by its mutations, can cause a change in more 

 than one character, and in some of these cases not all the effects run 

 parallel, as one allele may show character a more affected than b while 

 another allele shows b more affected than a. These and related facts 

 give evidence of the complexity of the individual gene and of the multi- 

 plicity of the types of change that it can undergo. 



Although differences in the dosage of hypomorphic mutant genes for 

 visible characters usually occasion marked differences in the phenotype 

 (the higher doses being more nearly normal in expression) , changes in the 

 dosage of the corresponding normal genes usually have extremely little or, 

 most often, no effect at all that is detectable by ordinary inspection. 

 This is connected with the fact that the phenotypes of hypomorphic 

 mutants also show a tendency to be readily influenced by differences in 

 the environmental conditions existing during development and by differ- 

 ences (caused by mutation) in numerous other genes, which do not per- 

 ceptibly affect the given character when the normal allele of the hypo- 

 morph is present. In other words, the effect of the more weakly acting 

 gene, the hypomorph, is more variable than that of the normal gene. 



These results are understandable, as pointed out independently by 

 Plunkett (1932) and by the present writer (1932b, 1935b, 1950a), when 

 it is realized that these dosage studies show that increase in gene dosage, 

 i.e., in gene concentration, or in its equivalent, gene activity, is usually 

 accompanied in its early stages by an approximately proportionate 

 increase in the phenotypic effect, but that as larger and larger doses or 

 greater gene activities come into play the effect increases ever more 

 slowly. That is, the curve relating the effect (as ordinate) to the gene 

 dosage or activity (as abscissa) (see Fig. 7-6) rises at first in a straight 

 line from the base line at the origin, but as it proceeds to the right its 

 slope gradvially decreases, tending toward the horizontal as a kind of 

 saturation level of effect is approached. This falling off in efficiency is a 



