398 RADIATION BIOLOGY 



in question. This is to be concluded from the fact that a change in 

 dosage of the mutant gene in such a case does not influence the degree of 

 expression of the given character. In many of these cases hypomorphic 

 alleles of the gene have been found as well, which show a similar but less 

 marked difference from the normal and respond to dosage increases by 

 causing an approach to the normal phenotype. In other cases, where 

 hypomorphic alleles have not been found, the fact that the amorph 

 stands at the zero level on the scale of activity of the type responsible for 

 the differences studied can be deduced from the fact that reduction in 

 dose of the normal gene is manifested by a character change similar in 

 kind, but usually much smaller in degree, than that found in the presence 

 of the amorph. 



There appears to be a much rarer type, termed an antimorph, which 

 has an action opposite in direction to that of the normal gene, in that an 

 increase in dosage of the mutant gene, when the normal gene is not pres- 

 ent, causes a greater departure from the normal phenotype. Some cases 

 previously considered as antimorphs probably belong in other categories, 

 however, since at first it was not realized that a mutant allele could, 

 without being an antimorph, actively compete with the normal in the 

 determination of the phenotype in individuals having both genes (see 

 p. 404). 



There is no doubt that changes in the hypermorphic direction can occur 

 as well, although, for reasons to be given in Sect. 14, such mutations of 

 normal genes would usually be very difficult to detect. Certainly differ- 

 ences, ordinarily subliminal in their effect on the phenotype, have been 

 found between the normal alleles of different populations, of such a 

 nature as to show that one of these alleles was hypermorphic in relation 

 to the other. From the standpoint of the more effective gene on the 

 other hand the less effective one (although "normal" for its population) 

 would be hypomorphic. Although in these cases one could not know 

 which one represented more nearly the ancestral condition the answer to 

 this question in any given case is relatively unimportant in view of the 

 evidence, reviewed in the following paragraph, that mutations (both 

 spontaneous and radiation-induced) can take place in each of two opposite 

 directions. 



Among the types of change that can be brought about by gene muta- 

 tion are transformations of a mutant gene, usually of a hypermorphic 

 nature, which cause it to give a phenotype more nearly like, or even in 

 some cases sensibly identical with, that produced by the normal gene 

 from which the mutant gene had been derived. It is in line with the con- 

 ception, previously presented, of the greater likelihood of a mutation 

 causing a degradation rather than an improvement or increase in gene 

 functioning, that these reverse or normad mutations arise, in the case of 

 most genes, with much lower frequency than the so-called direct or 



