404 RADIATION BIOLOGY 



parison with the usual normal, so that in the heterozygote, or in single 

 dose, the phenotypic level is distinctly below the saturation level, yet in 

 the homozygote, i.e., in double dose, it appears to attain that level. In 

 the case of certain genes (e.g., the normal allele of garnet eye in Drosoph- 

 ila) these cryptic mutants arise relatively frequently. 



On the interpretation of dominance above given, it is to be expected 

 that when two hypomorphic alleles of qualitatively the same type, lying 

 at different points in the rapidly ascending portion of the phenotype- 

 dosage curve, are crossed to each other or to an amorphic allele of the 

 same gene, no marked dominance of one over the other would usually be 

 shown. That is, the heterozygote or corn-pound (as an individual hetero- 

 zygous for two different mutant alleles of the same gene is called) would 

 usually have a phenotype approximately intermediate between those of 

 the two homozygous mutants. This has long been known to be the case, 

 and is caused by the fact that both mutant genes in such a case are far 

 from saturation potency, acting in the nearly straight-line portion of the 

 curve relating gene dose or activity to amount of effect. Thus the addi- 

 tion of the two actions results in an intermediate amount of effect. 



In some compounds, however, it has been found that the amount of 

 effect cannot be explained as due simply to an addition of the actions of 

 the two genes, working independently. As Stern and his co-workers 

 (Stern, 1943; Stern and Heidenthal, 1944) have shown, the results in 

 some compounds show an active interference or competition between the 

 two alleles, indicating that there is in these cases a limited amount of sub- 

 strate available, and that the higher ability of a given allele (or its 

 products) to preempt this substrate, thus removing the latter from the 

 possibility of being acted on by the other allele, is not always associated 

 with a greater ability of the former allele to convert the substrate in a 

 manner suitable for the production of the phenotypic effect that is under 

 observation. Hence a more hypomorphic mutant allele sometimes has 

 greater dominance than a less hypomorphic one. This has been found 

 more especially in cases of changes in gene functioning caused by a posi- 

 tion effect {positional alleles). Such results therefore occur oftener when 

 mutants produced by ionizing radiation are used, since more of these than 

 of spontaneous mutants result from structural changes rather than gene 

 mutations. It is probable, however, that some gene mutations also have 

 this kind of behavior. Effects of this kind could usually be detected 

 readily only in compounds between two mutant alleles, since in hetero- 

 zygotes having one normal allele the potency of the latter would usually 

 be so high as to mask the effect of competition by the mutant allele. 

 Some of these cases of interfering action appear to have a complicated 

 basis, so that a series of compounds formed by a group of multiple 

 alleles taken two at a time seems to show no consistent seriation of 

 effects. 



