378 Action of the Genetic Material 



thus is also conferred upon heterozygotes of the frequently produced 

 mutants. Clearly, such an explanation of the origin of dominance pre- 

 supposes a type of genie action of the general kind just reviewed, and 

 developed in detail in my book of 1927. If such a system underlies 

 genie action, phylogeny of dominance may be conceived in Fisher's 

 sense but, also, in other different ways. Muller (1933), Plunkett 

 (1932), and Wright (1934a) argue, more or less identically, that 

 dominance modifiers may have been selected, to provide a margin of 

 stability and security, to insure the organism against weakening 

 genetic or environmental variability. Quoting Muller: "These modi- 

 fiers must so affect the reaction set in motion by the primary gene in 

 question as to cause this gene, when in two doses, to be near an upper 

 limit of its curve of effectiveness, that is in a nearly horizontal part of 

 the curve, not so readily subject to variation by influences in general, 

 including reduction in the primary dosage of the gene." Plunkett and 

 Wright express the same idea (the physiological part of which is 

 identical with mine) in somewhat different language. 



One of the corollaries of the foregoing discussions is that differ- 

 ent wild-type alleles ( hyper- wild ones ) must exist above the threshold 

 for wild type, though they are distinguishable only in special combina- 

 tions. Mohr, Haldane, and I have used this notion in theoretical 

 discussions. Muller (1935a) discussed it and tried a conclusive experi- 

 ment, which in principle is of the same general type as Stern's 

 subsequent work with ci isoalleles. He compared in triploid condition 

 white eye mutants of different origin and known to be somehow 

 genetically different (in regard to mutability). He found that the 

 American white locus is less dominant than the Russian, and attributes 

 this to different potency, based upon different levels above a threshold. 

 Thus a very large and diversified body of facts fits together into a 

 single picture of genie action of the type developed in this chapter. 



It is only a small step from a discussion of dominance as a dosage 

 phenomenon (or potency phenomenon) to the problem of penetrance, 

 which sometimes overlaps with dominance. Timofeeff-Ressovsky 

 (1927) introduced this term for cases in which a mutant did not 

 produce its phenotype in all individuals, some of the individuals being 

 normal. Thus we may speak of mutants with penetrance varying 

 from 1 per cent to 100 per cent, the last being the "good" mutants, 

 with which the classic geneticist likes to work. The former mutants 

 with low penetrance, though less "useful" for crossover work, are 

 actually of greater importance for the theory of genie action. Many 

 mutants of this type are known. From the point of view of genie ac- 



