PHYSIOLOGY OF GENE ACTION IN HYBRIDS 101 



homozygous with sufficient frequency to bring about the elimination of 

 many of them. 



As a matter of observation, it would seem that a comparison of the occur- 

 rence and degree of heterosis in different species, along with a consideration 

 of the reproductive mechanisms in the various species, supports the proposal 

 that heterosis in many cases is the result of the covering uj) in the hybrids of 

 deleterious recessive alleles with a consequent return to vigor. The often 

 stated argument that hybrids of corn, for instance, frequently are more 

 vigorous than the original open-pollinated populations from which the in- 

 breds used in their production were derived, has no validity with respect to 

 this situation. In the production of the inbreds there is invariably a reassort- 

 ing of the alleles of the open-pollinated populations. 



It is highly improbable, however, that dominant alleles operating either 

 because of certain inherent favorable characteristics of their own, or simply 

 to prevent the deleterious activity of recessives, present the only genetic 

 basis of heterosis. Dominance is by no means the clear-cut feature described 

 in Gregor Mendel's original paper. The dominance of a particular allele may 

 be conditioned by the environment, or it may depend upon the genetic 

 background in which the allele exists. A completely dominant effect of one 

 allele over another, in the classic sense of our utilization of the word domi- 

 nance, is by no means universal. 



Rather unfortunately the so-called heterozygosity concept of heterosis has 

 usually been introduced as being in opposition to the dominance explanation. 

 Because the concepts of the features of dominance and recessiveness early 

 put them into rigid categories, it has been difficult to postulate how a hetero- 

 zygous condition with respect to one or more genes could render an organism 

 more vigorous than the homozygous condition, usually of the dominant 

 alleles. 



Evidence of significance for the interpretation of the importance of hetero- 

 zygosity in heterosis has been accumulated slowly. There is now, however, a 

 fairly long list of instances in many different species in which the heterozy- 

 gous condition for certain alleles is known to be superior to either the homo- 

 zygous recessive or the homozygous dominant condition (Stubbe and 

 Pirshcle, 1940; Singleton, 1943; Karper, 1930; Robertson, 1932; Robertson 

 and Austin, 1935; Gustafsson, 1938, 1946; Nabours and Kingsley, 1934; 

 Masing, 1938, 1939a, 1939b; Rasmusson, 1927; and Timofeef-Ressovsky, 

 1940. 



The accumulation of data on these cases followed a long period during 

 which all the investigations reported seemed to indicate no marked differ- 

 ences between organisms heterozygous for certain alleles and those with the 

 dominant homozygous condition for these same alleles. At least, in no in- 

 stance, was there any marked superiority referable to the heterozygous 

 condition. Most of the genes involved in the more recent findings have been 



