102 W. GORDON WHALEY 



catalogued as having at least moderately deleterious effects in the mutated 

 state. The characteristics controlled by them include: chlorophyll deficien- 

 cies, modifications of leaf form and pigmentation, stalk abnormalities, flower- 

 ing pattern, and time of flowering. 



The extent to which the actual nature of the genetic situation has been 

 analyzed varies, but in several of the cases it seems clear that the mutation 

 of a single gene is involved and that the Fi hybrids are heterozygous only 

 with respect to the alleles at this particular locus. The amount of heterosis 

 manifest also varies greatly. Because of experimental differences, no accurate 

 comparisons can be made, but in some instances the amount of hybrid vigor 

 appears to be nearly comparable to that which occurs in crosses involving 

 large numbers of allelic differences. The situation appears to be one in which 

 a mutation takes place, and the mutated allele is definitely deleterious when 

 homozygous. In individuals heterozygous for the particular gene, there ap- 

 pear none of the deleterious effects. Instead, a definite heterotic effect ap- 

 pears. Dominance is of no apparent importance, and the distinction between 

 the vigorous hybrids and the less vigorous non-hybrids rests upon hetero- 

 zygosity. 



Jones (1944, 1945) has reported several cases of what he has called heter- 

 osis resulting from degenerative changes. He first suggested that these cases 

 represented instances of heterosis with a genetic basis in the heterozygosity 

 of certain of the mutated genes. More recently (private communication) 

 Jones has concluded that these cases involve more than single gene differ- 

 ences, and that the results may be explained on the basis of an accumulation 

 of favorable dominant effects. 



The case of a single locus heterosis reported by Quinby and Karper (1946) 

 involves alleles which do not produce any detectable deleteriousness, but in 

 certain heterozygous combinations produce hybrid vigor comparable in 

 amount to that in commercial hybrid corn. Quinby and Karper have referred 

 the hybrid advantage in this case to a stimulation of meristematic growth in 

 the heterozygous plants. 



All of these instances involve specific allelic interactions and not superior- 

 ity resulting from heterozygosity per se — as was postulated by some of the 

 earlier workers concerned with the genetic interpretation of heterosis. These 

 examples contribute to the increasing realization that the phenomenon of 

 dominance is perhaps of less importance with respect to heterosis than has 

 been supposed. There is no a priori reason why the interaction of a so-called 

 recessive allele and a so-called dominant allele should not give results differ- 

 ent from and metabolically superior to those which are conditioned by either 

 two recessives or two dominants. 



This situation bears closely upon the interpretation of heterosis set forth 

 by East in 1936. East postulated that at the loci concerned with the 

 mechanism of heterosis there might be a series of multiple alleles — with the 

 combinations of different alleles giving results metabolically superior to 



