PHYSIOLOGY OF GENE ACTION IN HYBRIDS 103 



those determined by the combinations of like alleles, and with no considera- 

 tions of dominance being involved. In the light of existing evidence it seems 

 a safe assumption that a considerable portion of hybrid vigor is the result 

 of allelic interaction between different alleles at the same locus. Although the 

 evidence as yet is scanty, it is certainly pertinent to suggest that some 

 heterosis may result from the interaction of alleles at different loci, when 

 such alleles are brought into new combinations in the hybrids. 



Most of the recent studies of the relation of heterozygosity to heterosis 

 have been concerned with the results of the action of single genes. Such 

 studies have emphasized that heterosis need not have its basis in the action 

 of large numbers of genes but can be, and apparently frequently is, a result 

 of the combining of different alleles of a single gene. Any considerable amount 

 of hybrid vigor resulting from the action of single genes would seem to indi- 

 cate the involvement either of multiple effects of single genes or of genie 

 action in the control of relatively fundamental metabolic processes. Both are 

 likely probabilities. 



The metabolic system of any organism which grows and functions in a 

 satisfactory manner is an exceedingly complicated mechanism with a great 

 number of carefully balanced, interrelated processes. The mutation of any 

 gene which has control over any of the key processes or functions will almost 

 certainly be reflected in a number of processes and activities. For example, if 

 a change in the character of some fundamental enzyme system is involved, 

 either the addition or subtraction of a functional step, or of a substance 

 produced at a particular developmental stage, would be likely to enhance or 

 inhibit a number of important processes in the general metabolism of the 

 organism. 



The equilibrium factor in genie action is obviously a consideration of 

 great importance. If a mutation disturbs this equilibrium after it has become 

 fairly well established through selection and elimination processes, the con- 

 sequences may reduce the organism's vigor. If, in a hybrid, the mutation is 

 then brought together with the original wild type or normal allele, the sum 

 total of the actions of the mutated allele and the original allele may well be 

 such as to exceed that of two copies of the original allele in the production 

 of vigor in the organism. 



When we give attention to physiology of gene action in hybrids which are 

 heterotic, we must concern ourselves with all of these considerations in- 

 cluding the fact that a single gene, the mutation of which affects some 

 processes in a sufficiently fundamental stage of the organism's formation, 

 may well have a greater end effect than a number of genes whose functions 

 are concerned with more superficial developmental processes. 



SEED AND EMBRYO DEVELOPMENT 



The literature on heterosis contains a number of discussions concerning 

 the relation between seed and embryo size and heterosis (Kiesselbach, 1926: 



