PHYSIOLOGY OF GENE ACTION IN HYBRIDS 113 



insistence upon attempts to find a single genetic mechanism. It has suffered, 

 too, from faikire to recognize that between the gene and the final mature 

 organism there lies a system of developmental processes of great complexity. 

 The complexity of this system is formidable but it surely can be analyzed, 

 at least with respect to its most significant features, if it is taken part 

 by part. 



SUMMARY 



The evidence relating to heterosis suggests that the phenomenon is to be 

 explained genetically in terms of various recombination effects. In some cases, 

 dominance is the important consideration, while in other cases, hetero- 

 zygosity must be considered. In any event, it is the resulting specific gene 

 action which lies at the basis of the physiological advantage or advantages 

 which give rise to hybrid vigor. One or many genes may be involved. Con- 

 siderations of genetic balance and genotype-environment balance are im- 

 portant. Probably most cases of heterosis are to be explained physiologically 

 in terms of differences in the more fundamental aspects of the metabolic pat- 

 tern, particularly those concerned with enzyme, auxin, and other growth 

 substance activity in plants and with enzyme and hormonal activities in 

 animals. 



To clarify the mechanism further, studies must be concerned primarily 

 with the genetics and physiology of early development. We have been con- 

 cerned with mature characteristics of size and yield, with the inheritance of 

 so-called quantitative genes, and with analyses by the classic methods of 

 genetics. These studies have brought us close enough to an understanding 

 of the phenomenon of heterosis to indicate that its further analysis by 

 techniques now at hand will uncover facts of tremendous importance for 

 genetics, physiology, and other studies of development, some of them con- 

 siderably afield from heterosis itself. 



