W. GORDON WHALEY 



The Plant Research Insfifufe of fhe Universify of Texas and 

 the Clayton Foundation for Research 



Chapter 6 



Physiology of Gene 

 Action in Hybrids 



The physiology of gene action in hybrids is not a subject apart from the 

 physiology of gene action in organisms in general. The approach to specific 

 problems of gene action is probably better made in non-hybrid organisms 

 than in hybrids. Hybrids do, however, represent one type of genetic situation 

 which in certain instances is particularly favorable for the study of gene 

 action. Most useful in this respect are those hybrids which exhibit the phe- 

 nomenon referred to, often rather loosely, as hybrid vigor. The terms hybrid 

 vigor and heterosis often are used synonymously. A more precise usage, and 

 one in accord with the original definitions, refers to the developed superior- 

 ity of hybrids as hybrid vigor, and to the mechanism by which the superior- 

 ity is developed as heterosis. By this definition, hybrid vigor is heterosis 

 manifest. Because in studies of growth and development it is often desirable 

 to distinguish clearly between mechanism and end result, this use of the two 

 terms will be followed in this chapter. 



Heterosis has been the subject of many experiments and a great deal of 

 speculation on the part of geneticists. The concern has been mostly with the 

 genetic bases of heterosis, and relatively little attention has been given to the 

 physiological mechanisms involved. As a matter of fact, the literature on 

 heterosis mirrors faithfully the changing emphasis in genetics in the last two 

 or three decades. Practically all of the early investigations of heterosis had 

 to do with the comparison of mature characteristics of inbred lines and their 

 vigorous hybrids, and then with attempts to formulate genetic schemes in 

 explanation of the differences. Gradually, the focus of investigation has 

 turned to a study of developmental differences responsible for the hybrid 

 vigor, and more recently to the gene action bases of these developmental 

 differences. 



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