PHYSIOLOGY OF GENE ACTION IN HYBRIDS 9^ 



It is a fair hope that from detailed studies of the nature and development 

 of heterosis, much will in time be revealed about specific gene action. Un- 

 fortunately, most of the studies up to the present time have been directed 

 to general rather than to specific considerations. It has been necessary to deal 

 in terms of size differences, yield differences, and growth rate differences, un- 

 til enough of the pattern should appear to indicate what specific physio- 

 logical considerations are likely to be involved in heterosis. Because we have 

 come only to this point and have proceeded but a little way in an analysis of 

 these specific physiological considerations, this chapter will have to deal 

 more with suggestions of the likely mechanisms than with data from investi- 

 gations of them. 



It is neither possible nor desirable to separate wholly the consideration of 

 the physiological mechanisms of heterosis from the genetic bases. Our main 

 concern will ultimately be with the genes involved and the nature of their 

 action. 



The word hybrid has no good, definitive genetic meaning. It can be used 

 with equal propriety to refer to organisms which approach complete hetero- 

 zygosity or to organisms which are heterozygous for only a small number 

 of genes. 



There is at least a rough relationship between the amount of heterosis in a 

 hybrid and the extent of the genetic differences between the parents. Physio- 

 logical and morphological diversity are dependent both upon the number of 

 allelic differences between organisms and upon the nature of the action of 

 the particular genes among which these allelic differences exist. It is quite 

 possible that organisms differing by only a few genes may be more widely 

 separated in certain characteristics than are organisms differing by many 

 more genes — the actions of which are of less fundamental significance for 

 the control of the developmental pattern. 



In our approach to questions of hybrid vigor, we may be concerned with 

 different degrees of hybridity. Consideration of this factor must involve not 

 only the number of genes but also the nature of the action of the particular 

 genes. Nor is this all, for the action of any specific allele is conditioned by the 

 genetic background in which it occurs in a particular individual. Hence, the 

 relations among genes may often be of critical importance. 



Of tremendous import, too, are the interactions between the activities of 

 the genes and the environment. In speaking of hybrid vigor, we are general- 

 ly concerned with such characteristics as size and yield, but these are merely 

 end products of the metabolic processes. Patterns of these metabolic proc- 

 esses are set by the genes, but the processes themselves may be either ac- 

 celerated, inhibited, or otherwise modified by the effects of environmental 

 factors. Hybrids which are particularly vigorous under certain conditions 

 may show relatively little vigor under other environmental conditions. It is 

 true that the enhanced vigor of hybrids frequently gives to them a wide 



