GENE INTERACTION IN HETEROSIS 327 



combination of alleles at each bottleneck locus. There are presumably many 

 loci that never act as bottlenecks in any part of the reaction system affecting 

 vigor or yield, no matter which allele or combination of alleles happens to 

 occupy such a locus. 



11. The difference between the weakest inbred and the most vigorous hy- 

 brid is merely one of degree. Each represents an integration of the many 

 genetic-environmental bottleneck effects under which it has labored. The 

 weak inbred has been throttled down by one or more bottlenecks to a low 

 level. The superior hybrid is able to go much further, even attaining what we 

 might concede to be extreme vigor. But both the weak inbred and the vigor- 

 ous hybrid have throughout their lives been held down to their respective 

 levels by their genetic -environmental bottlenecks.^ 



MISCELLANEA 



The fourth and last group of postulates comprise a heterogeneous popula- 

 tion randomly listed as separate topics for discussion. 



1 . If each step in a complex physiological process such as photosynthesis is 

 conditioned by the action of a specific gene, and if each successive step in the 

 chain of reactions is contingent upon the successful completion of the pre- 

 ceding steps, it follows that in attempting a biomathematical analysis of the 

 inheritance of quantitative characters such as yield we may not be justified 

 in assuming, as a basis for our calculations, that each of the genes concerned 

 is independent in its action. 



2. Since our efforts to "improve" the genotype are constantly being 

 thwarted by bottleneck genes, we may be tempted to damn all such genes as 

 inventions of the Devil. No doubt there are many defective genes that would 

 have to be classed as liabilities under any normal environment. But certainly 

 there are many bottleneck genes that are indispensable to survival — genes 

 that act as governors in regulating physiological reactions and in fitting the 

 organism to its particular ecological niche. A mouse or a moss can survive and 

 reproduce where larger organisms would perish. And a mouse which, as a 

 result of changes in certain of its adaptive bottleneck genes attained the size 

 of a rat, might find itself at a disadvantage in a community of normal mice. 



3. If we are correct in assuming that even a single major bottleneck locus 

 can act as a limiting factor in the development of an otherwise superior geno- 



* Certain of the foregoing postulates pertaining to the role of bottleneck genes in quanti- 

 tative inheritance may be guilty of gross over-simplification. So complex is the physiological 

 reaction system of even the simplest organism that we are only now beginning to gain an 

 inkling of the extent of its complexity. These postulates may also be guilty of exaggeration. 

 Because we believe that the action of limiting factors in quantitative inheritance has not re- 

 ceived the attention that it deserves, we have intentionally stressed the importance of the 

 bottleneck locus, even at the risk of over-emphasis. Furthermore, we have pictured the 

 limiting factor at a given moment as pertaining to a single bottleneck locus. This may or 

 may not be the rule. It would not be difficult to imagine a bottleneck which pertains to sev- 

 eral loci and which could be relieved or eliminated by substituting a more effective allele at 

 any one of these loci. 



