GENE INTERACTION IN HETEROSIS 325 



1. The adequacy of a diet is determined not by those constituents which 

 are present in ample amounts, but by those which are deficient to the point of 

 acting as limiting factors. Similarly the excellence of a genotype is deter- 

 mined not by its strongest but by its weakest links. The term weak link as 

 here employed refers to a gene pair at a particular locus which at some mo- 

 ment in the life of the organism proves so inadequate in performing the task 

 required of that locus as to act as a limiting factor — a bottleneck in an essen- 

 tial physiological process. A bottleneck efifect may result from a deficiency of 

 an essential gene product or from an excess of a gene product. 



2. At each moment throughout its life the physiological processes of even 

 the most vigorous organism are held down to their prevailing rates by bottle- 

 necks or limiting factors. We are merely rephrasing a genetic axiom when 

 we say that a bottleneck in the physiological reaction system is neither purely 

 genetic nor purely environmental. The physiological bottleneck at any given 

 moment results from the interaction of a particular locus (which we may for 

 convenience refer to as the bottleneck locus) with the remainder of the geno- 

 type and with the environment of that moment. When we speak of an en- 

 vironmental bottleneck, we are merely focusing attention upon the environ- 

 mental component of the genetic-environmental bottleneck. When we speak 

 of a bottleneck gene, we are referring to the genetic component of the genetic- 

 environmental bottleneck. 



3. The value of an otherwise perfect diet would be seriously impaired by 

 the omission of a single essential element. Similarly an otherwise superior 

 genotype could be rendered mediocre or worse by a single bottleneck. A po- 

 tentially superior genotype is unable to manifest its potentialities so long as it 

 is being throttled by a genetic-environmental bottleneck. A breeder looks at 

 the bottleneck and sees the need of a better allele at the bottleneck locus. 

 An agriculturist looks at the same bottleneck and sees the need for correcting 

 its environmental component. Bottlenecks relating to climatic limitations 

 usually can be most economically dealt with by breeding.- On the other hand, 

 bottlenecks resulting from nutritional deficiencies can often be advantage- 

 ously dealt with by correcting the environment. 



4. The substitution, at a bottleneck locus, of a better combination of al- 

 leles^ will result in an improvement in yield providing that no other limiting 

 factor, genetic or environmental, asserts itself before an appreciable gain has 

 been realized. 



5. The substitution of potentially better alleles at loci other than bottle- 

 neck loci cannot substantially improve yields any more than the addition of 

 calcium to the diet of a plant or an animal can relieve the effect of a phos- 

 phorus deficiency in that diet. We take it for granted that each essential 



2. This rule is not without exceptions. For example, a bottleneck resulting from a 

 deficiency of rainfall can sometimes be economically eliminated by irrigation. 



3. As already indicated, the best combination of alleles may be AA, Aa, or aa depend- 

 ing upon the particular locus. 



