GENE INTERACTION IN HETEROSIS 329 



larities noted by East between the beneficial effects of heterosis and those of 

 improved nutrition are more than coincidental. The first prerequisite for en- 

 hanced well-being is the removal of the bottlenecks that stand in the way- 

 This can sometimes be accomplished by improving the nutrition, sometimes 

 by substituting more efficient alleles at the bottleneck loci, and sometimes by 

 both. 



9. The term heterosis remains ambiguous in spite of the many attempts to 

 define it. It continues to have different meanings for different workers. 



10. If heterosis is to be measured by comparing performance of offspring 

 with performance of parents, then the higher the standing of the two parents 

 in the scale of measurement, the lower the degree of heterosis to be expected 

 in their offspring. Conversely the lower the standing of the parents, the great- 

 er the heterosis to be expected. (Exceptions to the latter rule will occur when 

 both parents owe their enfeeblement to the same bottleneck genes.) 



1 1. Success in crop and livestock production depends largely upon the skill 

 of the grower in detecting, diagnosing, and correcting the environmental com- 

 ponents of the bottlenecks affecting yield. Success in developing higher yield- 

 ing genotypes depends largely upon the ability of the breeder to substitute 

 more effective alleles at the bottleneck loci, and to accomplish this without 

 establishing new and equally serious bottlenecks at other loci. 



