326 A. J. MANGELSDORF 



chemical element has its specific role to perform in the physiological reaction 

 system. Similarly we accept as well established the thesis that gene action is 

 likewise specific — that a particular gene can perform its particular function, 

 and that function only. Nevertheless we sometimes engage in speculations 

 which ignore these convictions and which appear to assume that genes affect- 

 ing quantitative characters such as yield are freely interchangeable, one with 

 another, and that one yield gene can serve as well as another, regardless of 

 its locus or function. 



6. A bottleneck locus may act as such throughout the life of the individual 

 or it may act as a limiting factor only for a short period and under specific 

 conditions, such as drought, nitrogen deficiency, or excessively high or low 

 temperatures. Under a varying environment the bottleneck of one moment 

 may be superseded by a different bottleneck at the next moment. 



7. The physiological bottleneck may be ameliorated or removed by correct- 

 ing the particular feature of the environment contributing to the bottleneck. 

 In the examples cited above this would entail supplying moisture, or nitrogen, 

 or lowering or raising the temperature. Or the bottleneck may be ameliorated 

 or removed by substituting a more effective allele at the bottleneck locus, 

 providing that such an allele is available. 



8. As already indicated, the amelioration or removal of a bottleneck, either 

 by improving the environment or by substituting a better allele at the 

 bottleneck locus, will permit a rise in the rate of the essential physiological 

 processes. This rise may be small or it may be large, depending upon the 

 point at which the next ensuing bottleneck begins to make itself felt. The 

 substitution of a more efficient allele at a bottleneck locus in a certain geno- 

 type, under a particular environment, may result in a large gain. The substi- 

 tution of the same allele in a different genotype or under another environ- 

 ment may result in little or no gain. It is not strange that difficulty should 

 be encountered in analyzing the inheritance of genes affecting yield and other 

 quantitative characters which are subject to the influence of a varied and 

 fluctuating array of genetic-environmental bottlenecks. 



9. A diet that is low in calcium may supply calcium at an adequate rate so 

 long as growth is being retarded by a lack of phosphorus. But once phos- 

 phorus is supplied at an adequate rate, calcium deficiency becomes a bottle- 

 neck which limits the rate of growth. Similarly a mediocre gene m at one 

 locus may be adequate (not a bottleneck) so long as the rate of physiological 

 activity of the organism is being throttled by environmental limitations or 

 by a bottleneck gene at some other locus. But once the other genetic-environ- 

 mental limiting factors have been removed, the mediocre gene m is unable 

 to handle the increased load and becomes the bottleneck in the reaction 

 system. 



10. The maximum vigor or yield possible under a given environment will 

 be attained when the organism is endowed with the best available allele or 



