102 PHYSIOLOGICAL GENETICS 



sary assumption that there is a chain of reactions between the 



gene and the end product and that any condition that makes 

 one link in the chain act in the heterozygotes as it acts in the 

 homozygotes will result in dominance. He assumes that the 

 gene acts as a catalyst controlling the rate of production of some 

 substance (in harmony with Goldschmidt's basic assumptions) 

 by a chain of irreversible transformations, the intermediary sub- 

 stances being in a flux equilibrium. These reactions may be 

 treated as monomolecular and dependent upon the joint con- 

 cent rat ion of catalyst and substrate. The rate in this case 

 varies directly with the concentration of the catalyst, and the 

 curve expressing the relation of variation in the product with 

 variation in the catalyst is a hyperbole, asymptotic at its upper 

 limit. Therefore with increasing activity of the gene its hetero- 

 zygous effect ought to approach dominance. Another pos- 

 sibility is that a bimolecular reaction is involved. The general 

 consequences are shown to be the same, though a number of 

 new possibilities appear, e.g., one part of the reaction setting a 

 threshold for another. A third possibility is assumed, viz., 

 that the rate depends not separately on concentration of sub- 

 strate and enzyme but on their compound. Also, in this case, 

 a similar result will follow, viz., a relation between the difference 

 in the quantity of the catalyst and the amount of dominance. 

 Without going into further details of Wright's calculations, we 

 realize that in his view again dominance is dependent upon the 

 same elementary gene-controlled processes as are involved in the 

 model (Fig. 23), emphasizing more the behavior of the primary 

 chain of reactions. Thus we see that dominance is best con- 

 ceived as a consequence of the quantitative aspects of genie 

 action and their quantitatively fixed interplay in time and 

 space during development. If this is correct, dominance, vice 

 versa, furnishes important facts toward the understanding of 

 genie action. To these facts we shall turn now. 



A. The Influence of the Environment upon Dominance 



If dominance is controlled by a developmental system, as 

 indicated above, it ought to be possible to shift the phenotype of 

 the heterozygote by environmental action in the same way as 

 the mutant phenotype may be secured as a phenocopy. This is 

 especially to be expected when the mutant phenotype appears 



