THE Ml TA TED GENE 1 2 1 



sarily be dominant. This model will account for all similar 

 cases, whatever the details of the chemism may be. 



There is no need for a special explanation of the cases of 

 pleiotropy, in which each series of actions of the same gene 

 has its own dominance relations. We have already pointed out 

 (page 89) that the independent behavior of these different 

 effects in regard to sedation is dependent upon the special 

 developmental conditions of the different parts of the body. 

 This means for the case of dominance that the variables of our 

 model (Fig. 23) may be different for each organ involved with 

 all the consequences for dominance, already reported. The 

 same type of reasoning applies to those cases in which different 

 alleles cause very different processes, e.g., spineless aristapedia. 

 But here another additional problem is involved, viz., the problem 

 of pattern, to which we shall return in another chapter. 



F. The Phylogenetic Theory of Dominance 



Phylogenetic deliberations have nothing to do directly with 

 the physiology of gene action. But, in connection with domi- 

 nance, they may at least be mentioned, because the phylogenetic 

 argument has silently assumed that dominance is a phenomenon 

 of such a physiological type as had been developed in Gold- 

 schmidt's interpretation of dominance (1927c). As a matter of 

 fact, Wright (1934(2) clearly stated that this is the case and worked 

 out his theoretical formulation of the physiology of dominance 

 (see page 102) with the intention of furnishing a basis for a 

 phylogenetic theory. 



Fisher (1928-1932) has developed the theory that the domi- 

 nance of Wild type over mutations is the result of a selection of 

 modifiers which tend to shift the phenotype of the heterozygote 

 toward resemblance to Wild type. He assumes that recessive 

 mutations must have occurred always in sufficient number and 

 therefore that heterozygotes were always present. If those 

 heterozygotes which most resembled Wild type had a selective 

 advantage, this would work finally to a selection of complete 

 dominance by selection of genes modifying this dominance. 



Wright (1929) and Haldane (1930) have published objec- 

 tions to this theory which, after all, it will be difficult to prove or 

 disprove. But, as stated above, such a theory depends com- 



