GENE CONTROLLED PROCESSES 



185 



4. Dominance 



Dominance of hypo-hypermorphic genes is a question of the dose- 

 effect curve. Stem, Muller, and others have shown that this curve has 

 an approximately hyperbohc form, rising rapidly for small doses, but 

 flattening out and approaching an asymptote as the doses become 

 larger. Wright^ has argued that this is a simple consequence of the 

 ordinary chemical dynamics of the gene-reaction system, assuming 

 gene-quantity to correspond to the quantity of enzyme or catalyst. 



Fig. 87. The Evolution of Dominance. — Fisher suggests the selection of modifiers 

 which push up the early part of the dose-effect curve; Muller suggests a similar 

 pushing down of the later part; Haldane suggests the adoption of a more efficient 

 wild-type allelomorph. 



An allelomorph A is dominant to a if A A Ues on the horizontal part 

 of the dose-effect curve and if the compound Aa also comes in the 

 horizontal part of the curve and thus shows the same effect as A A. If 

 A A and Aa Ue in any other part of the curve they will show some sort 

 of incomplete dominance, i.e. the difference between AA and Aa will 

 be less than that between Aa and aa (because the slope of the curve 

 falls off). Thus some degree of dominance is to be expected as a general 

 rule, particularly for large mutations, but it is probably necessary to 

 provide some explanation of the great frequency of complete domi- 

 nance; i.e. some mechanism by which A A is pushed so far along the 

 horizontal part of the curve that Aa is also on it. (The problem is 

 exacdy the same as that of dosage compensation in the X, p. 166). The 

 alternatives are (i) direct selection of modifiers which make Aa similar 

 to AA, i.e. steepen the early part of the dose-effect curve so that the 

 flat part occurs early (Fisher);^ (2) selection of normal wild type genes 



^ Wright 1934. 



2 Fisher 1928, 193 1. 



