The molecular basis of dominance relationships: the case of some recent adaptive genes

A controversial debate in evolutionary biology has been to explain why deleterious mutations are usually recessive to their wild-type allele. For Fisher, dominance of the wild-type is the result of selection, whereas for Wright it is a mere consequence of the biochemical properties of physiological pathways. Over time, Wright's theory has appeared as the most appropriate, and Kacser and Burns explained why the widespread occurrence of recessive mutants is the inevitable consequence of the kinetic structure of enzyme networks. Does Wright and Kacser and Burns (W-K-B)'s theory apply for newly arisen adaptive genes? A survey of more than 70 studies shows that pesticide resistance conferred by mutations decreasing the affinity of the pesticide target-sites varies from complete recessivity to complete dominance. This review shows that dominance always has a purely physiological explanation that can be roughly, but not simply, predicted by W-K-B's theory. Thus, although W-K-B's theory remains powerful for predicting the recessivity of deleterious mutations involved in enzymatic pathways, no general theory emerges from the study of other situations, and molecular explanations are to be sought on a case by case basis.