Why are most mutations recessive?

Abstract

It is a long-standing observation that most mutations are recessive. That is, they do not lead to visible phenotypic effects when in heterozygous combination with the wild-type allele. The reason for this has long been debated. Fisher (1930) attributed the observed dominance of the wild type to the action of natural selection at modifier loci. Wright (1929) on the other hand asserted that dominance did not have a selective functionper se, but was a more-or-less automatic offshoot of genetic regulatory mechanisms. The present essay discusses these explanations from a contemporary standpoint and suggests that neither is likely to be valid exclusively. In particular, even when physiology appears to offer a sufficient explanation, evolution of dominance cannot be ruled out.