F . Dove A Metabolic Basis for Dominance and Recessivity

T HE reasons for the existence of genetic dominance have provoked much debate in the literature, starting with FISHER’S (1928) paper on a possible evolutionary explanation for dominance. Any theory for the basis of dominance and recessivity, whether evolutionary or physiological, should explain three patterns in dominance relationships of the diploid phenotypes. The first is MENDEL’S observation that the heterozygote is often indistinguishable in phenotype from one or the other of the homozygotes, and we now know that in the vast majority of cases the wild type is dominant over the mutant. Second, in the few cases where measurement of degrees of dominance for mutations of small phenotypic effect has been carried out, it appears that the heterozygote phenotype is on average close to intermediate (CROW and SIMMONS 1983). Third, there are several interesting series of mutations at the same locus that produce different phenotypes, e.g., the albino series of mutants in rodents. In such cases, heterozygotes between “lower” mutant alleles of the series are usually intermediate in phenotype, whereas the wild type is usually dominant over all of them. In 1981 H. KACSER and J. A. BURNS published in GE-