Hitherto, we have treated dominance as a fixed property of the alleles concerned, both in cases where we were discussing dominance in fitness and in cases where we discussed dominance at the gross phenotypic level, as in the case of industrial melanism. However, there is much experimental evidence to indicate that the degree of dominance of one allele over another at a given locus (the “main” locus) can be affected by the presence or absence of specific alleles (“ modifiers of dominance”) at other loci. Ford (1940) showed that, in the currant moth, yellow wing ground colour could be made either near-dominant or near-recessive to white ground colour by selection of the appropriate genetic background. Fisher and Holt (1944) showed that, in the mouse, the mutant Danforth’s short tail, which normally shows marked expression in the heterozygote, could be made near-recessive, again by selection of the right genetic background. Ford (1955) showed that in the lesser yellow underwing moth, the near-dominance of dark to light in two widely separated natural populations depends on the presence of modifiers specific to the population from which moths were taken. Kettlewell (1965) showed that the dominance of dark to light in the peppered moth collapsed when the English background genotype was replaced with that from a Canadian stock. Clearly, modifiers of dominance are of widespread occurrence and, in some cases at least, are responsible for the presence of dominance as observed in nature.
KeywordsModifier Locus Metrical Character Main Locus Advantageous Mutant Mutant Homozygote
Unable to display preview. Download preview PDF.