ISO-Alleles and the response to selection

A IVth chromosome containing the semi-dominant alleleci w was introduced into a strain, the rest of the genotype being that of an inbred Canton wild type. Selection was practised for the degree of expression of the heterozygote. Response to the selection occurred. It was found that after some forty generations the response had not involved the production of any new iso-alleles at theci locus. If other loci behave in the same way the mutation rate to iso-alleles cannot be great enough for the mutations of any one locus to play an important role in response to selection.