Stability Of Iso-Alleles

THE high stability of most genes when kept under natural conditions is deduced from the relative rarity of spontaneous mutations which produce striking deviations from normality. The existence of wild-type iso-alleles, that is, of different alleles usually causing the same normal phenotype although distinguishable by their effects under special conditions, raises the question whether mutations from one iso-allele to another are more frequent than mutations with more obvious new effects.