In Muller's classification, an isomorph is described as a gene mutation that expresses a nonsense point mutant, with expression identical to the original allele.
Therefore, in respect to the relationships between the original and mutated genes, it is difficult to ascertain the effects of dominanceness and/or recessiveness.
Muller's classification of mutant alleles CategoryAlternative functionWild typeReferent gene expression, normal expression of parent alleleAmorph Dysfunctional, with null expression Hypomorph Reduced or partially reduced gene activityHypermorph Increased or partially increased parent gene activity Neomorph Novel function, compared to the new propertyAntimorphInterfering with gene activityIsomorphExpression identical to the original (parent) allele, mostly resulting from silent point mutations
See also Allele Mutation Muller's morphs
References