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.[1] [2] [3]
| Category | Alternative function |
| Wild type | Referent gene expression, normal expression of parent allele |
| Amorph | Dysfunctional, with null expression |
| Hypomorph | Reduced or partially reduced gene activity |
| Hypermorph | Increased or partially increased parent gene activity |
| Neomorph | Novel function, compared to the new property |
| Antimorph | Interfering with gene activity |
| Isomorph | Expression identical to the original (parent) allele, mostly resulting from silent point mutations |