In genetics, heterogeneity (from the Greek hetero = different) refers to the phenomenon that the same characteristics (phenotypes) are genetically completely different. Different mutations can therefore lead to a similar or identical phenotype. Different mutations at the same locus (gene locus) are referred to as "allelic heterogeneity", heterogeneity caused by mutations at different loci as "locus heterogeneity".
Diseases that are caused heterogeneously, i.e. by mutations in different genes, include albinism, hereditary deafness, tuberous sclerosis (chromosome 9q and 16p), Lynch syndrome (chromosome MLH1, MSH2, PMS2 or MSH6) or Ehlers-Danlos syndrome (COL5A2, COL5A, COL1A1 etc.). Heterogeneity can make the molecular genetic analysis of hereditary diseases considerably more difficult, as it contributes to variations in the appearance of a disease in addition to the presence of different alleles and environmental factors.