Genetic Heterogeneity is a phenomenon in which a single phenotype or genetic disorder may be caused by any one of a multiple number of alleles or non-allele (locus) mutations.[1] This is in contrast to pleiotropy, where a single gene may cause multiple phenotypic expressions or disorders. Genetic heterogeneity can be classified as either "allelic" or "locus".
Allelic heterogeneity means that different mutations within a single gene locus (forming multiple alleles of that gene) cause the same phenotypic expression. For example, there are over 1000 known mutant alleles of the CFTR gene that cause cystic fibrosis.
Locus heterogeneity means that variations in completely unrelated gene loci cause a single disorder. For example, retinitis pigmentosa has autosomal dominant, autosomal recessive, and X-linked origins. However, only one mutant locus is needed for the phenotype to manifest.