The production of the same or similar phenotypes (observed biochemical, physiological, and morphological characteristics of a person determined by his/her genotype) by different genetic mechanisms. There are two types: (1) allelic heterogeneity ??? when different alleles at a locus can produce variable expression of a condition; and (2) locus heterogeneity ??? the term used to describe disease in which mutations at different loci can produce the same disease phenotype.