Genetic heterogeneity

Genetic heterogeneity is common and involves multiple alleles
at a single locus as well as multiple loci for some disorders.
Allelic heterogeneity implies that many different mutations can
occur in a disease gene. It is common for affected individuals to
have two different mutations in the disease-causing gene and
these people are referred to as compound heterozygotes. The
severity of the disorder may be influenced by the particular
combination of mutations present. Locus heterogeneity, where
a particular phenotype can be caused by different genes, is seen
in some autosomal recessive disorders. A number of recessive
genes at different loci cause severe congenital deafness and this
affects recurrence risk when two affected individuals have
children