A number of disorders are known to be caused by duplication of genes, but these are all rare events. However, there is evidence that polymorphic gene duplication may be common and a growing number of genes are known to be duplicated in a polymorphic manner although phenotypes cannot be associated with most of these. Gene duplication occurring due to cytogenetic abnormalities such as Down syndrome predisposes the patients to a variety of complex disorders. It is possible therefore that many complex disorders and variable phenotypes are associated with duplication of genes.