obsolete complex genetic disease true disease_ontology A genetic disease that require multiple abnormal alleles to manifest are complex. These diseases are termed multifactorial or polygenic. Expressivity or penetrance of the disease is complex and is associated with multiple genes in combination with lifestyle and environmental factors. Multifactoral genetic diseases include heart disease and diabetes, although a subset of diabetes may be classified as a monogenic disease (due to the presence of a single abnormal allele). DOID:0050178