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A gene is said to be polymorphic if more than one allele occupies that gene’s locus within a population. For example in dogs the E locus, which controls coat pattern, can have any of five different alleles, known as E, Em, Eg, Eh, and e.
A polymorphic variant of a gene may lead to the abnormal expression or to the production of an abnormal form of the gene; this may cause or be associated with disease. For example, a polymorphic variant of the enzyme CYP4A11 in which thymidine replaces cytosine at the gene's nucleotide 8590 position encodes a CYP4A11 protein that substitutes phenylalanine with serine at the protein's amino acid position 434. This variant protein has reduced enzyme activity in metabolizing arachidonic acid to the blood pressure-regulating eicosanoid, 20-Hydroxyeicosatetraenoic acid; humans bearing this variant in one or both of their CYP4A11 genes have an increased incidence of hypertension, ischemic stroke, and coronary artery disease.
Most notably, the genes coding for the Major Histocompatibility Complex (MHC) are in fact the most polymorphic genes known. In fact there are more than 800 different alleles of human MHC class I and II genes.