Pleiotropy is the phenomenon in which a single gene exhibit multiple phenotypic expressions. The gene exhibiting pleiotropy are called pleiotropic genes. Pleiotropism occurs mainly because of mutation in a particular gene, e.g., phenyl ketonuria which is a disorder caused by mutation in the gene coding for the enzyme phenylalanine hydroxylase. In the absence of this enzyme, phenylalanine is not converted into tyrosine and accumulation of phenylalanine takes place. The affected individual shows hair and skin pigmentation and mental problems.