A single gene can exhibit multiple phenotypic expressions. Such a gene is called a pleiotropic gene. A classic example of pleiotropy is the human disease PKU (phenylketonuria). This disease can cause mental retardation and reduced hair and skin pigmentation and can be caused by any of a large number of mutations in a single gene that codes for an enzyme (phenylalanine hydroxylase) that converts the amino acid phenylalanine to tyrosine, another amino acid. Hence, a pleiotropic gene controls multiple traits in an individual.