A genetic disorder in humans is caused by a mutation in the gene that codes for the enzyme phenylalanine hydroxylase. This single gene mutation manifests in multiple phenotypic expressions, including mental retardation and a reduction in hair and skin pigmentation. This illustrates the phenomenon of:

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The genetic disorder described is Phenylketonuria (PKU), which is caused by a mutation in the gene coding for the enzyme phenylalanine hydroxylase. This single gene mutation leads to multiple phenotypic expressions, such as mental retardation and a reduction in hair and skin pigmentation. The phenomenon where a single gene exhibits multiple phenotypic expressions is known as pleiotropy. Answer: Pleiotropy