Phenotypic manifestations of phenylketonuria areA. Mental retardation                                   B. Reduction in hair pigmentationC. Reduction in skin pigmentation.

Phenylketonuria is due to the effect of a gene on metabolic pathway related to Tyrosine formation from Phenyl Alanine. In such individuals there is no synthesis of phenyl alanine hydroxylase. This single gene mutation manifests through phenotypic expression characterized by mental retardation, reduction in hair and skin pigmentation.