Phenylketonuria is a hereditary disease. Phenylketonuria is an inherited error of metabolismcaused by a deficiency in the enzyme phenylalaninehydroxylase. It results in mental retardation and is inherited as an autosomal recessive trait. It is ahereditary human condition resulting from the inabilityto convert phenylalanine into tyrosine. This changecan be traced to a tiny mutation in a single gene onchromosome 12 .