Phenylketonuria is an autosomal recessive disorder. In this disease, the patient cannot produce enzyme phenylalanine hydroxylase, which helps in conversion of phenylalanine to tyrosine. Thus, there is accumulation of phenylalanine and its derivatives in the body. Accumulation of these in brain results in mental retardation. These are also excreted through urine because of its poor absorption by kidney. The gene coding for phenylalanine hydroxylase is located on chromosome number 12.