Medical Genetics 2.1 Phenylketonuria, PKU Phenylketonuria is one of the commonest inherited disorders-occurring in approximately 1 in 10,000 babies born in theu s. it occurs in babies who inherit two mutant genes for the enzyme phenylalanine hydroxylase(PAh). This enzyme normally breaks down molecules of the amino acid phenylalanine that are in excess of the body,'s needs for protein synthesisMedical Genetics 2.1 Phenylketonuria，PKU Phenylketonuria is one of the commonest inherited disorders - occurring in approximately 1 in 10,000 babies born in the U. S. It occurs in babies who inherit two mutant genes for the enzyme phenylalanine hydroxylase (PAH). This enzyme normally breaks down molecules of the amino acid phenylalanine that are in excess of the body's needs for protein synthesis