Phenylketonuria (PKU) is located in the q arm of chromosome 12. It is caused by a mutation of the (PAH) phenylalanine hydroxylase gene. The (PAH) gene produces the enzyme phenylalanine hydroxylase and it then converts the phenylalanine to tyrosine.enetic Testing. The phenylalanine tolerance test is one form of genetic testing for PKU. It is performed by giving a person a measured amount of phenylalanine and the concentration of phenylalanine in the blood plasma is measured.