Mendel described the inheritance of recessive and dominant genes. Phenylketonuria (PKU) is an autosomal recessive mutation of gene on chromosome 12. PKU results when there is a deficiency of liver enzyme phenylalanine hydroxylase that converts phenylalanine into tyrosine. There is a high level phenylalanine in their blood interferes with brain development, muscles and cartilages of the legs may be defective and the patients cannot walk properly.