Describe the recessive disorder, phenylketonuria. Explain how it occurs and how it is treated

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One of the most frequently occurring recessive disorders is phenylketonuria, or PKU, which affects the way the body breaks down proteins contained in many foods. Infants born with two recessive alleles lack an enzyme that converts one of the basic amino acids that make up proteins (phenylalanine) into a byproduct essential for body functioning (tyrosine). Without this enzyme, phenylalanine quickly builds to toxic levels that damage the central nervous system. By age 1, infants with PKU can become permanently mentally retarded.
All U.S. states require that each newborn be given a blood test for PKU. If the disease is found, doctors place the baby on a diet low in phenylalanine. As long as dietary treatment begins early and continues, children with PKU usually attain an average level of intelligence and have a normal lifespan. Treated children with PKU nevertheless show mild deficits in certain cognitive skills, such as memory, planning, decision making, and problem solving.