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PKU is caused by a mutation in a single gene. In normal circumstances, this gene contains the instructions for making phenylalanine hydroxylase (PAH) — an enzyme necessary for processing the amino acid phenylalanine, which is found in significant amounts in all proteins and some artificial sweeteners. In a person with PKU, this gene is defective, causing a complete or near-complete deficiency of the enzyme. This makes it impossible to effectively break down the amino acid. When a person with PKU eats foods that contain proteins, such as milk, yogurt, shrimp or sausage, a dangerous buildup of phenylalanine can result, eventually leading to mental retardation and other serious problems.

The PAH gene mutation is passed from generation to generation in a pattern of inheritance called autosomal recessive. This means that both the mother and father must pass on the defective form of the gene for a child to be affected. Most often, PKU is passed down the family tree by parents who are carriers of the disorder but don't know it.

The risk of inheriting PKU really comes down to genetics. Children with two parents who carry at least one copy of the mutated gene for PKU are the only ones at risk of developing this condition. The gene defect occurs mainly in whites of Northern European ancestry. It's much less common in black babies and infants of Asian descent.

Children of mothers who have PKU but who didn't follow the recommended PKU diet during pregnancy also may be affected. These children don't inherit the genetic predisposition for this disorder, but they suffer consequences of the high level of phenylalanine in the mother's blood, which also circulates through the system of the fetus and produces toxicity to the developing nervous system.

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