A genetic mutation causes PKU. The defective gene contains the instructions for making an enzyme needed to process the amino acid called phenylalanine. Amino acids are the building blocks for protein. In a person with PKU, this gene is defective, causing a complete or near-complete deficiency of the enzyme. Without the enzyme necessary to process phenylalanine, a dangerous buildup of this amino acid can develop when a person with PKU eats foods that are high in protein, such as milk, cheese, nuts or meats. This can eventually lead to serious health problems.

For a child to inherit PKU, both the mother and father must have and pass on the defective gene. This pattern of inheritance is called autosomal recessive. It's possible for a parent to have the defective gene, but not have the disease. This is called being a carrier. Most often, PKU is passed to children by parents who are carriers of the disorder, but don't know it.

Nov. 17, 2011