A genetic mutation causes phenylketonuria. In a person with PKU, this defective gene causes a lack of or deficiency of the enzyme that's needed to process an amino acid called phenylalanine. A dangerous buildup of phenylalanine can develop when a person with PKU eats foods that are high in protein, such as milk, cheese, nuts or meat.

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. If only one parent has the PKU gene, there's no risk of passing PKU to a child, but it is possible for the child to be a carrier.

Most often, PKU is passed to children by two parents who are carriers of the disorder, but don't know it.

Nov. 26, 2014