Both parents must pass along a copy of the mutated PKU gene for their child to develop the condition. If only one parent has the PKU gene, there's no risk of passing PKU to a child.

The gene defect occurs mainly in people from Europe and the U.S. The disorder is much less common in Asians and Latinos. Africa has the lowest rates of phenylketonuria.

Children of mothers who have PKU but who didn't follow the PKU diet during pregnancy also may be affected. Although these children don't often have PKU, they do have serious consequences of the high level of phenylalanine in the mother's blood.

Nov. 17, 2011