Phenylalanine hydroxylase (PAH) is a key enzyme in the catabolism of phenylalanine, and mutations in this enzyme cause phenylketonuria (PKU), a genetic disorder that leads to brain damage and mental retardation if untreated.
The history of PKU is one of science in the discovery of an inborn error of metabolism and a chemical cause of mental retardation; and also one of technology with the development of methods to prevent disease.
The dysgenic effect (increase in prevalence of the PKU gene) from this reproduction will have negligible influence on the frequency of mental retardation from PKU over the next few centuries.