Phenylketonuria
Phenylketonuria (or PKU) is an inherited disorder, that, left untreated, changes cells inside the brain and can lead to mental retardation. This dramatic change in behaviour can be traced to a tiny mutation in a single gene on chromosome 12. Fortunately, through routine newborn screening, almost all affected newborns are now diagnosed and treated early, allowing them to grow up with normal intelligence.
Due to this missing or deficient enzyme, children with PKU cannot process a part of the protein called phenylalanine, which is present in nearly all foods. Without treatment, phenylalanine builds up in the bloodstream and causes brain damage and mental retardation.
All newborn babies are tested for PKU, using a heel-prick blood test taken during the first few days of life. About one in 10,000 newborn babies are affected with PKU.