As an autosomal recessive disorder, two PKU alleles are required for an individual to experience symptoms of the disease. For a child to inherit PKU, both the mother and father must have and pass on the defective gene. If both parents are carriers for PKU, there is a 25% chance any child they have will be born with the disorder, a 50% chance the child will be a carrier and a 25% chance the child will neither develop nor be a carrier for the disease.PKU is characterized by homozygous or compound heterozygous mutations in the gene for the hepatic enzyme phenylalanine hydroxylase (PAH), rendering it nonfunctional. : 541  This enzyme is necessary to metabolize the amino acid phenylalanine (Phe) to the amino acid tyrosine (Tyr). When PAH activity is reduced, phenylalanine accumulates and is converted into phenylpyruvate (also known as phenylketone), which can be detected in the urine.Carriers of a single PKU allele do not exhibit symptoms of the disease but  appear to be protected to some extent against the fungal toxin ochratoxin A. This accounts for the persistence of the allele in certain populations in that it confers a selective advantage—in other words, being a heterozygote is advantageous.The PAH gene is located on chromosome 12 in the bands 12q22-q24.2. As of 2000, around 400 disease-causing mutations had been found in the PAH gene. This is an example of allelic genetic heterogeneity.
A 1987 study from Slovakia reports a Roma population with an extremely high incidence of PKU (one case in 40 births) due to extensive inbreeding. It is the most common amino acid metabolic problem in the United Kingdom. History
Before the causes of PKU were understood, PKU caused severe disability in most people who inherited the relevant mutations. Nobel and Pulitzer Prize winning author Pearl S. Buck had a daughter named Carol who lived with PKU before treatment was available, and wrote an account of its effects in a book called The Child Who Never Grew. Many untreated PKU patients born before widespread newborn screening are still alive, largely in dependent living homes/institutions.Phenylketonuria was discovered by the Norwegian physician Ivar Asbjørn Følling in 1934 when he noticed hyperphenylalaninemia (HPA) was associated with intellectual disability. In Norway, this disorder is known as Føllings disease, named after its discoverer. Følling was one of the first physicians to apply detailed chemical analysis to the study of disease. In 1934 at Rikshospitalet, Følling saw a young woman named Borgny Egeland. She had two children, Liv and Dag, who had been normal at birth but subsequently developed intellectual disability. When Dag was about a year old, the mother noticed a strong smell to his urine. Følling obtained urine samples from the children and, after many tests, he found that the substance causing the odor in the urine was phenylpyruvic acid.