PKU is caused by mutation to the phenylalanine hydroxlase (PAH) gene on chromosome 12q. It has autosomal inheritance. Incidence is ~1:10,000 – 1:15,000 (USA).
Early and life-long treatment with a low phenylalanine diet prevents serious outcomes (intellectual disability, seizures). Temporary non-adherance to the diet leads to behavioural changes, which may appear like ADHD.
References: Genetics Home Reference. Phenylketonuria. Available online at: http://ghr.nlm.nih.gov/condition/phenylketonuria