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What causes mental illnesses like schizophrenia and autism? We have long known that both tend to run in families and that if one of two identical twins has such a disorder, there is a much higher than average probability that the other will too. Autism is sometimes associated with genetic syndromes, such as Rett, Down, and Turner's, Phenylketonuria, and Tuberous Sclerosis. The clearest single-gene cause of autism spectrum disorder (ASD) is Fragile X syndrome, with a wide range of severity in symptoms and 25-47 per cent of affected males meeting the criteria for autism. But neither autism nor schizophrenia obeys classical Mendelian laws of inheritance in the way that Cystic Fibrosis or some types of colour blindness do.