ABSTRACT
Biological theories of autism have attempted to account for the condition through reference to genetic factors, intrauterine environmental factors, perinatal complications, neuroanatomical factors, neurochemical factors, physiological factors or some combination of these (Gillberg & Coleman, 2000). Evidence from twin and family studies show that genetic factors contribute to the development of autism and that the mode of transmission is quite complex and probably involves multiple genes. In a small proportion of cases (5-10%) autism may be due to single gene disorders or chromosomal abnormalities (Johnstone & MRC Autism Review Group, 2001) including Fragile X anomaly and tuberose sclerosis. Tuberose sclerosis is a neurocutaneous disorder characterized by skin lesions and neurological features; both epilepsy and learning difficulties also occur in many cases. Past reports of an association between autism and phenylketonuria have not been replicated (Volkmar et al., 2004a). The risk of parents having a second child with autism is between 3 and 7%.
