Are autistic spectrum disorders genetic? If so, why do they vary in severity?
Hallmayer: Autism is strongly genetic. If one identical twin individual is affected the chance that the co-twin is also affected is between about 70 and 90 percent vs. about 3 to 5 percent in the case of a non-identical twin. However, this does not mean that environmental factors are unimportant. For example, the specific symptoms experienced by affected individuals within a family or twin pair can vary widely. This leads us to believe that, while certain genes increase the risk that a child may develop some type of autism, the presence and severity of particular symptoms experienced by that child depends on other genetic and environmental factors. Finally, about 10 percent of autism cases can be ascribed to single-gene disorders (such as fragile X syndrome, tuberous sclerosis complex and Rett syndrome), or to chromosomal abnormalities – all of which affect brain development. Abnormal brain development early in life is probably another risk factor for developing clinical symptoms seen
