Expert: Ettina - 1/30/2010

Question
my sons girlfriend had a baby and the baby has autism, i was wanting to no how close in the family is it heriditary?

Answer
The cause of autism is unknown in most cases. It seems most likely that there are many different causes of autism, and we only know a fraction of them.
Genetics is a major factor in most cases, but it's usually not a single-gene condition (there are exceptions, such as Fragile X Syndrome, which can be detected by a genetic test). Typically, it's a polygenetic trait, meaning that you need to have several genetic traits together in order to be autistic. This means that although you can see some autistic traits in the child's relatives, the chance of one of them being autistic is quite low. For example, about 2-3% of full siblings of autistic kids are also autistic. If there's more than one autistic person in the same family, of course, the recurrance rate gets higher, because that's clear evidence that genetics is a strong factor in that family's autism.
It also seems to be relatively common for autistic people who have children to have autistic children. In fact, there are some people who are only diagnosed after they have an autistic child.