Ok, will try to be more specific this time - trying to find where Ive read thing on this! In George and Sam, by Charlotte Moore, she's written a chapter on "does it run in the family" and she mentions going to the Maudsley hospital for genetic counselling. This is after she's had 2 boys who are autistic.
They asked her to compile a list of all family members, back to g/parents and spot any autistic traits, or others along behaviour,eccentricities etc. She came up with eccentricities but no obvious autism other than her 2 sons, and was given a 1 in 20 chance of it happening again with her 3rd child.