I am a professional human geneticist and I admit can't give a reliable estimate. For all I know, there are so far 4 unlinked genetic loci associated with Asperger's and two more which seem to be associated with X-linked Asperger's syndrome, but I don't think that there is a lab or clinic in the world who would claim that they offer prenatal diagnosis or at least risk assessment.
For all I know, in about 30 % of all families with a family member with ASD, there are more than one than one which are affected by the disorder, but this might not be true for all ethnic and racial groups.
Overall, the risk of having a child with Asperger's is higher if you have one or more children already who have either Asperger's or autism, or if you have another blood relative with these disorders, especially parents or siblings. How much higher, however, no-one can tell at the moment. Incalculable, as other members already said.
Note: Don't try to apply Mendel's laws to inheritance of complex traits as ASD, pgd. They won't work as they are only good enough for simple traits that are associated with a specific gene and are not influenced by other genes and/or the environment. Even in monogenic diseases (that is, one mutated gene causes a specific disease, e.g haemophilia, beta-thalassemia, etc.), the clinical course can differ depending on the nature of the mutations causing it (despite the fact that they affect the same gene) and the influence of other genes.
Last edited by Severus on 22 Sep 2010, 3:56 pm, edited 1 time in total.