Lack of evidence for increased genetic loading for autism among families of affected females: a replication from family history data in two large samples.

Both the broad and narrow phenotypes of autism have been consistently observed in family members of affected individuals. Additionally, autism spectrum disorders (ASDs) present four times more often in males than in females, for reasons that are currently unknown. In this study, we examined whether there were differences in familial loading of ASD among families of male versus female probands. Analyses were conducted with existing data from two distinct samples. The first sample contained 417 individuals with autism and Asperger's disorder and included information on the ASD diagnoses of their first- and second-degree relatives. The second sample consisted of 405 sibships participating in the Autism Genetic Resource Exchange, of which one or more siblings had an ASD diagnosis. Results from both samples did not suggest significant differences in the prevalence of ASD among relatives of affected males versus females.