Constantino, the Blanche F. Ittleson Professor of Psychiatry and Pediatrics and director of the William Greenleaf Eliot Division of Child and Adolescent Psychiatry at Washington University and psychiatrist-in-chief at St. Louis Children's Hospital, says the findings also suggest that in many families, the transmission of autism is the result of the effects of many genes not just one with each contributing a small proportion of risk.
Prior estimates of the extent to which autism is influenced by genetic factors are derived from studies of identical and fraternal twins where one, or both, are affected by the disorder. Since identical twins share 100 percent of their genes, and fraternal twins share 50 percent, inherited conditions tend to be twice as common in an identical twin pair compared to a fraternal twin pair. But twin studies of autism are too small to give precise estimates about how the disorder is inherited.
"The largest studies have included less than 300 clinically affected twin pairs," Constantino says. "And they include girls, boys and mixed twin pairs, which complicates the testing of models of inheritance in autism because the disorder is much more common in boys than girls."
Other studies have focused on siblings of children with autism, looking at how much more common autism recurrence is in siblings than the general population. But to derive more information on genetic structure from their family studies, Constantino's group looked at autism recurrence in half siblings and compared it to that in full siblings.
The researchers studied over 5,000 families in which there was a child with autism and at least one additional sibling the families were enrolled in a national volunteer, Internet-
|Contact: Judy Martin|
Washington University in St. Louis