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When a child has autism, siblings are also at risk for the disorder, according to a new study.

According to John N. Constantino, the Blanche F. Ittleson Professor of Psychiatry and Pediatrics at Washington University, the research adds to recent evidence that even though autism is far more common in males, females still can inherit and pass along genetic risk for autism.

"We found that autism risk for half siblings is about half of what it is for full siblings," Constantino said. "Most of the half siblings we studied had the same mothers. Given that half of the risk of transmission was lost and half was preserved among those maternal half siblings, mothers and fathers appear to be transmitting risk equally in families in which autism recurs."

Constantino 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.

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-based family registry for autism, the Interactive Autism Network (IAN). Among those families, 619 included at least one maternal half-sibling.

The team focused on maternal half-siblings rather than paternal half siblings because these children were more likely to live full-time with their biological mothers and to share the same environmental influences between the time they were born and the age of two, the time at which the onset of autistic syndromes occur. They compared autism recurrence among the 619 maternal half siblings to the rate among 4,832 full siblings.

The team also looked at a group of local St. Louis families in which maternal half siblings also were known to have been raised in the same household.

After analyzing both sets of families, the team found that 10 percent to 11 percent of full siblings had been diagnosed with autism, compared to 5 percent to 7 percent of half siblings.

"If transmission of autism risk was occurring equally from unaffected mothers and fathers, you would predict that maternal half siblings' risk of autism would be about half of what we saw in full siblings," the lead author said. "And that's exactly what we found."

Constantino says that according to the new findings, more than 60 percent of autism cases are likely to be inherited on the basis of genetic variations inherited from unaffected mothers and fathers.

"In 15 to 20 percent of children with autism, it appears that genetic problems aren't inherited, rather that genes become altered in sperm cells, egg cells or in the developing embryo," Constantino said.

"The recent discovery of these kinds of abnormalities have raised questions about the interpretation of twin studies and the extent to which autism is inherited. The current study, however, supports inheritance as a central cause for a majority of autistic syndromes and encourages a new focus on the mechanisms by which genetic susceptibility to autism can be silenced in some individuals, especially females who typically exhibit symptoms of autism at only one third of the rate seen in males," he added. 

The study is published online in the journal Molecular Psychiatry.