Adding to confusion about the roots of autism, new research suggests that varying genes are often responsible for the disorder even among siblings who share a diagnosis.

Kids who have an older bother or sister with autism are known to be at higher risk for the developmental disorder and scientists have thought this heightened occurrence was due to shared genes.

But a study released Monday throws a kink in that theory. After sequencing the whole genomes of individuals from dozens of families — each with at least two children on the spectrum — researchers found that less than a third of affected siblings shared the same autism-associated gene variations.

“We knew that there were many differences in autism, but our recent findings firmly nail that down,” said Stephen Scherer of the University of Toronto and Toronto’s Hospital for Sick Children, who led the study published in the journal Nature Medicine. “We believe that each child with autism is like a snowflake — unique from the other.”

For the study, researchers used a technique called whole genome sequencing to analyze the complete DNA of individuals from 85 families. They looked for roughly 100 gene variations that are known to be associated with autism.

In the majority of cases — 69 percent — the study found that siblings on the spectrum carried different autism-related mutations.

What’s more, researchers said that siblings with different types of gene variations were more likely to present with contrasting clinical profiles while those who shared a genetic marker often exhibited similar symptoms.

The findings highlight just how unique the genetics behind each person with autism may be.

“This means we should not be looking just for suspected autism-risk genes, as is typically done in diagnostic genetic testing,” Scherer said. “A full assessment of each individual’s genome is needed to determine how to best use knowledge of genetic factors in personalized autism treatment.”

The study is considered the largest ever to look at the complete genomes of people with autism. It also marks the first step in a collaboration between Autism Speaks and Google.

Researchers said data from the study is part of the first upload to the MSSNG project, an online portal on the Google Cloud Platform that aims to make the full genomes of more than 10,000 people with autism openly available for research.

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