Siblings Sam and Max Drohan both have autism and struggle with social interactions and communicating with others.

"It was really devastating to have them both being diagnosed at the same time, it was more devastating knowing our youngest son really was on a great trajectory and then all of a sudden something went awry," their mother Jenn Drohan, told CBS News.

Autism is know to run in families, so for some time experts have believed that when more than one offspring develops the disorder it's due to the same genetic mutations. But new research funded by the advocacy group Autism Speaks suggests the genetics of autism are more complicated than previously thought.

In the largest-ever autism genome study, published in the journal Nature Medicine, researchers sequenced 340 genomes from 85 families with two affected children.

The researchers focused on 100 different genetic variations in the genomes sequenced. They found 70 percent of siblings had little or no overlap in the gene variations that contribute to autism.

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"We frequently refer to them as snowflakes. Snowflakes are very unique from flake to flake and autism is very much the same thing at a genetic level," Dr. Robert Ring, chief science officer at Autism Speaks, told CBS News. "Each child is very different."

The researchers say they are now making the 1,000 genome maps from their study available to other scientists around the world through the Google Cloud platform, as a way to crowd-source and encourage more progress in autism genetic research.

"There is probably not one autism. There are probably hundreds of different autisms," said Ring.

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Genetics of autism more complicated than thought, study finds

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