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Guidance on Genetic Testing for Younger Sibling of Child with Autism

“Our first child has autism, and our pediatrician recommends genetic testing for our baby. Why?”

This week’s “Got Questions” response is from clinical geneticist Kandamurugu Manickam, M.D., of Nationwide Children’s Hospital, in Columbus, Ohio. Nationwide Children’s Hospital is one of 17 centers in Autism Speaks Autism Treatment Network.

In recent decades, we’ve learned that autism spectrum disorders (ASD) have a strong genetic component. For example, if one identical twin has autism, there’s a higher chance that the other twin will, too. We know that genes play a particularly strong role because this overlap is higher between identical twins, who share the same DNA, than with other siblings, who share roughly half their genes.

For many years this was the extent of the guidance that we could provide to families. Now, advances in understanding ASD and in genetic technology have greatly improved genetic testing’s usefulness and affordability.

Why we test baby siblings
We know that younger siblings of children on the spectrum have close to a 20 percent chance of developing ASD.  For babies with more than one sibling on the autism spectrum, the risk is even higher—nearly 30 percent. So if your older child has autism, there are several potential benefits for testing your younger child.

First, learning that your baby has genetic risk factors for autism encourages closer monitoring of developmental milestones by you and your child’s physician. It can likewise encourage earlier intervention should early warning signs appear. We encourage earlier therapy with “at-risk” children because we know that it encourages optimal development.

In addition, some types of autism are caused by genetic conditions that have associated medical problems. Fragile X and Angelman syndrome are two examples. Early identification of these syndromes enables you and your child’s doctor to be on the lookout for related medical problems. This will help ensure prompt treatment that reduces or prevents complications.

Currently genetic testing can identify one or more known autism risk genes in around 20 percent of affected individuals. However, I’ve seen this estimate range up to 30 percent, depending on the study.

Genetic risk is NOT certainty
Importantly, the finding of autism-risk genes indicates predisposition. It does NOT mean that you baby will certainty develop autism.

The highest genetic risk for autism is related to fragile X, a rare disorder. Roughly a quarter of children with fragile X develop autism. But not all children with autism carry the fragile-X gene.  Many of the more common autism risk genes carry much less risk.

It’s important to remember that there’s no single genetic cause for autism. We believe that it often results from the interaction of risk genes and other influences (known and unknown) during brain development.

For these reasons, we encourage early monitoring for autism symptoms for all baby siblings of children with autism. This is true even if genetic testing fails to find autism risk genes. After all, families tend to share both genes and a common environment.

Genetic testing after diagnosis
In general, genetic testing in a family should begin with the person who is already diagnosed with autism. In your case, this would be your older child. Indeed, we recommend genetic testing as part of the initial evaluation for all individuals diagnosed with ASD. Sometimes, the test results help us understand the underlying causes and guide treatment. As mentioned above, this is particularly true when autism is related to a genetic syndrome.

Thank you for your question, which I know is shared by many families. I wish you and your children all the best.

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The Autism Speaks blog features opinions from people throughout the autism community. Each blog represents the point of view of the author and does not necessarily reflect Autism Speaks' beliefs or point of view.