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In This Article
Autism spectrum disorder (ASD) affects about 1 in 36 children in the U.S. For many families, this diagnosis brings a wave of questions: Why did this happen? Was it genetic? Could it happen again?
This guide offers clear, research-backed answers about the genetics of autism - and what it means for your family.
Autism is a neurodevelopmental condition. That means it affects how the brain develops and functions, especially in areas like communication, behavior, and social skills.
But autism is a spectrum. Some people with ASD need a lot of daily support, while others live independently. Each personโs strengths, challenges, and sensory experiences vary.
ASD typically appears before age 3. Common signs include:
Medical professionals now diagnose ASD based on levels of support needed, ranging from Level 1 (needs some help) to Level 3 (needs substantial support).
Some myths about autism still linger - letโs set the record straight:
ASD is a biological condition, not the result of poor choices or bad luck.
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Scientists agree that genetics plays a major role in autism. But itโs not about one โautism gene.โ Itโs a complex picture involving many genes - and in some cases, spontaneous genetic changes.
Letโs break it down.
Twin studies give us some of the clearest evidence:
Because identical twins share all their genes, and fraternal twins only half, these patterns suggest that genetic factors account for most of the risk. In fact, researchers estimate autismโs heritability at around 80%.
That means while the environment matters, genetics is the dominant influence.
Thereโs no single gene that โcausesโ autism. Instead, most people with ASD inherit a combination of many common genetic variants, each adding a small amount to their overall risk. This is called polygenic inheritance.
Polygenic means โmany genes.โ It refers to traits - like autism - that arise from the combined effect of lots of genes rather than one.
Researchers now use polygenic risk scores to estimate how all these tiny genetic differences might work together.
Some rare genetic events also play a role:
In many cases, these risk factors are passed down from both parents - even if the parents donโt have autism themselves.
Some of these inherited factors can now be measured through tools like polygenic risk testing, but they canโt always predict outcomes.
Sometimes autism appears as part of a larger genetic condition. These account for a smaller slice of cases (about 20%), but they provide valuable insights.
Syndrome | Gene(s) Involved | % with ASD |
Fragile X Syndrome | FMR1 | ~40% |
Tuberous Sclerosis Complex (TSC) | TSC1/TSC2 | 16โ61% |
Rett Syndrome | MECP2 | 17โ58% |
Smith-Lemli-Opitz Syndrome | DHCR7 | 50โ60% |
22q11.2 Deletion Syndrome | Multiple | ~20% |
These conditions are often diagnosed through genetic testing, especially if a child has other medical signs alongside ASD.
While most people with autism donโt have one of these syndromes, understanding them helps researchers uncover the broader genetic architecture of ASD.
While genes are the biggest piece of the autism puzzle, they donโt work in isolation. Environmental factors - especially those during pregnancy - can interact with genetic susceptibility and increase the risk of autism in some children.
Here, โenvironmentโ mostly refers to prenatal conditions - that is, what the baby is exposed to before birth. These may include:
Not every child exposed to these factors develops ASD, but they may raise the odds when genetic risk is already present.
Genes set the stage - but the environment can pull the trigger.
Plain-Language Guard: Geneโenvironment interaction means that certain environmental influences may only increase risk in people with specific genetic traits.
For example, a child with certain genetic variants may be more sensitive to maternal infection or medication during pregnancy. Another child without those variants might not be affected the same way.
This explains why autism risk varies so much - even between siblings - and why no one environmental factor can fully โexplainโ autism on its own.
Many families want to know: Can genetic testing explain why our child has autism? And will future children have it too?
The answers arenโt simple, but research and clinical tools can provide guidance.
In some cases, yes. Genetic testing can reveal a known mutation or chromosomal change linked to autism. But for most individuals, results are either negative or inconclusive.
Still, testing is worthwhile - especially if a child has other health or developmental concerns.
The American College of Medical Genetics recommends several tests:
Plain-Language Guard: A chromosomal microarray is a test that scans for large chunks of missing or duplicated DNA.
Using these tools, a genetic cause can be found in about 25% of people with ASD. That number may rise with future technologies - but testing doesnโt always yield answers today.
For more on what these tools can (and canโt) tell you, read about false positives in genetic testing.
If you already have one child with autism, the chance of having another is higher than average - but far from certain.
Family Scenario | Estimated Risk |
One child with ASD | ~20% |
Two children with ASD | ~37% |
Male sibling | ~25% |
Female sibling | ~13% |
Several factors influence these numbers. For example, siblings of girls with autism tend to have a higher risk than those of boys - likely due to different genetic mechanisms.
Also, if a specific genetic cause is identified in your child, risk for future children will depend on how that condition is inherited.
Navigating autism genetics is complex. Thatโs where a genetic counselor can help.
These specialists explain testing options, interpret results, and walk families through medical, emotional, and reproductive decisions.
If you're considering genetic testing - or planning another pregnancy - a counselor can help make sense of whatโs known and whatโs still uncertain.
To learn more about this process, consider reading our full guide: What is a genetic counselor?
Understanding the causes of autism can be overwhelming. Hereโs what matters most:
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