What Causes Autism, and What Can Genetics Tell You?

What genetic changes can contribute to autism?

Several kinds of genetic changes can contribute to autism risk. Many common variants, each with a very small effect, can add up; less-common gene variants or chromosome changes may have a stronger effect in particular people. The important distinction is cumulative risk versus a single cause.

MedlinePlus Genetics reports that changes in more than 1,000 genes have been reported in connection with autism, although many reported associations are not confirmed and no single gene explains most autism. It estimates that genetic factors overall contribute 40–80% of autism risk. That range describes genetic influence across populations, not the probability that a specific person will be autistic.

Rare gene variants or chromosome abnormalities are considered the cause in about 2–4% of people with autism, often as part of a broader genetic syndrome, according to MedlinePlus Genetics. “Cause” here refers to a recognized finding that may explain an individual’s presentation; it does not mean that most autism has one identifiable mutation.

A study summarized by the National Institute of Mental Health estimated that about 52% of autism risk was associated with common and rare inherited genetic variation, while spontaneous mutations contributed 2.6% of total risk. These are estimates from a particular study at the population level. They are not an individual risk score, and they should not be added to the separate MedlinePlus estimate as if the measures were interchangeable.

“Although each exerts just a tiny effect individually, these common variations in the genetic code add up to substantial impact, taken together.”

Researchers quoted by the National Institute of Mental Health

A practical way to read these findings is to separate two questions: does a genetic change contribute to likelihood, and does it explain a particular person’s autism? The first can be true even when the second remains unknown.

Is autism inherited from one or both parents?

Autism-related genetic influences can be inherited from either parent, and risk may reflect contributions from both sides of a family. There is no single usual inheritance pattern for autism, and carrying an associated change generally means increased likelihood rather than certainty.

MedlinePlus Genetics puts it plainly: “People with gene changes associated with ASD generally inherit an increased risk of developing the condition, rather than the condition itself.” A parent may carry a variant without being autistic because the variant is not a guarantee. The effect also depends on the broader combination of genetic influences; a single family observation cannot identify that combination on its own.

This is why a family tree can provide useful context without supplying a simple answer. A clinician or genetics team may consider which relatives have relevant diagnoses or features, what testing has already found, and whether a known genetic condition is present. The result is a more informed discussion, not a certainty about what will happen in a future pregnancy.

Can a child be autistic if neither parent is autistic?

Yes. A child can be autistic even when neither parent is autistic, because genetic risk is not inherited as a straightforward, guaranteed condition and some changes arise newly.

Common inherited variants can contribute small effects without producing autism in a parent. In addition, a de novo change may appear in the child rather than being detectable in either parent. Autism can also reflect a complex mix of influences that current genetic testing does not fully identify.

For families considering another child, a group recurrence statistic can be a starting point, not a personal forecast. A prospective study reported in Pediatrics followed 1,605 infants with an older autistic sibling at 18 sites; 20.2% were diagnosed with autism by age 3, with a 95% confidence interval of 16.6%–24.3%. The estimate differed with factors including the infant’s sex and having more than one older autistic sibling.

That 20.2% is an average for the study group. It does not determine an individual family’s likelihood, and a specific genetic finding or family history may change how a genetics professional interprets the situation. The study authors recommend developmental monitoring for younger siblings, which gives families a concrete next step without treating a population estimate as a diagnosis.

What does it mean when an autism-related variant is de novo?

A de novo variant is a genetic change detected in a child but not in either parent’s tested sample. It may arise in an egg or sperm cell, or shortly after fertilization; “new” describes when it appears in a family, not how strongly it affects a person.

MedlinePlus Genetics’ guide to mutations and disorders explains that a parent can have germline mosaicism: some egg or sperm cells carry a variant even though the variant is not found in other cells. This means a change described as de novo can, in some circumstances, recur in a family. The label alone does not settle recurrence risk.

Testing also has limits. According to American Academy of Pediatrics HealthyChildren.org, testing may identify a pathogenic variant, a variant of uncertain significance, or no finding. No single test checks for every genetic condition; which tests are appropriate depends on the child and family history.

A variant of uncertain significance is not a confirmed explanation. A negative result also does not rule out a genetic condition: a test may not detect the cause, or researchers may not yet know how to interpret it. A genetics team can explain what a result does and does not establish and discuss implications for relatives or future pregnancies.