Not in the way the question usually implies. Autism is not carried by a single gene that lies dormant for a generation and then reappears. What passes through families is a genetic predisposition built from many small contributions, and that predisposition can be present in someone who is never diagnosed, then show up in their child.
So a family tree with a gap in it does not mean anything was skipped. It means the risk was there the whole time and did not reach the threshold for a diagnosis in that person. Our ABA therapy services page covers what support looks like once a diagnosis is in hand.
What "Skipping a Generation" Actually Means
The phrase comes from how we learn genetics at school, where a recessive trait disappears in one generation and returns in the next. Eye colour works roughly like that. Autism does not.
Autism involves many genes, each contributing a small amount to overall likelihood, interacting with each other and with environmental factors. There is no on-off switch to hide and reappear. There is a cumulative load that varies between relatives, and a threshold effect that means two people carrying similar risk can have very different outcomes.
How Strong Is the Genetic Link?
Strong, and now measured at scale. A study published in JAMA Psychiatry followed more than two million people across Denmark, Finland, Sweden, Israel, and Western Australia, including 22,156 with an autism diagnosis. It estimated the heritability of autism at approximately 80%.
Two findings from that study are worth stating plainly, because both correct common assumptions.
There was no support for a maternal effect. Estimates ranged from 0.4% to 1.6% with confidence intervals including zero. Autism does not come from the mother in any meaningful sense, and anything suggesting that something a mother did or did not do during pregnancy caused her child's autism is not supported by this evidence.
Shared family environment contributed minimally. How a family raises children is not what is driving this.
Why Autism Does Not Follow a Simple Inheritance Pattern
Many Genes, Small Effects
Genome-wide studies have identified a large number of genetic variants that appear more often in autistic people. Each one contributes a small amount. Having several does not produce a diagnosis on its own, and no single variant has been found that causes autism by itself.
Carrying Risk Without a Diagnosis
This is the mechanism behind the apparent skipping. Someone can carry variants associated with autism, show some traits, or show none at all, and never be diagnosed. They can still pass those variants to a child, who may inherit a different combination, or combine them with variants from the other parent, and reach the diagnostic threshold.
Nothing skipped. The grandparent's contribution was present and simply did not add up to a diagnosis in them.
Spontaneous (De Novo) Mutations
Some autism arises from mutations that occur during the formation of an egg or sperm cell. These are not inherited from either parent and are unique to that child, which is why autism appears in families with no history of it at all.
It is worth being direct about this: these are random events. They are not caused by anything a parent did before or during pregnancy.
What the Family Numbers Show
The most useful evidence for families comes from following younger siblings of autistic children from infancy. The Baby Siblings Research Consortium did exactly that, and published an update in Pediatrics in 2024 covering 1,605 younger siblings.
Group | Proportion later diagnosed |
All younger siblings | 20.2% |
Younger brothers | 25.3% |
Younger sisters | 13.1% |
Families with one autistic child | 21.2% |
Families with more than one autistic child | 36.9% |
Roughly one in five, then, with the likelihood about double for boys, and markedly higher where more than one child is already diagnosed.
Two things make this figure more trustworthy than most. It comes from following children prospectively rather than asking families to recall, and it has barely moved since the 2011 estimate of 18.7%, despite large increases in general-population prevalence over the same period. Diagnostic expansion does not appear to explain it.
Read the number the right way round, though. One in five means four in five younger siblings were not diagnosed.
What Twin Studies Do and Do Not Tell Us
Identical twins share all of their DNA effectively and show much higher concordance for autism than fraternal twins, who share about half. That comparison is the strongest evidence for a genetic contribution, and it is sound.
A comparison you will sometimes see made between fraternal twins and non-twin siblings does not work the same way. Fraternal twins and full siblings share the same expected proportion of DNA, about 50%, so any difference between them cannot be explained by extra shared genes. It is also worth noting that the large cohort study above found shared environmental contributions to be minimal, which makes that particular comparison a weak place to rest an argument.
Environmental Factors
Genetics accounts for most of the variation, but not all of it, and some prenatal exposures have been associated with increased likelihood.
The best established is valproic acid, an anticonvulsant, where prenatal exposure carries a recognised association. Certain maternal infections during pregnancy, including rubella and cytomegalovirus, have also been linked in the literature. Other proposed factors, including air pollution and pesticide exposure, remain under investigation with mixed findings.
Two cautions matter here. Association is not causation, and the research in this area is genuinely unsettled. And none of this is a list of things parents should be auditing themselves against retrospectively. Discuss specific concerns about medication during pregnancy with the prescribing clinician, who can weigh them against the risks of stopping treatment.
What This Means If You Are Planning a Family
If you have one autistic child and are thinking about another, the roughly one-in-five figure is the honest starting point, adjusted upward if you already have more than one autistic child or if the next baby is a boy.
A genetic counsellor can give you something better than a population average. They can take your actual family history, discuss whether genetic testing would tell you anything useful in your case, and explain what the results would and would not mean. That is a more productive conversation than trying to read a family tree yourself.
Our posts on how autism develops and autism statistics in North Carolina cover adjacent ground, and IQ in nonverbal autistic people covers why presentation varies so much even within one family.
If You Have a Younger Child
This is where the research becomes something you can act on, and the mechanism is the same in both states: you do not need a diagnosis, or even a confirmed concern, to get a young child evaluated.
In North Carolina, the Infant-Toddler Program serves children up to age three. Once a referral is made, development of the initial Individualized Family Service Plan must happen within 45 days, and services in the plan should begin no later than 30 days after it is signed. The plan is reviewed at least every six months. Evaluations and service coordination are provided at no charge.
Virginia works the same way through the Infant & Toddler Connection, which serves children from birth to 36 months who are not developing as expected, with no diagnosis required and services available regardless of a family's ability to pay.
For a family who already has one autistic child, this is the practical upshot of a one-in-five figure. You are not waiting for certainty before anyone will look. Early intervention ABA is built to work alongside an existing plan rather than duplicate it, and families we work with through our Charlotte office often start that conversation well before any formal evaluation.
Conclusion
Autism does not skip generations, because it was never following the one-gene pattern that idea depends on. Heritability is high, estimated at around 80% in a cohort of more than two million people, but it is spread across many genes that can be carried without producing a diagnosis and recombined differently in the next generation.
For families, the figure that matters is a roughly one in five chance that a younger sibling will also be diagnosed, higher for boys and higher again where more than one child already is. That is a reason to know what early evaluation routes exist in your state, not a reason to assume an outcome. And it is worth repeating that no credible evidence supports maternal behaviour during pregnancy as a cause.
Ready to Take the Next Step?
Connect n Care ABA provides in-home ABA therapy, school-based ABA, and ABA parent training to families across North Carolina and Virginia.
If you have one child diagnosed and questions about a younger one, contact our team.
SOURCES:
- https://jamanetwork.com/journals/jamapsychiatry/fullarticle/2737582
- https://publications.aap.org/pediatrics/article/154/2/e2023065297/197777/Familial-Recurrence-of-Autism-Updates-From-the
- https://www.ncdhhs.gov/nc-itp-step-step/open
- https://dbhds.virginia.gov/behavioral-health/child-and-family-services/
- https://www.cdc.gov/autism/signs-symptoms/index.html
- https://www.cdc.gov/autism/hcp/diagnosis/index.html
