Can a Twin Be Autistic and the Other Twin Be Neurotypical Even With Identical DNA? Yes, and this is one of the clearest pieces of evidence that autism isn’t purely deterministic genetics.

Identical twins share DNA, but not fate. Learn how epigenetics and non-shared environments explain autism discordance in monozygotic pairs.

It is possible for only one twin to have autism, a phenomenon known as “discordance” in twin studies. This occurs when one twin is diagnosed with a condition while the other is not, indicating that autism’s origins are not solely genetic, even with high genetic similarity.

Variable expressivity, a fundamental concept in genetics, directly addresses the observation that identical twins can display differing phenotypes, including one being diagnosed with autism spectrum disorder (ASD) while the other is not.

For instance, consider a set of identical twins where one exhibits the characteristic social communication difficulties and repetitive behaviors associated with ASD, while the other displays neurotypical development. Understanding why this divergence occurs is crucial for several reasons.

Myth, If autism runs in the family or shows up in one identical twin, the other twin is guaranteed to develop it too. Reality, Even with identical genes, a substantial share of identical twin pairs are discordant for autism.

Identical twins with autism spectrum disorder (ASD) often experience large differences in symptom severity even though they share the same DNA, according to an analysis funded by the National Institutes of Health.

Can one identical twin have autism and the other not? Yes, in roughly 10-25% of cases, one identical twin remains neurotypical, proving that DNA is not the only factor.

Twin studies offer critical insight into autism, defining the balance between high genetic heritability and non-shared environmental influences.

For decades, researchers have looked at twins to understand why autism happens, how much of it is genetic, and what it means when one twin is diagnosed. The answers are nuanced, sometimes surprising, and — most importantly — genuinely useful for parents like us.