# Inheritance & Genetics This page explains how PRRT2-related conditions are passed through families — and why the same gene can look so different from one relative to the next. ## Two copies, and one is enough Every person carries two copies of the PRRT2 gene, one inherited from each parent. PRRT2-related conditions follow a pattern geneticists call **autosomal dominant** inheritance. *Autosomal* means the gene is not on a sex chromosome — it affects everyone equally, regardless of sex. *Dominant* means that **just one altered copy is enough** to cause a condition; a person does not need both copies to be affected. ## The 50% chance Because only one altered copy is needed, a parent who carries a PRRT2 variant has a **one-in-two (50%) chance of passing it to each child**. Each pregnancy is independent — having one child who inherits the variant says nothing about the next. Some people, though, are the *first* in their family to carry a PRRT2 variant. This is called a **de novo** ("new") variant — a change that arose fresh rather than being inherited. From that person on, it can be passed down in the usual way. ## Why the same gene looks different in different people This is one of the most important — and most confusing — features of PRRT2. Two ideas explain it. **Reduced penetrance.** Not everyone who carries a PRRT2 variant develops symptoms at all. Some carriers go through life completely unaffected, while still able to pass the variant to their children. Penetrance for PRRT2 is high, but **incomplete**. **Variable expressivity.** Among those who *are* affected, the condition does not show up the same way. Within a single family, carrying the very same variant, one person may have paroxysmal kinesigenic dyskinesia, another infantile seizures, another migraine — and another may have nothing at all. Together, these two features mean a PRRT2 variant can move quietly and unpredictably through a family tree. ## The common variant Many different changes in the PRRT2 gene have been identified, but one is far more common than the rest: a variant known as **c.649dupC**. It accounts for a large share of all cases. There is a reason it recurs so often. That part of the gene contains a short repeated run of a single DNA letter, and stretches like that are prone to small copying errors when DNA is duplicated. The same mistake therefore happens independently, again and again, in unrelated families — which is why c.649dupC turns up worldwide. Most PRRT2 variants, including this one, are **loss-of-function** changes: they reduce or remove the working protein, leading to the haploinsufficiency described in [How the PRRT2 Gene Works](/prrt2-gene-overview/how-the-gene-works.md). Rarely, a person inherits an altered copy from *both* parents. This is uncommon, and tends to cause a more severe condition. ## What this means for families Because PRRT2 is inherited and often under-recognized, a diagnosis in one person is rarely just about that person. It can be a clue to a whole family. Relatives with seemingly unconnected histories — a childhood seizure here, episodes of odd movement there, migraines in someone else — may in fact share the same underlying variant. Genetic testing and genetic counseling can help a family understand its situation. The practical side of testing is covered in [Diagnosis & Genetic Testing](/diagnosis-and-genetic-testing/how-to-get-tested.md). {% hint style="success" %} **🧭 PRRT2.org Perspective** *PRRT2.org's own perspective — our synthesis and lived experience, offered alongside the established science, not as established medical fact.* Reduced penetrance and variable expressivity are not just technical genetics terms. They are, in our view, a large part of *why PRRT2 stays hidden*. When the same variant produces a seizure in one relative, a movement problem in another, a migraine in a third, and nothing at all in a fourth, families — and even doctors — rarely connect the dots. Each person is assessed alone, given a separate label, and the single thread running through the family is never seen. PRRT2.org's view is that **family history is one of the most powerful and most underused clues** for recognizing PRRT2 — and that testing a single person can sometimes bring an entire family's scattered history into focus. If your relatives have unexplained paroxysmal symptoms, that pattern is worth paying attention to. {% endhint %} ## Sources The established science on this page is drawn from: * [MedlinePlus Genetics — PRRT2 gene](https://medlineplus.gov/genetics/gene/prrt2/) — U.S. National Library of Medicine, NIH * [GeneReviews — Familial Paroxysmal Kinesigenic Dyskinesia](https://www.ncbi.nlm.nih.gov/books/NBK1460/) — University of Washington / NCBI * [OMIM — Entry 614386](https://omim.org/entry/614386) — Johns Hopkins University For the complete set of authoritative references, see [Official Resources](/resources/official-resources.md). ## Continue reading * [How to Get Tested](/diagnosis-and-genetic-testing/how-to-get-tested.md) — pursuing a genetic diagnosis * [The PRRT2 Spectrum](/associated-conditions/prrt2-spectrum.md) — the conditions associated with the gene * [Introduction to PRRT2](/prrt2-gene-overview/introduction.md) — the overview, if you'd like to start from the beginning --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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