Huntington’s disease follows an autosomal dominant inheritance pattern, meaning a child needs only one copy of the altered gene to develop the condition. If one of your parents carries the Huntington’s mutation, you have a 50% chance of inheriting it. About 90% of people with Huntington’s inherited the gene from an affected parent, while roughly 10% develop it through a new genetic change with no prior family history.
The Gene Behind Huntington’s Disease
Huntington’s disease is caused by a mutation in the HTT gene on chromosome 4. Inside this gene, a short DNA sequence (CAG) repeats itself over and over, like a stutter in the genetic code. Everyone has some CAG repeats in this gene. The question is how many.
The number of repeats determines whether someone will develop the disease:
- 26 or fewer repeats: Normal. No risk of Huntington’s.
- 27 to 35 repeats: Intermediate. The person won’t develop symptoms, but the repeat count can expand when passed to a child.
- 36 to 39 repeats: Reduced penetrance. Some people in this range develop symptoms and others don’t.
- 40 or more repeats: Full penetrance. The person will develop Huntington’s if they live long enough.
When the repeat count crosses into the disease range, the HTT gene produces an abnormal version of a protein called huntingtin. This defective protein accumulates in brain cells over time, gradually damaging them and causing the movement, cognitive, and psychiatric symptoms associated with the disease.
Why Only One Parent’s Gene Matters
You carry two copies of nearly every gene, one from each parent. With autosomal dominant conditions like Huntington’s, a single defective copy overrides the normal one. That’s why having just one affected parent creates a 50/50 chance for each child, regardless of whether the affected parent is the mother or the father. The gene sits on chromosome 4, not on the X or Y chromosome, so sex plays no role in who inherits it.
If the affected parent carries one normal copy and one expanded copy (which is the typical situation), each pregnancy is essentially a coin flip. Having one unaffected sibling doesn’t change the odds for the next child. Each conception is independent.
How Repeats Expand Across Generations
One of the more unsettling features of Huntington’s inheritance is that the CAG repeat count can grow from one generation to the next. A parent with 42 repeats might pass on 45 or 50 to a child. This means the child can develop symptoms earlier and more severely than the parent did. Geneticists call this phenomenon “anticipation.”
Anticipation is far more pronounced when the father carries the mutation. Research using data from the U.S. National Huntington’s Disease Roster found that children of affected mothers generally developed symptoms around the same age as their mothers. But a subset of children who inherited the gene from their fathers showed onset roughly 24 years earlier. This happens because sperm production involves many more rounds of DNA copying than egg production, creating more opportunities for the repeat sequence to expand.
This is why juvenile Huntington’s disease, which strikes before age 20, is most often inherited from the father. These cases typically involve very high repeat counts, sometimes 60 or more, that expanded dramatically during paternal transmission.
New Mutations and Intermediate Alleles
About 10% of Huntington’s cases appear in people with no known family history of the disease. These cases often trace back to a parent who carried an intermediate allele, in the 27 to 35 repeat range. The parent never developed symptoms, but the unstable repeat sequence expanded into the disease-causing range when passed to their child.
Not all intermediate alleles carry the same risk. Repeats at the lower end of the intermediate range (27 to 32) are relatively stable. But at 33 repeats and above, the risk of expansion climbs sharply. A parent with 34 CAG repeats has about a 2.4% chance of the repeat expanding into the disease range in a child. At 35 repeats, that risk jumps to 21%. These expansions are more likely to occur during sperm production, which again makes paternal transmission the primary route for new mutations.
Reduced Penetrance: The Gray Zone
People with 36 to 39 CAG repeats fall into a gray zone. Some will develop Huntington’s symptoms during their lifetime and others won’t. The reasons aren’t fully understood, but likely involve a combination of other genetic factors and the degree of repeat instability in different brain regions. If you or a family member has a genetic test result in this range, the uncertainty can be particularly difficult to navigate, and genetic counseling becomes especially important for understanding what it means in practical terms.
Genetic Testing and Family Planning
Predictive genetic testing can tell you whether you carry the expanded HTT gene before symptoms appear. This is a straightforward blood test that counts the number of CAG repeats. The decision to get tested is deeply personal: some people at risk want to know so they can plan, while others prefer not to live with that knowledge.
For people who carry the mutation (or are at risk) and want biological children without passing on the gene, several reproductive options exist. The most established is preimplantation genetic diagnosis (PGD), which involves creating embryos through IVF, testing each embryo’s DNA for the CAG expansion, and transferring only unaffected embryos. This approach has been available at specialized centers since the late 1990s.
There’s also a more unusual option called exclusion testing, designed for people who are at risk but don’t want to learn their own genetic status. Instead of testing for the mutation directly, this method tracks which copy of chromosome 4 each embryo inherited. Embryos that received their copy from the unaffected grandparent are selected for transfer. The at-risk parent never finds out whether they actually carry the gene. This approach requires DNA samples from family members, including the affected grandparent, which isn’t always possible. Prenatal testing during pregnancy (with the option of termination) is also available in many countries, though most couples who pursue genetic selection now choose the IVF-based route.
What 50% Risk Actually Means in a Family
When a parent has Huntington’s, each child independently has a 50% chance of inheriting the expanded gene. In a family with four children, it’s possible that all four inherit it, none do, or any combination in between. The 50% figure is a probability per child, not a guarantee that exactly half will be affected.
Grandchildren of an affected person also face risk, but only if their parent (the affected person’s child) carries the gene. If your parent was at risk but tested negative, your risk drops to essentially zero. If your parent was at risk and has not been tested, your statistical risk is 25%, half of your parent’s 50% chance.
One nuance worth knowing: the gene cannot skip a generation. If your parent truly does not carry the expanded repeat, they cannot pass it on. The only exception is the rare scenario involving intermediate alleles, where a parent with a borderline repeat count might produce a child whose repeats have expanded into the disease range. But this is a new mutation, not a skipped generation.