Huntington’s disease is autosomal dominant. That means you only need one copy of the mutated gene, inherited from one parent, to develop the condition. If one of your parents carries the mutation, you have a 50% chance of inheriting it. Unlike recessive conditions, where you need two copies of a faulty gene (one from each parent) before symptoms appear, a single copy of the Huntington mutation is enough to cause disease.
What “Autosomal Dominant” Means in Practice
Your DNA contains two copies of nearly every gene, one from each parent. In a recessive disorder, the healthy copy can compensate for the faulty one, so you’d need both copies to be mutated before you’d get sick. Huntington’s works differently. The mutated copy of the HTT gene on chromosome 4 produces an abnormal protein that is actively toxic to brain cells. Having a perfectly normal second copy doesn’t protect you, because the damage comes from what the mutant protein does, not from what’s missing.
This is called a “toxic gain of function.” The mutant huntingtin protein interferes with the brain’s normal protective mechanisms. It weakens the cell’s ability to block self-destruction pathways and allows harmful processes to ramp up, particularly in the striatum, the brain region that coordinates movement. The normal copy of the gene keeps doing its job, but it simply can’t counteract the damage the abnormal protein causes.
Huntington’s is also described as having “complete penetrance” at higher mutation levels. That means if you carry a large enough mutation, you will develop the disease. It’s not a matter of probability or lifestyle factors. The mutation guarantees it.
The CAG Repeat: Why Some People Get It and Others Don’t
The mutation behind Huntington’s is an unusually long stretch of repeated DNA letters (C-A-G) inside the HTT gene. Everyone has some CAG repeats in this gene. The number you have determines your risk:
- 10 to 26 repeats: Normal. No risk of Huntington’s and no risk of passing it on.
- 27 to 35 repeats: You won’t develop Huntington’s yourself, but the repeat can expand when passed to your children, potentially putting them at risk.
- 36 to 39 repeats: A gray zone. Some people in this range develop symptoms and others never do. This is called “reduced penetrance.”
- 40 or more repeats: Full penetrance. You will develop Huntington’s disease.
People with Huntington’s typically carry between 36 and 120 or more repeats. The length of the repeat also influences when symptoms start. Longer repeats generally mean earlier onset. The juvenile form of the disease, which begins before age 20, is usually associated with more than 60 repeats.
Why Having Two Copies Isn’t Worse Than One
Here’s something that surprises even geneticists. For almost every other dominant disorder ever studied, people who inherit the mutation from both parents (homozygotes) have more severe symptoms than people who inherit it from just one parent. Huntington’s breaks this rule. Research identifying individuals who carry two mutant copies found that their symptoms and disease progression look no different from people with just one. Huntington’s appears to be the first genetically confirmed human disease that shows complete phenotypic dominance, meaning one copy produces the full effect and a second copy adds nothing extra.
Anticipation: Repeats Can Grow Across Generations
CAG repeats are unstable. They can expand when the gene is passed from parent to child, a phenomenon called genetic anticipation. When this happens, the next generation may develop symptoms at a younger age than their affected parent did.
The risk of significant expansion is higher when the gene is inherited from the father. In studies of families with Huntington’s, offspring of affected mothers tended to develop symptoms around the same age as their mothers. But a subset of children who inherited the gene from affected fathers showed onset roughly 24 years earlier. This is one reason juvenile Huntington’s most often traces back through the paternal line. The instability appears to be linked to how DNA is processed during sperm development, where the long repeat sequences are more prone to expanding.
This also explains why people in the 27 to 35 repeat range, who will never get Huntington’s themselves, can still have children who do. Their borderline-length repeats can stretch past the critical threshold in the next generation.
Genetic Testing and What It Involves
Because the inheritance pattern is so clear cut, genetic testing for Huntington’s is straightforward in principle. A blood test counts the number of CAG repeats in your HTT gene. If you have 40 or more, the result is definitive.
The decision to get tested, however, is anything but simple. International guidelines recommend that predictive testing for people without symptoms be requested through a clinical geneticist, with comprehensive genetic counseling before and after. Testing is generally offered to people who have a 50% risk (one affected parent) or a 25% risk (one affected grandparent). Testing of minors under 18 is considered appropriate only in exceptional circumstances, since there is currently no way to prevent or cure the disease and the psychological burden of a positive result is significant.
If the familial diagnosis hasn’t been confirmed by genetic testing in an affected relative, a negative result rules out Huntington’s but doesn’t necessarily rule out whatever condition is actually running in the family. This distinction matters and is something genetic counselors will explain.
Family Planning Options
The 50/50 inheritance risk makes family planning a major concern for carriers. Preimplantation genetic testing (PGT-M) allows couples to use IVF, test embryos for the Huntington mutation before implantation, and transfer only unaffected embryos. This avoids the difficult decision of terminating a pregnancy after a positive prenatal diagnosis. Prenatal testing through amniocentesis or chorionic villus sampling remains an option as well, though it carries the emotional weight of deciding what to do if results come back positive.
Some people who are at risk but don’t want to learn their own genetic status can still use PGT-M through a process called exclusion testing, which uses genetic markers to determine whether an embryo inherited the at-risk chromosome without revealing the parent’s own status. These decisions are deeply personal and vary by individual circumstances, but the dominant inheritance pattern at least makes the genetic testing itself reliable and clear.
Current Treatment Landscape
Because a single dominant gene drives the entire disease, Huntington’s has become a prime target for gene-silencing therapies. Several clinical trials are testing treatments designed to reduce the amount of toxic huntingtin protein the brain produces. One approach uses RNA interference, a technique that intercepts the gene’s instructions before the harmful protein is made. Another uses antisense oligonucleotides, short pieces of synthetic genetic material that bind to the gene’s messenger RNA and mark it for destruction. As of early 2025, multiple trials are underway across North America and Europe, testing both selective approaches (targeting only the mutant copy) and non-selective ones (reducing all huntingtin protein, normal and mutant alike). None have reached approval yet, but the dominant, single-gene nature of the disease makes it more targetable than conditions involving dozens of genes.