Huntington’s disease is an autosomal dominant genetic disorder, meaning it takes only one copy of the mutated gene to cause the condition. If one of your parents carries the mutation, you have a 50% chance of inheriting it. Unlike many genetic conditions that require both parents to pass along a faulty gene, Huntington’s needs just one copy to eventually produce symptoms.
What Autosomal Dominant Means
Your DNA is organized into 23 pairs of chromosomes. “Autosomal” means the gene sits on one of the 22 non-sex chromosomes, so Huntington’s affects men and women equally. “Dominant” means a single altered copy overrides the normal copy you got from your other parent. There’s no carrier state where you silently pass the gene along without being affected yourself. If you have the mutation, you will almost certainly develop the disease at some point in your life.
Most people with Huntington’s inherit it from an affected parent. About 10% of cases, however, appear without any known family history. These arise when a borderline-length gene expansion in a parent, one that wasn’t long enough to cause disease in them, stretches further during reproduction and crosses the threshold in their child.
The CAG Repeat Expansion
The specific mutation behind Huntington’s is an abnormal stretch of repeating DNA in the HTT gene on chromosome 4. A short segment of genetic code, the letters C-A-G, repeats over and over. Everyone has some CAG repeats in this gene. The number of repeats determines whether the gene functions normally or causes disease.
- 26 or fewer repeats: Normal. No risk of Huntington’s.
- 27 to 35 repeats: Intermediate. You won’t develop Huntington’s, but the repeat count is unstable and could expand into the disease range when passed to your children.
- 36 to 39 repeats: Reduced penetrance. Some people in this range develop the disease, others don’t.
- 40 or more repeats: Full penetrance. The disease will develop, typically in midlife.
- More than 60 repeats: Associated with juvenile-onset Huntington’s, where symptoms appear before age 20.
The HTT gene normally provides instructions for making a protein called huntingtin, which plays a role in nerve cell function. When the CAG repeat expands beyond the normal range, the gene produces a misshapen version of this protein that gradually damages neurons, particularly in areas of the brain that control movement, thinking, and emotion.
Genetic Anticipation and Paternal Inheritance
One of the more striking features of Huntington’s is a phenomenon called genetic anticipation: the disease can appear earlier and more severely in each successive generation. This happens because CAG repeats are unstable. When the gene is copied and passed to the next generation, the repeat count can grow.
Paternal transmission drives this process far more than maternal transmission. Research from Johns Hopkins University confirmed that when fathers pass on the expanded gene, the repeat count tends to increase, and longer repeats correlate directly with earlier symptom onset. Mothers with Huntington’s can pass the gene to their children, but the repeat length typically stays more stable. This is why juvenile-onset cases, those with very large expansions above 60 repeats, overwhelmingly come from affected fathers.
How Genetic Testing Works
The gene responsible for Huntington’s was identified in 1993, and a direct genetic test has been available since then. The test counts the number of CAG repeats in your HTT gene using a simple blood draw. Results fall into the ranges described above, giving a clear answer in most cases.
Testing protocols typically include genetic counseling before and after the test, a neurological exam, and a psychological assessment. These steps exist because learning you carry the mutation means learning you will develop a progressive, currently incurable neurological disease. Not everyone at risk chooses to be tested, and guidelines strongly recommend that minors not be tested unless they’re already showing symptoms, since the information can’t be acted on medically in childhood and removes the individual’s future choice about whether to know.
For people who already show movement problems, cognitive changes, or psychiatric symptoms and have a family history of Huntington’s, genetic testing confirms the diagnosis. For those without symptoms but with an affected parent, predictive testing tells them whether they carry the expansion.
Reproductive Options for Carriers
Because the transmission risk is 50% for each pregnancy, people who carry the Huntington’s mutation have several options if they want biological children without passing on the gene. Preimplantation genetic testing (PGT-M) allows couples using IVF to test embryos for the CAG expansion before implantation. Only embryos without the mutation are transferred, effectively eliminating the risk of transmission. Prenatal testing through amniocentesis is another option, though it carries the difficult possibility of deciding whether to continue a pregnancy based on the results.
PGT-M is also available for individuals at 25% risk, those with a grandparent who had Huntington’s but a parent who hasn’t been tested. In some cases, the testing can be structured to determine whether the embryo inherited the at-risk chromosome without revealing the parent’s own status, preserving their choice not to know.
How Common Is Huntington’s Disease
Huntington’s is rare but not vanishingly so. A 2023 meta-analysis found prevalence of about 8.87 per 100,000 people in North America and 6.37 per 100,000 in Europe. The disease is far less common in Asian and African populations, with rates below 1 per 100,000. This geographic variation likely reflects differences in the background frequency of intermediate-length CAG repeats across populations, meaning some gene pools carry more borderline-length repeats that are closer to expanding into the disease range.
Symptoms most commonly appear between ages 30 and 50, though the juvenile form can start in childhood or adolescence. The disease progresses over 10 to 25 years, gradually affecting movement, cognition, and behavior. Because onset typically happens after the prime reproductive years have begun, many people unknowingly pass the gene to their children before they develop symptoms themselves, which is one reason the mutation persists in the population despite its severity.