Huntington’s disease follows an autosomal dominant inheritance pattern, meaning a child needs only one copy of the altered gene to be at risk. If one parent carries the Huntington’s mutation, each child has a 50% chance of inheriting it. The gene sits on chromosome 4, and because it’s not on a sex chromosome, it affects men and women equally.
The Gene Behind Huntington’s Disease
Huntington’s disease is caused by a mutation in a gene called HTT, which provides instructions for making a protein called huntingtin. Inside this gene, a short sequence of DNA building blocks (C, A, and G) repeats itself in a row, much like a word typed over and over. In most people, this sequence repeats 10 to 35 times, and the gene functions normally.
In people with Huntington’s disease, that same sequence repeats 36 to more than 120 times. This expansion causes the body to produce an abnormally long version of the huntingtin protein. The oversized protein gets cut into smaller, toxic fragments that clump together inside nerve cells, gradually disrupting their function and eventually killing them. This is what drives the progressive movement problems, cognitive decline, and psychiatric symptoms of the disease.
What the Repeat Count Means
Not every expanded gene leads to the same outcome. Genetic testing measures the exact number of repeats, and the results fall into distinct ranges:
- 26 or fewer repeats: Normal. No risk of developing or passing on Huntington’s disease.
- 27 to 35 repeats (intermediate): The person won’t develop Huntington’s, but the gene is somewhat unstable. In rare cases, it can expand into the disease-causing range when passed to a child.
- 36 to 39 repeats (reduced penetrance): The person may or may not develop symptoms during their lifetime. Some people in this range live without ever showing signs of the disease.
- 40 or more repeats (full penetrance): The person will almost always develop Huntington’s disease at some point in their life.
Higher repeat counts tend to correlate with earlier symptom onset. People with very high counts, sometimes 60 or more, can develop a juvenile form of the disease that appears before age 20.
The 50/50 Chance, Explained
Everyone carries two copies of chromosome 4, one inherited from each parent. A person with Huntington’s disease typically has one normal copy of the HTT gene and one expanded copy. When they have a child, they pass on one of those two copies at random. That’s where the 50% risk comes from: it’s essentially a coin flip for each pregnancy, and the outcome for one child has no effect on the odds for the next.
If a child doesn’t inherit the expanded gene, they can’t pass it on. The chain stops with them. If they do inherit it, they carry the same 50% chance of passing it to their own children.
Why the Gene Can Get Worse Over Generations
One of the more unsettling features of Huntington’s disease is a phenomenon called genetic anticipation. The CAG repeat sequence is unstable, and it can grow longer as it’s passed from parent to child. When the repeat count increases, the next generation may develop symptoms earlier or more severely than the parent did.
This expansion is more likely to happen when the gene is inherited from the father. During sperm production, the repeat sequence is more prone to expanding than during egg production. This is why cases of juvenile Huntington’s are more commonly traced to paternal inheritance. Research in the Journal of Medical Genetics has shown that the tendency toward anticipation is inherited through the male line, though the mechanism involves changes in how the gene is chemically tagged during development rather than a change in the DNA sequence itself.
Cases Without a Family History
About 10% of Huntington’s disease cases appear without any known family history. These are sometimes called de novo cases, and they typically arise when a parent with an intermediate repeat count (27 to 35) passes on a gene that expands into the disease-causing range during reproduction. The parent never had symptoms themselves, so the diagnosis in their child comes as a complete surprise.
In other cases, a family history exists but was hidden. A parent or grandparent may have died young from other causes before symptoms appeared, or the diagnosis may have been attributed to something else. Genetic testing has made it easier to trace these hidden lineages, but it also means a negative family history doesn’t completely eliminate the possibility.
Genetic Testing and Family Planning
Predictive genetic testing is available for adults who have a parent with Huntington’s disease but haven’t developed symptoms. The test measures the exact CAG repeat count from a blood sample. Because the results are life-altering and irreversible, genetic counseling before and after testing is standard practice. Some people choose not to be tested, preferring to live without knowing their status.
For those who know they carry the expanded gene and want biological children, preimplantation genetic testing offers one option. During IVF, embryos are screened for the Huntington’s mutation before being transferred to the uterus. This form of testing targets single-gene disorders specifically. It allows a carrier to have a biological child without passing on the expanded gene, though false positives and false negatives can occur. Prenatal diagnostic testing through procedures like amniocentesis is typically offered as confirmation during pregnancy.
There’s also a testing approach for people who don’t want to learn their own genetic status. Called exclusion testing, it uses DNA from the at-risk person’s parents to determine whether an embryo or fetus inherited the chromosome 4 that came from the affected grandparent, without revealing whether that chromosome actually carries the expansion. This preserves the parent’s choice not to know while still reducing the risk for the next generation.