The genetic mutation that causes Huntington disease is located on chromosome 4. It involves repetition of a particular section of the genetic code in the DNA. The larger the number of repeats, the earlier symptoms start.
The gene for Huntington disease is dominant. Thus, having only one copy of the abnormal gene, inherited from one parent, is sufficient to cause the disease. Almost all people with the disease have only one copy of the abnormal gene. Children of such people have a 50% chance of inheriting the abnormal gene and thus the disease.
People who have a parent or grandparent with Huntington disease can find out whether they have inherited the gene for the disease by taking a genetic test. For the test, a blood sample is taken and analyzed. Such people may or may not want to know whether they have inherited the gene. This issue should be discussed with an expert in genetic counseling before genetic testing.