HD is caused by a genetic defect on chromosome 4. It causes a part of DNA, called a CAG repeat, to occur many more times than it's supposed to. Normally, this section of DNA is repeated 10 to 28 times, but for a person with HD, it is repeated 36 to 120 times. It is an inherited disease, so if one parent has it there is a 50% chance of the child having it. It is not a sex-linked disease, which means that it is autosomal. This means that men and women are at an equal risk of getting it. HD is a dominant gene, so most people have one gene of each. However, if someone has a parent who is suffering the disease, but that person doesn't inherit it, they is no way for them to pass it on.