What causes Huntington disease? How does the mutation occur?

Huntington's disease (Huntington's chorea) is an inherited disease in which people in midlife begin having occasional jerks, or spasms and gradual loss of brain cells, progressing to chorea,  athetosis,  and mental deterioration.

The gene for Huntington's disease is dominant; therefore, children of people who have this disease have a 50 % chance of developing it. Because Huntington's disease begins subtly, the exact age of onset is difficult  to determine. Symptoms usually begin between ages of 35-40.

The gene mutation that causes Huntington's disease has been identified .Of the 23 pairs of human chromosomes, chromosome 4 carries the defective gene; a person who has HD carries the defective gene on one of the two copies of chromosome 4. But the crucial question is whether the normal or the abnormal chromosome 4 was passed on to a child---the odds are 50/50.

People who have a parent with HD can find out whether they have inherited the disease. Usually the DNA next to the HD gene on the parent's abnormal chromosome 4 is different from the corresponding segment of DNA on the parent's normal chromosome 4. Blood tests can determine whether a person has inherited the neighboring DNA fragment from the abnormal or the normal chromosome 4.

The odds are high that a person who has inherited the DNA next to the HD gene has also inherited the defective gene. New tests make it possible to determine whether the HD gene itself has been inherited. 

Huntington’s disease is a genetic disorder. It is a rare dominant mutation meaning if you have the mutation, you will develop Huntington’s. If a parent has Huntington’s, then their kids have a 50% chance of having Huntington’s as well. 

I don’t think they know what caused the mutation, but once that gene is mutated it moves down their genetic line. Huntington’s doesn’t tend to show symptoms until your 30s or 40s meaning you may already have children before you know you have it. The gene is known and can be found through genetic testing if there is a history of Huntington’s in their family which can be done before symptoms show up. This can help make decisions of whether or not to have kids. 

I attached a site that you might find helpful. 

It’s genetic.