The genetics of Huntington’s disease

A gene is a piece of biological information you inherit from your parents. It is present in every cell of the body and tells cells what to do. Genes control cells by producing proteins; each gene is really a kind of recipe for making a certain protein.

Genes are usually attached to a chromosome (a strand of DNA containing many different genes). Each human cell contains around 25,000 genes and most have 23 pairs of chromosomes.

The Huntington’s gene - the gene that determines whether you will develop Huntington’s disease - is attached to chromosome pair number 4.

Some facts about genetics and Huntington’s:

  • If you have Huntington’s disease, this means you inherited a faulty version of the Huntington’s gene and the recipe for the protein it produces is incorrect.
  • The faulty gene that causes Huntington’s repeats a particular coding sequence known as CAG (cytosine-adenine-guanine) too many times - a bit like adding too much of one ingredient in a recipe. This means the protein it makes damages nerve cells in the brain.
  • If you have 40 or more CAG repeats, it is certain that you will develop Huntington’s at some point.
  • Huntington’s is what’s known as an “autosomal dominant disorder”. In plain English, this means that you can inherit the gene, and therefore the disease, from only one parent. One of your parents is likely to have Huntington's too.
  • Every child conceived naturally to a parent who has the faulty gene has a 50 % chance of inheriting it and the disease. If both parents have the faulty gene the child has a 75 % chance of inheriting it.

You can find out if you carry the faulty gene by taking a blood test known as a predictive test. You need to be 18 years old to take the test.