All humans possess two alleles for the Huntingtin (HTT) gene. This gene provides instructions for making the protein huntingtin (Htt). Part of the HTT gene contains repeats of the trinucelotide cytosine-adenine-guanine (CAG).

This repeat varies in length between individuals, but when it reaches a certain level of expansion, a mutated huntingtin protein (mHtt) is produced that is no longer normal and is damaging to brain cells. As the abnormal protein accumulates, it causes pathological changes that lead to the symptoms of Huntington’s disease.

Huntington’s disease is inherited in an autosomal dominant manner. All nucleated cells of the human body contain 46 chromosomes, with 23 derived from each parent. Of these 23 pairs of chromosomes, one pair is the sex chromosomes (X and Y chromosomes) and the other 22 pairs are the autosomal chromosomes.

Autosomal inheritance means that disease is inherited through a mutation that occurs in one of these autosomal chromosomes. In the cases of Huntington’s disease, the specific chromosome involved is chromosome 4.  Dominant inheritance means that only one mutated copy of the gene needs to be passed on to offspring in order for the disease to manifest.  This form of inheritance pattern means that each child born to a parent who has the mutation is at a 50% risk of inheriting the condition. As Huntington’s disease is inherited via an autosome rather than a sex chromosome, men and women are affected equally by the disease.

As expanded copies of HTT are passed on through generations, the number of repeats can increase in successive generations causing a more severe disease to occur at an earlier age of onset than in previous generations. This is referred to as genetic anticipation. In around 6% of cases, Huntington’s disease develops in people aged under 21 years. This rapidly progressing form of the condition is referred to as juvenile, akinetic-rigid or Westphal variant HD.