Inheritance of HD

Genes come in pairs, so every individual has two copies of every gene, including the HD gene. If one of pair of the HD genes has a genetic change, or mutation, this individual will develop HD. An individual only needs one gene to contain a mutation to develop the disease.

Parents pass on one copy of each pair of their genes to their children, so that each child receives one copy of every gene from their mother and one from their father. This process is random; meaning that if a parent has an HD gene with a mutation there is a 50:50, or 1 in 2 chance to pass on the gene with a mutation to each child. This is illustrated in the following family tree, also known as a ‘pedigree’.

Sometimes an individual is diagnosed with HD and they have no family history of the condition. This can happen because previous family members may have carried the HD causing gene but not been diagnosed with the disease (or misdiagnosed with another condition), or because they died before onset of HD symptoms. HD can also be caused by a new change (or mutation) in the HD gene in that individual which was not inherited, but occurred by chance during their fertilization. Speaking to a genetic counselor can help you understand your specific situation.

Grandchildren of a person affected with HD have a 25%, or 1 in 4 chance of inheriting the disorder, if the genetic status of the parent is unknown.