HD is caused by a defective gene. A gene is a basic unit of inheritance. Genes are the means by which a child inherits all his or her characteristics. For example, there are genes for height, eye colour and baldness. A person inherits his or her genes at the time of conception and carries this set of genes all his or her life.

The gene for HD is passed from parent to child without regard for sex. In other words, it can be transmitted from mother or father to either son or daughter. Huntington disease affects men and women equally. Each child of a person with the Huntington gene has a 50 percent chance of inheriting the defective gene. This means that each child has a one in two chance of inheriting the gene, independent of whether or not other siblings have the gene. It follows that each child has a 50 percent chance of inheriting the normal gene. It does not matter whether or not the parent is displaying symptoms of HD when the child is conceived - what matters is whether or not they have the HD gene in their genetic make up.

If a person does not inherit the HD gene, it is not possible for that person to pass it on to his or her children. For this part of the family line, HD has ended.

Everyone who inherits the HD gene will eventually develop the disorder unless he or she dies from some other cause before the signs appear. Sometimes when this occurs, because the person had the gene in his or her genetic make-up, it may have been passed on to offspring without knowledge that the gene was present.

It is most common for people with the Huntington gene to show symptoms when the person at risk is aged between 35 and 45 years. It is however, possible for people younger than 35 and older than 45 to begin showing symptoms of HD.