Huntington's disease is caused by a faulty gene that is found on chromosome number four (see box, right).
The normal copy of the gene produces a protein called huntingtin, but the faulty gene is larger than normal and produces a larger form of huntingtin.
Cells in parts of the brain – specifically, the basal ganglia and parts of the cortex – are very sensitive to the effects of the abnormal huntingtin. This makes them function poorly and eventually die.
The brain normally sends messages through the basal ganglia. When this part of the brain is damaged, it causes problems with control of movement, behaviour and thinking.
It is still unclear exactly how abnormal huntingtin affects the brain cells and why some are more sensitive than others.
Chance of inheriting the disease
A parent with the Huntington's gene has one good copy of the gene and one faulty copy of the gene. Their child will inherit one of these genes. Therefore, there is a 50:50 chance that the child will get the faulty gene. If they get the faulty gene, they will develop Huntington's disease.
There is a 50:50 chance that the child with Huntington's disease will pass the faulty gene on to their future child.
This pattern of inheritance is called autosomal dominant.
In as many as 3% of cases, there is no family history of the disease. This could be due to adoption or because relatives with the disease died early from other causes. Or, it could be a new genetic mutation (a new error in a gene). For example, there may be a fault in the genetic material of the father's sperm.
