There are a few very rare cases where Alzheimer's disease does run in families. In these cases there is a direct link between an inherited mutation in one gene and the onset of the disease. These tend to be cases of 'early onset' Alzheimer's disease, which affects those under the age of 65. In these cases, the probability that close family members (brothers, sisters and children) will develop Alzheimer's disease is one in two.
Most cases of Alzheimer's disease are not of the type that is passed on directly in this way. If a family member has a normal form of Alzheimer's disease, the risk to close relatives is around three times higher than the risk for a person of a similar age who has no family history of the disease. It is thought that in these cases a person's genes may contribute to the development of the disease but do not cause it directly.