If you or someone in your family has Alzheimer's disease, relatives have a higher risk than the general population for developing the disease. In fact people with at least one first-degree relative (parent or sibling) with Alzheimer's disease are 3.5 times as likely to develop the disease.

This risk increases for each additional relative with Alzheimer's disease. This strong tendency of Alzheimer's disease to run in families is due, in part, to mutations in PS1 and PS2.

http://www.genetichealth.com/ALZ_Gene ... _Alzheimers_Disease.shtml