Research has identified four genes that influence disease development. Three of these genes affect younger people, and one affects older people.Early onset Alzheimer'sThe three genes that have a major effect on risk of Alzheimer's disease are the amyloid precursor protein (APP) gene and two presenilin genes (PSEN-1 and PSEN-2). People with any of these genes tend to develop the disease in their 30s or 40s, and come from families in which several members also have early onset Alzheimer's disease. It is very rare type of Alzheimer's and accounts for only a very very small number of cases worldwide.
Our knowledge of genetic risk factors for Alzheimer's is increasing all the time. It is thought that people who have a parent or sibling with Alzheimer's have a 3.5 fold higher risk of developing the disease. It is believed that if both parents have the disease then you have a greater risk factor.
Most researchers are convinced that there are many more genes involved than is presently known and that other conditions must also be present for the disease to develop. We still do not know why some people with apparent gene abnormalities develop Alzheimer's and others do not.
Here is a link to genetic risk factors and Alzheimer's disease theat will give you more information.
At this time in Alzheimer's disease research, there is the thinking that there are two distinct categories of the disease: "sporadic" or "late-onset" Alzheimer's disease, which affects people in older adulthood, usually after age 65, and which progresses gradually; and a rarer form of the disease, "familial" or "young-onset" Alzheimer's disease, which affects someone younger than 65. Late-onset Alzheimer's disease accounts for roughly 95% of the diagnosed cases of the disease. Roughly 20% of families with a history of late-onset Alzheimer's disease are at greater risk for developing the illness in future generations. Young-onset Alzheimer's disease, on the other hand, has a strong history in families, and the odds of children developing the illness are significantly higher than the odds for someone without such a family history. However, please note that a family history of young-onset Alzheimer's does not indicate that everyone else in the family will develop the illness, only that the odds are greater.
As you've seen, they generally are increased, but it depends on the genes you carry and how many copies of each of several genes (according to the lastest studies that I've read - this changes all the time) you carry.
Do all you can to take care of your body. It may help, and won't hurt. Still, there are no guarantees.
The previous answers are spot on! I also wanted to add that I went to a presentation a couple of years ago by Dr. David Snowdon, who was the lead researcher on the landmark Nun Study. He found that autopsies of some of the nuns' brains showed extensive Alzheimer's plaques; however, when they were alive, these nuns did not show any symptoms of the disease. So this still remains a very mysterious disease.
Take care and keep us posted!