Familial or early-onset Alzheimer's disease is inherited and develops in people between the ages of 30 and 60. If even one of three gene mutations that causes the disease is inherited from a parent, the individual will almost certainly develop Alzheimer's disease. However, less than five percent of patients have early-onset Alzheimer's disease.
Late-onset is usually developed after age 60; this is the most common form of the disease. Its cause is not known and no pattern of inheritance has yet been discovered, although clusters of cases are seen in some families. One particular gene carried by about 25 percent of the American population increases the risk of developing Alzheimer's disease, while another carried by a small proportion of the population substantially protects against the disease. Scientists have identified other genes that may influence the risk of contracting the disease, and further research is ongoing.
Since genetic risk factors are not enough to cause late-onset Alzheimer's disease, researchers are also studying education, diet, and other factors to see if they play a role in developing the disease.
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