Scientists don't yet fully understand what causes Alzheimer's disease (AD). There is some concern about the earlier-onset Alzheimer's disease "running in families."
According to the National Institutes of Health, some cases of early-onset AD, called familial AD (FAD), are inherited. FAD is caused by a number of different gene mutations on chromosomes 21, 14, and 1, and each of these mutations causes abnormal proteins to be formed.
Most cases of Alzheimer's are of the late-onset form, developing after age 60. Scientists studying the genetics of AD have found that the mutations seen in early-onset AD are not involved in this form of the disease. Sometimes if a member of the family has the disease, the siblings show a higher risk – but this isn't always true.
Genetic markers have been identified in some which may predispose to this condition, but one clear cause in all Alzheimer's disease sufferers has not been identified. Whether or not Alzheimer's disease is hereditary is not as clear-cut a link as in some diseases, but there does appear to be a pattern in some cases.