Scientists don't yet fully understand what causes Alzheimer's disease (AD), but what is known is that most forms of the disease show a combination of genetic, lifestyle, and environmental factors. In the majority of cases, Alzheimer's disease is not hereditary.
However, there is some concern about a very early-onset form of 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.
Whether or not Alzheimer's disease is hereditary is not as clear-cut a link as in other diseases. Most cases of Alzheimer's are of the late-onset form, developing after age 60. Sometimes if a member of the family has Alzheimer's there is a slightly higher genetic risk to other family members, but scientists studying the genetics of AD have found that the mutations seen in early-onset familial Alzheimer's (eFAD) are not involved in the later form of the disease.
Although symptoms are outwardly the same, late onset and eFAD have distinctly different underlying causes. Families facing earlier age of onset do have some cause for concern as there does appear to be a hereditary pattern in the case of early-onset familial Alzheimer's disease, however eFAD only accounts for 2 to 3% of all Alzheimer's cases.
Source: National Institute on Aging, https://www.nia.nih.gov/health/what-causes-alzheimers-disease