Hereditary Alzheimer’s is rare– accounting for only about 5% of all Alzheimer’s cases. This is the familial form of the disease and if you have the gene for it, you will develop Alzheimer’s (usually relatively early).
You may also inherit a risk factor–Apoe-4 or Sorl 1. These genes can predispose someone to develop it, but like risk factors for cancer that run in the family, it doesn’t mean that a person will produce it. These risk factors are involved in the late-onset form of the disease (after age 65) and are in combination with other environmental and health factors that may dispose one person more than another to develop Alzheimer’s. At this stage, we still do not fully understand the complex interplay of all these factors.