Genetics is most important in families with a history of early-onset Alzheimer's (occurring before age 50) stretching back for several generations. (The early-onset form accounts for less than 1% of all Alzheimer's cases.) Mutations in three genes are known to cause this type of Alzheimer's: amyloid precursor protein gene (APP), presenilin 1, and presenilin 2. All three genetic mutations increase the production of beta-amyloid, which is deposited in the plaques found in Alzheimer's disease. Excessive amounts of beta-amyloid fragments are thought to be toxic to nerve cells.
If one parent has any of these mutations, each child has a 50% chance of inheriting the mutated form. A child who inherits the mutated gene will inevitably develop early-onset Alzheimer's disease.