Although people with a family history of Alzheimer's disease are generally considered to be at a higher lifetime risk of developing the disease themselves, many people who have relatives with Alzheimer's disease never develop the disease, and many without a family history of the disease get it.
If you have a close relative who has been diagnosed with Alzheimer's disease your lifetime risk increases by about 30%. This is a relative risk increase, meaning a 30% increase in your existing risk. If you are age 65, the risk of being diagnosed with Alzheimer's is 2% per year. Family history raises the 2% annual risk by about 30%, to 2.6% per year. Therefore, the absolute increase is relatively small.
Most cases of Alzheimer's disease are late-onset occurring after mid-60s and are not linked to a specific genetic mutation. However, there are genetic factors that appear to increase a person's risk of developing late-onset Alzheimer's. For example, a gene called apolipoprotein E4 (APOE4) is associated with greater risk for dementia. If you inherit one copy of APOE4, your risk triples. If you have two copies (which is rare), your risk is 8-12 times higher . Having the gene puts you at a higher risk, but it does not mean that you will get dementia. About 25 per cent who age normally into their 70s have one or more copies of the risk gene. On the other hand, about 35 per cent of people with Alzheimer's do not have one of the risk genes. Therefore, genetic testing for APOE4 is not helpful and is not recommended. Over 20 other genetic variants have also been identified as risk variants for Alzheimer’s disease. However, most of these variants only increase a person’s risk of developing Alzheimer’s very slightly.
Early-onset Alzheimer's disease (when symptoms begin before age 65) is fortunately rare and can be caused by mutations or changes in certain genes. As they are inherited in a manner called “autosomal dominant”, if one of the parents carries the changed gene, each child has a 1 in 2 chance of inheriting the gene. If the gene mutation is passed down, the child will usually (but not always) have the disease. For these families, counselling and genetic testing may be helpful.