My father has Alzheimer’s disease. Does that mean my children and I will eventually develop it too?

DEAR DOCTOR K:

My father has been diagnosed with Alzheimer’s disease. Does that mean that my children and I will eventually develop Alzheimer’s too?

DEAR READER:

Many people worry that if a parent had Alzheimer’s disease, they are doomed. But that’s not true. Having a first-degree relative (parent, sibling or child) with Alzheimer’s disease increases a person’s risk by about 30 percent. That sounds like a lot, and therefore sounds scary. But what you really want to know is: What is my risk in the first place? If it’s a very low number, then raising a low risk by 30 percent won’t be a big deal.

Here are the numbers. If you are age 65, the risk of being diagnosed with Alzheimer’s is 2 percent per year. (This also means a 98 percent chance per year of not developing Alzheimer’s.) In absolute numbers, a 2 percent annual risk means that two out of 100 65-year-olds will develop dementia every year.

Family history raises the 2 percent annual risk by about 30 percent, to 2.6 percent per year. That means going from 20 cases in a group of 1,000 to 26 in 1,000, or six additional cases in 1,000.

Age raises the chance of Alzheimer’s more than family history. People in their 70s have a 5 percent chance of being diagnosed. That’s more than twice that of people in their 60s. Family history raises this by 30 percent, from 5 percent to 6.5 percent. Again, the absolute change is relatively small.

When a relative is diagnosed with dementia later in life, family members often wonder if they should be tested for the “Alzheimer’s gene.” The short answer is no, because the test won’t tell you whether you will develop the disease. It will only tell you if you are at a greater or lower risk.

For Alzheimer’s disease that begins later in life, as it does in most cases, a gene called apolipoprotein E (APOE4) is associated with greater risk for dementia. If you inherit one copy of APOE4, your risk triples. If you have two copies, your risk is 10 to 15 times higher (this is rare).

But having APOE4 does not mean you will definitely develop dementia. Among people age 70 who have no signs of Alzheimer’s disease, about 25 percent have one or more copies of the risk gene. Nor does the absence of APOE4 protect you: About 35 percent of people with Alzheimer’s don’t carry the risk genes.

Some forms of Alzheimer’s disease begin early in life, even in a person’s 30s. These forms are uncommon, but they more often run in families. The more common form of older-onset Alzheimer’s disease that typically begins after age 65 is what we’ve been talking about.

There are many diseases like older-onset Alzheimer’s disease in which a person’s risk is increased if a first-degree relative has the disease. Yet that does not mean a person with an affected relative is destined to get the disease — or that a person without an affected relative is guaranteed protection from the disease.