Of the many forms of dementia, Alzheimer’s disease has received the most research attention. People are often worried they will inevitably contract Alzheimer’s disease if a parent did. But increasingly, scientists have found this is not necessarily the case.

Processes in our bodies are regulated by genes, but not everyone has exactly the same version of each gene. These different versions are called alleles. Different alleles carry different risks for physical conditions in the body, including Alzheimer’s disease and other dementias.

One of the most strongly replicated findings for genetic risk of Alzheimer’s disease in late life involves the APOE gene, and the ε4 allele. You have two alleles for every gene. For the APOE gene, you may have the ε2, ε3, or ε4 alleles. The ε2 allele appears to be protective; people who carry it are less likely to develop Alzheimer’s disease. Having one allele with ε4 increases your risk, however, and having both alleles with ε4 raises it even more.

Distributions of ε4 differ based on ancestry groups, but in general, it is much less common than ε3 (the most common). Moreover, the APOE alleles are only risk and protection factors; they do not determine whether you get the disease. Even people with the highest risk—double ε4—only develop Alzheimer’s disease about 40% to 60% of the time by age 80. This chart presents a simplified example of the distribution of the alleles and the likelihood of developing Alzheimer’s disease by age 80. This chart is based on multiple studies; actual rates differ by age, sex, ancestry, and other factors.

A disclaimer: Early-onset Alzheimer’s disease—which manifests before age 60—has a strong genetic influence. The early form is relatively rare. For example, fewer than 1 in 100,000 adults in their 40s have Alzheimer’s........

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