The answer depends heavily on which genetic category applies to a family.

Sporadic Alzheimer’s disease (no known family history).
A first-degree relative, meaning a parent, sibling, or child, of someone with sporadic, late-onset Alzheimer’s disease faces a lifetime chance of about fifteen-to-thirty-nine-in-one-hundred, often quoted as roughly twenty-to-twenty-five-in-one-hundred, compared with about ten-in-one-hundred in the general population. This is an elevated risk but not a predictable, gene-by-gene inheritance pattern.

Late-onset familial Alzheimer’s disease (three or more affected relatives, onset after age sixty-five).
This form is thought to involve many genes interacting together rather than one dominant change. First-degree relatives face roughly one-and-a-half-to-two times the general population’s risk, or about fifteen-to-twenty-five-in-one-hundred over a lifetime. When both parents have had Alzheimer’s disease, the risk to their children is thought to be at least double the general population’s risk.

Early-onset familial Alzheimer’s disease (APP, PSEN1, or PSEN2).
This form follows autosomal dominant inheritance, meaning a change in just one of the two gene copies a person carries is enough to cause disease. Each child of an affected parent has a fifty-in-one-hundred, or one-in-two, chance of inheriting that same gene change. A few practical points matter here:
• Affected parents are often no longer living by the time of diagnosis. Because onset is typically in early adulthood and progression can be relatively fast, a parent who carried the gene change may have passed away before their child’s symptoms even began, which can make a family history look falsely reassuring.
• A “negative” family history is not proof. Reduced penetrance, meaning a gene carrier does not always show symptoms, along with early parental death or missed diagnoses, means an apparently clean family history cannot be fully trusted unless parents were specifically evaluated or tested.
• Siblings of an affected person remain at meaningful risk even if parents seem unaffected, because of the possibility that a parent carries the change without ever having shown symptoms.

Because the practical meaning of “risk” is so different across these three situations, meeting with a genetics professional to correctly classify a family’s situation is often the most useful first step before estimating a specific number.