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Alzheimer’s disease is a condition that slowly damages the brain, leading to memory loss and, over time, difficulty with thinking, judgment, and language. At the level of brain tissue, it involves a build-up of sticky protein clumps called amyloid plaques and twisted fibres called neurofibrillary tangles, which interfere with how brain cells communicate and survive. But the genetic picture behind that process looks quite different from one family to the next.
Most Alzheimer’s disease, roughly three-quarters of cases, is described as sporadic. This means it arises from a combination of aging, general genetic background, and other factors that are not yet fully understood, rather than from a single identifiable gene change. There is no clear inheritance pattern in these cases, and no single gene to blame.
About one-in-four cases are called familial, meaning three or more relatives in a family have had Alzheimer’s disease. Even here, most familial cases are not explained by one dominant gene. Instead, research suggests that many genes, each contributing a small amount of risk, act together with age and environment to produce the disease. Scientists have identified around twenty of these “susceptibility genes,” but no single variant in them is considered to cause Alzheimer’s disease on its own, and testing for them is not currently recommended as a diagnostic tool.
A distinctly different, smaller group is early-onset familial Alzheimer’s disease, making up fewer than two-in-one-hundred cases overall. This form is caused by a pathogenic variant, a disease-causing change, in one of three specific genes: APP, PSEN1, or PSEN2. Unlike sporadic or the broader familial form, changes in these three genes follow a clear, predictable, single-gene inheritance pattern.
One additional cause is worth knowing about: nearly everyone with Down syndrome develops the brain changes of Alzheimer’s disease after age forty. This happens because the APP gene sits on chromosome 21, and having three copies of that chromosome, as occurs in Down syndrome, leads to lifelong overproduction of the amyloid protein.
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