This is the 88th instalment in a series on dementia, including the research into its causes and treatment, advice for carers, and stories of hope.

I have lost count of the number of times somebody has said to me, during a conversation on dementia, “Oh, it runs in my family.”

Like red hair. Or long fingers.

The only type of Alzheimer’s disease that really does is familial Alzheimer’s disease (FAD). It is caused by mutations in the amyloid precursor protein (APP), presenilin 1 (PSEN1) and presenilin 2 (PSEN2) genes.

These are dominant mutations, so each child of an affected parent has a 50 per cent chance of inheriting the genes. If they do, their risk of developing Alzheimer’s, usually at a young age, is high. Symptoms usually show in a person’s forties or fifties.

Mercifully, FAD is very rare – it accounts for less than 1 per cent of all Alzheimer’s cases.

There could be more than 100 risk genes associated with greater susceptibility to the disease. But Professor Alison Goate at the Icahn School of Medicine at Mount Sinai in New York, who was part of the team that identified the APP gene, says their impact is quite small.