Since the 1980s, the average life expectancy of a person with Down’s syndrome has more than doubled, from less than 30 to well into their 60s. Living longer, though, has revealed a tragic twist—the vast majority of people with Down’s will go on to develop Alzheimer’s disease by the age of 65. That’s because people with Down's syndrome, who have an extra copy of chromosome 21, produce increased amounts of beta-amyloid, a protein that is implicated as a possible cause of Alzheimer’s. To make things worse, people with Down’s syndrome often cannot access the dementia drugs that might help them—doctors are reluctant to prescribe them because people with Down’s haven't been allowed to take part in the clinical trials for Alzheimer’s drugs. People with Down’s think that should change. In doing so, could scientists unlock the mystery of Alzheimer’s itself?

Alok Jha, The Economist’s science and technology editor and Emilie Steinmark, our science correspondent, follow the story of Kate Olmstead, who has Down’s syndrome, and her mother, Amy. Emilie also interviews John Hardy, a neuroscientist at University College London who proposed the amyloid hypothesis for Alzheimer’s in the 1990s. Thanks also to Frank Stephens of the Global Down Syndrome Foundation.

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