Doctors took an hour to realise Sarah Merriman had Down’s syndrome after her birth in January 1992. By then, her father, Andy, had phoned friends and family to tell them his wife, Alison, had given birth to a healthy baby. His happy news was dashed. “It was a real shock,” Andy recalls. “From the start, we were warned about the difficulties and troubles that lay ahead for Sarah. Then she was diagnosed as having a hole in her heart. The worry, for the first years of her life, was constant.”

Sarah’s heart healed. She did well at her school in Haringey, north London, and went on to pass the equivalent of four GCSEs. Today, she is studying catering and lives with other students near her college in Somerset. “Sarah is independent and copes with life in a way we could never have imagined just after she was born,” says Andy.

It is a reassuring story, although one major worry still besets the Merriman family: Sarah’s long-term future and her susceptibility to Alzheimer’s disease, a form of dementia that leads to complete loss of memory, speech and awareness and which is closely linked to Down’s syndrome. Among members of the general population, the risk of getting Alzheimer’s before the age of 65 is less than 5%. For a person with Down’s syndrome the figure is 50%.

Could Down’s syndrome point the way to preventing Alzheimer’s disease?