After performing all the usual tests, Tarby was stumped. He had never seen a case like it. Eventually he sent a urine sample to a lab that specialises in detecting rare diseases. They diagnosed “fumarase deficiency”, an inherited disorder of the metabolism. With just 13 cases known to medical science (translating into odds of one in 400 million), it was rare indeed. It looked like a case of plain bad luck.

But there was a twist. It turned out his sister, whom the couple believed was suffering from cerebral palsy, had it too. In fact, together with colleagues from the Barrow Neurological Institute, soon Tarby had diagnosed a total of eight new cases, in children ranging from 20 months to 12 years old.

In every case, the child had the same distinctive facial features, the same delayed development – most couldn’t sit up, let alone walk – and, crucially, they were from the same region on the Arizona-Utah border, known as Short Creek.

Even more intriguingly, this region is polygynous. In this small, isolated community of Fundamentalist Church of Jesus Christ of Latter-Day Saints (FLDS) – a group that split from the LDS in the early 20th Century – the likelihood of being born with fumarase deficiency is over a million times above the global average.

“When I moved to Arizona that’s when I realised that my colleagues here were probably the most familiar I’d ever met with this disease,” says Vinodh Narayanan, a neurologist at the Translational Genomics Research Institute, Arizona, who has treated several patients with fumarase deficiency.