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A polygamous community on the Utah-Arizona border is being decimated by a rare genetic disease. Sufferers are severely mentally and physically disabled and there’s currently no hope for a cure.

Rampant polygamy hinders genetic diversity. A conservative Mormon enclave on the Utah-Arizona border has learned this the hard way. A horrific disease has ravaged the community, with more and more babies falling prey each year.

Dr. Theodore Tarby discovered the problem almost 30 years ago when a local woman brought her son for treatment. The child was severely mentally and physically disabled. The doctor eventually realized that the boy was suffering from fumarase deficiency, a rare and crippling genetic disorder. Sufferers are missing chunks of their brains and usually have an IQ around 25.

When I moved to Arizona that’s when I realized that my colleagues here were probably the most familiar I’d ever met with this disease,” Vinodh Narayanan, a neurologist at the Translational Genomics Research Institute, told the BBC.

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The disease normally affects one in 400 million children. The Mormon communities polygamous lifestyle has left them far more exposed than the general population. The BBC report:

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Brigham Young, the founder of Mormonism, was a staunch polygamist. He had multiple wives, dozens of children, and told his followers that if they wanted to reach heaven they needed to do the same. Today most Mormons live in Utah.

This sudden explosion is down to exponential growth. Even with just one wife and three children, if every subsequent generation follows suit a man can have 243 descendants after just five generations. In polygynous families this is supercharged. If every generation includes three wives and 30 children, a man can – theoretically – flood a community with over 24 million of his descendants in the space of five generations, or little over 100 years. Of course this isn’t what actually happens. Instead, lineages begin to fold in on themselves as distant (and in the FLDS, not so distant) cousins marry. In polygamous societies, it doesn’t take long before everyone is related…”

Fumarase deficiency is rare because it’s a recessive trait. Both parents need to have the mutation in order for the child to have the disease. In societies where multiple women mate exclusively with the same man, it suddenly because easy for recessive genes to spread.

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Most families include at least three wives, because that’s the number you need to enter heaven,” an anonymous resident told reporters.

There aren’t enough women to supply every man in town with three wives. That’s why so many people end up courting close family members. There simply aren’t enough options. So a man who carries the recessive Fumarase trait might have children with women who, because they’re related to him, also share the recessive trait.

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The polygamist enclave being ravaged by a rare disease has only 7,700 residents. A few men are deciding the fate of the next generation by having huge numbers of children. It’s a perfect breeding ground for genetic disease.

With polygamy, you’re decreasing the overall genetic diversity because a few men are having a disproportionate impact on the next generation,” said Mark Stoneking, a geneticist at the Max Planck Institute for Evolutionary Anthropology, Germany. “Random genetic mutations become more important.”

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The community will collapse on itself unless it changes its practices or somehow gets an infusion of fresh blood. If residents keep mating with each other they’ll start bearing increasingly deformed children. Fumarase is a terrible condition. Sufferers can’t walk, talk, or feed themselves. Most are incapable of sitting up on their own.

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The spread of genetic disorders in Mormon communities is allowing researchers to expand their medical knowledge.

The disease is caused by a hiccup in the process that provides energy to our cells. In particular, it’s caused by low levels of an enzyme – fumarase – that helps to drive it. Since it was perfected billions of years ago, the enzyme has become a staple of every living thing on the planet. It’s so important, today the instructions for making it are remarkably similar across all species, from owls to orchids,” writes the BBC.

The sole benefit to the health crisis afflicting polygamous communities is the chance it gives researchers to identify disease traits. Scientists say that they now know more about Fumarase deficiency than ever before. Previous to the Utah outbreak there were only 13 known cases in medical literature.