A polygamist Mormon enclave on the Arizona-Utah border is seeing more and more children being born with an extremely rare disorder that causes severe mental and physical retardation.

Dr. Theodore Tarby, who specialized in rare childhood diseases, first discovered the problem in 1990, when a woman in the community brought her 10-year-old son to him.

The boy had unusual facial features, such as a prominent forehead, low-set ears, widely-spaced eyes and a small jaw. He was also severely physically and mentally disabled.

Dr. Tarby was stumped as to what condition the boy had until he sent a urine sample out for analysis.

Fumarase deficiency, an extremely rare disease, has become increasingly common in the fundamentalist Mormon enclave on the Arizona-Utah border (above)

Fumarase deficiency has become a common disease in the community, because of inbreeding and polygamy. Above, young girls in the commonity in 2004

That test came back position for fumarase deficiency, an extremely rare disorder that impacts the metabolism.

Fumarase deficiency happens when a person lacks the enzyme fumarase which helps drive energy to cells. It has the most severe impact on the brain, which takes up about 20 per cent of the body's energy.

Children who have it are missing parts of their brain, can't sit or stand without help, and often suffer from seizures. Their language skills are minimal and their IQ is usually around 25.

At that time, Dr. Tarby believed there were only 13 cases in the world, making the odds of having it about one in 400 million. But he soon discovered that his patient's sister, who was believed to suffer from cerebal palsy, also had the disease.

Dr. Tarby started working with the community after that and diagnosed eight more cases, in children ranging in age from 20 months to 12 years old.

All of the patients had the same facial characteristics, and most couldn't walk or even sit up.

Researchers found that the likelihood of having the disease was over one million times the global average.

The reason for this? Polygamy and the gradual inbreeding over generations.

The faulty gene traces back to one of the community's two founders Joseph Smith Jessop (seated, center right), and his first wife Martha Moore Yeates (seated center left). It's now estimated that 75 to 80 per cent of the community are related by blood to either Jessop or the other founder, John Y. Barlow

The polygamist twin cities of Hilldale and Colorado City were founded in the 1930s by two men, John Y. Barlow and Joseph Smith Jessop.

The two men had become pariahs by the main Mormon church, which outlawed polygamy in 1904. They moved to Hilldale/Colorado City to set up their own community where they could continue to practice plural marriage.

The group was then, and is now, very insular. New people are not welcomed into the community, and in fact, young boys are often kicked out so that a select few men can have an excess of wives.

That means that the gene pool is small. In other words, most of the people who are in the community are related to each other.

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

And that's a problem when it comes to recessive diseases like fumarase deficiency.

It's an exceptionally rare disease because it's recessive, meaning that in order to get it, both parents need to carry the gene.

The chances of that happening almost anywhere else in the world is extremely rare. But in the FLDS community in Hilldale and Colorado City, relatives marrying each other is a common occurrence.

It's said that 75 to 80 per cent of the population at the population in the community are blood related to one of the founders, either Barlow or Jessop.

The defective gene is traced all the way back to Jessop himself, and his first wife, Martha Yeates. Those who are descendants of Barlow are also likely to inherit the gene too, since he took one of Jessop and Martha's daughters as a bride.

And it's only going to get worse as the practice of polygamy continues, since a single man with the gene can pass it on to dozens of children and possibly hundreds of grandchildren by marrying multiple wives.

It's now estimated that thousands of people in the 7,700-person community have the gene.

Dr. Tarby told Time magazine in 2008 that the community doesn't believe that inbreeding and polygamy is making the issue worse.

'They have their mythology about the condition. They think it's something in the water, or something in the air,' he said.

Before he retired, he said he addressed FLDS leaders about measures they could take to stop the spread of the disease.

He said they should bar marriage between two people that have the recessive gene or ask the couple not to have children. If an already married couple have a child that presents the disease, they should discontinue having children in the future or undergo testing during pregnancy and abort any fetus that tests positive for the gene. The group refused to follow this advice

'It's not something they are willing to do,' Tarby said.