Those two towns are the home bases of the Fundamentalist Church of Jesus Christ of Latter Day Saints, a break away group from The Church of Jesus Christ of Latter-day Saints. The group is composed of many of the descendants of Joseph Smith Jessop one of the founders of the communities, and his wife either of which could have carried the gene.
The disability is caused by an enzyme irregularity that causes severe mental retardation brought on by cousin marriage.
"Arizona has about half the world's population of known fumarase deficiency patients," said Dr. Theodore Tarby, a pediatric neurologist who has treated many of the children at Arizona clinics under contracts with the state.
"It exists in a certain percentage of the broader population but once you get a tendency to inbreed you're inbreeding people who have the gene there, so you markedly increase the risk of developing the condition," he said.
It has become common enough among the group to be nicknamed "Polygamist's downs". The condition causes not only severe mental retardation but also unusual facial features, brain malformation, and epileptic seizures.
The two towns make up the community of about 10,000 people who shun outsiders. They broke away from the Mormon faith 72 years ago over their views of polygamy. About 85% of the residents are the blood relatives of two men, John Y. Barlow and Joseph Smith Jessop.
"There aren't any new people coming in. It's a closed door and that gene just keeps getting passed around," said Bruce Wisan, a court-appointed accountant overseeing a trust of the sect's assets.
They avoid all forms of media. The women dress in conservative 19th century clothing. They marry young and marrying a cousin is common practice.
"The disease itself is very rare in the rest of the world," said Dr. Vinodh Narayanan of Arizona's St. Joseph's Hospital & Medical Center and Barrow Neurological Institute. Doctors worldwide had only studied about 10 cases just a decade ago.
"Once you get people within in the same community marrying, then the chances grow of having two people carrying the exact same mutation."
The community doesn't believe that the children affected with the condition suffer because of inbreeding. They think it's in the water. They are wrong but with outdated theories which as that the disease will continue to crop up in town.