Fumarase deficiency in Colorado City

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Another reason to avoid inbreeding:

The twin border communities of Hildale, Utah, and Colorado City, Ariz., have the world's highest known prevalence of fumarase deficiency, an enzyme irregularity that causes severe mental retardation brought on by cousin marriage, doctors say.
"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.

Colorado City, AZ and adjacent Hilldale, UT form a closed community housing a formerly LDS splinter sect:

Local historian Benjamin Bistline said 75 to 80 percent of people in the area are blood relatives of two men — John Y. Barlow and Joseph Smith Jessop — who founded the sect on the remote desert plateau in the early 1930s.
"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.

Everybody carries some deleterious recessives; mating with close relatives produces the chance that a child will have the homozygote recessive genotype. But a lot of discussion of inbreeding neglects the community or cultural aspect -- in examples like this one, the fact that an entire community has adopted the practice of close relative marriage creates a pool of potential sufferers of the disorder. In a sense, the allele becomes emblematic of the group: here, fifty percent of cases of fumarase deficiency in the world belong to this one community of 10,000 people.