A reader pointed me to this fascinating, if tragic, article about the rise of rare recessive diseases among a schismatic Mormon sect which dominates Colorado City. This group has been in the news since the their “prophet” was just arrested. The article points out that because of the inbred nature of the community, and its small size, one particular rare disease, Fumerase Deficiency, has now become rather common. I have talked about inbreeding before. Most of us know the problems that crop up intuitively from experience, rare traits begin to spread in an inbred population. But, what needs to be emphasized is the greater problem from long term customary inbreeding, as is common in much of the Muslim world (and now in the Muslim Diaspora in the West), and in isolated cases as above.
First cousins have a coefficient of relatedness of 1/8, that means you can expect 1 out of 8 genes to be identical by descent from the same ancestor. But as a small community marries only among itself soon everyone is a first cousin in many different ways. In the United States most first cousin marriages are between individuals who share only one pathway of near genetic relationship, via one of their parents. In places like Colorado City this is not so, the family tree is reticulated and twisted back into itself multiple times. This results in a reduced long term effective population, a self-induced bottleneck as a few individuals population what should be a more diverse constellation of ancestors (e.g., your great-great-grand-father is actually your greater-great-grather-father multiple times). Low effective population increases stochastic effects, random genetic drift, and so you have deleterious alleles which can rise in frequency rather quickly. Because of low effective population selection is swamped.
To calculate an individual’s inbreeding coefficient, you take this equation:
FI = sum over all common ancestors[(1/2)i * (1 + FA)]2
You are summing over the paths to each common ancestor, FA, with i being the number of individuals in each path (obviously inbreeding via a common great-great-great grandparent is weighted far less than that via a grandparent). The key for the Colorado City Mormons, and many Muslim groups, is that tightly knit clans share many, many, recent common ancestors, so even individuals who are not technically first cousins may share more genetical similarities than the typical first cousin.
In the article above the anger and outrage at the “benign” neglect of the government at what was happening in Colorado City, and the refusal of the community to cease their inbreeding, is palpable. The Colorado City Mormons are dependent on the state for their well being, and their own actions have resulted in the generation of a dependent class of children who will never participate in society, as such. Though the case of Pakistani origin Muslims in Northern England is much milder per capita (due to the larger size of the community, and its more recent provenance1), the size of this community is going to have implications for the National Health Service.
1 – In many Muslim communities it seems that inbreeding has increased with “modernization” for a variety of reasons.