There's an article up about Samaritans. The community is small, down to 350, and traditionally endogamous. That's a problem:

To explain his decision, he points to his own family. When he was a young man, High Priest Elazar's father decreed that he should marry his cousin. It was a mistake, he says now. Two of his three sons were born deaf and mute. Two others died. Mr. Cohen is his only healthy child.

But note, this isn't just because the Samaritans are marrying first cousins. Rather, generations of endogamy has cranked up the coefficient of relationship so that deleterious alleles are now extant at an extremely high frequency. In the United States first cousins who marry are generally related because two of the parents are siblings. The other two parents are unrelated. When considering the possibility of the appearance of a rare deleterious recessive disease you only need to focus on one side of your family tree, you're safe not putting too much effort into the unrelated portion because you assume that they carry different rare alleles. This isn't true for the Samaritans, they're closely related every which direction. In any case, inbreeding reduces the effective population size and so cranks up the ability of random genetic drift to fix deleterious alleles. Consanguinity among obligately endogamous societies is a different order of inbreeding than what you might know from in the West. Though cousin marriage is not unknown (e.g., Charles Darwin), but isn't scaffolded by amplifying social customs (i.e., inbreeding vs. inbreeding^{generation n}).

Also, note that one generation of outbreeding can mask the deleterious alleles immediately. Nevertheless, many subsequent generations will of course still be subject to the recessive diseases of the Samaritans (though at a lower frequency) because one assumes that people of substantial Samaritan ancestry will still assortatively mate. And so they will bring together the deleterious alleles again.

H/T Ikram.

Labels: Population genetics