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Pure Jewish blood?

Gene ExpressionBy Razib KhanNovember 1, 2007 11:56 PM


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William Saletan has a piece up over at Slate, Jewgenics, which covers his reactions to a talk (you can view it online) sponsored by AEI around Jon Entine's book Abraham's Children: Race, Identity, and the DNA of the Chosen People. I've read the book, though I don't have time to listen to the talk right now. But I wanted to offer a quick perspective on one point in Saletan's piece:

According to Entine, the rate of Jewish "outbreeding"--procreating with non-Jews--is half a percent. That's the lowest rate of any population in the world today....

First, I don't know if that is the lowest rate of any population in the world today. There's a lot of populations out there. Additionally, the 0.5% is per generation over the last 2,000 years. Since American Jews outmarry at rates of 25-50%, and they are a substantial proportion of the world's Jewry, the 0.5% number doesn't make any contemporary sense. Rather, Jon is reporting data which specifically looks at the proportion of non-Jewish lineages that seem to have slipped into the Jewish gene pool over the past 2,000 years. But, that doesn't mean that in every generation only 0.5% of Jews married out, it means that 0.5% of non-Jews married in! There is plenty of historical data for Jews leaving Judaism through outmarriage, from Spain before the expulsion of 1492 to 19th and early 20th century Germany. The rates are well above 0.5%, but, these Jews interjected Jewish lineages into the gentile population. In contrast, it seems that far fewer gentiles married into the Jewish people over the past few thousand years. The reasons for this are two-fold: 1) Judaism makes it hard to convert and 2) Islam and Christianity have often imposed capital penalties upon those who convert to Judaism from their religions. The exceptions often prove the rule, there are instances where Christian converts to Judaism fled to Muslim Spain, as there the authorities would tolerate converts from Christianity to a non-Christian religion. I suspect that Jewish objections to converts is in part a cultural adaptation due to the long incubation of Jewish culture within Christendom and the Dar-al-Islam, when perceptions that Jews were poaching members of the dominant religion could result in communal retribution. Looking at the 0.5% per generation number it is also interesting to consider what that means genetically. Assuming discrete generations of 25 years and iterating the 0.5% gene flow number, one projects that over 2,500 years originally non-Jewish genes would form about 39% of the Jewish gene pool today. That would be a substantial amount in terms of ancestry, and certainly enough so that selection could operate and reshape Jewish phenotypes due to the introduction of non-Jewish polymorphisms. In other words, some of the characteristic diseases or Jews might very well have a non-Jewish origin and risen in frequency due to in situ evolutionary dynamics within the Jewish population. Via Ruchira Paul.

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