Legacy: A Genetic History of the Jewish People
Oxford University Press
2012, $24.95, pp. 288
The Wandering Gene and the Indian Princess: Race, Religion, and DNA
W.W. Norton & Co.
2012, $26.95, pp. 272
The question of who is a Jew has become a knot of complexities. I once heard Benjamin Netanyahu say in jest at a lunch at The New York Times--I think he was Israel's UN ambassador at the time--that the Cabinet had thrown up its hands over the issue: "We have decided to adopt Sartre's definition instead--anyone whom anyone else thinks is a Jew, is a Jew," he said.
The crux of the issue is whether Jewishness is just a religious and cultural trait or whether it also has a biological basis. rib undermine disastrous ideas about eugenics and racial purity, Jewish intellectuals from Franz Boas to Ashley Montagu have derided the idea of race. But just as they had succeeded in persuading social scientists to embrace the unlikely idea that race is a mere social construct, advances in decoding DNA began to show the opposite: Human populations do indeed fall into genetic clusters, and there is, after all, a genetic basis to Jewishness.
If Jews had married extensively outside their faith through the ages, there would be no Jewish genetics. But Jewish communities, it now turns out, have been largely endogamous, with very little intermarriage in each generation until the present. This practice has preserved Jews, if not as a race--that term is perhaps best reserved for the continent-based population groups--then as a biologically based ethnicity. And from the biology has emerged, mostly within the past 15 years, a trove of powerful genetic knowledge. Genetic diseases can now be screened for and diminished. Jewish history can be reconstructed deep into the past where all historical records are lost.
The development of Jewish genetics is expertly told by Harry Ostrer in Legacy. Ostrer is a New York medical geneticist who became intrigued by the power of DNA to peer into the Jewish past, particularly when comparing different communities. He has collected DNA samples from Jewish groups around the world. His book is a fine non-technical summary of his own and others' research in this fast-moving field, though arguably it has one major omission.
A century ago, the physical anthropologist Maurice Fishberg described the paradox that Jews throughout the world are distinctive, and yet all strongly resemble their host populations. Analysis of DNA has provided the explanation: Jewish communities are descended from a population that lived in the Middle East some 3,000 years ago. But the Jews in each country have intermarried to some extent with the local population. The rate per generation has been minuscule, but small differences accumulate. The Jews of North and Central Europe (Ashkenazis) and those who used to live in Spain and Portugal (Sephardis) have a proportion of European ancestry that ranges from 30 to 60 percent, Ostrer and colleagues have calculated.
As Ostrer notes, there is no unique gene or version of a gene that is found in all Jews. Rather, there are statistical regularities in DNA sequences that link Jewish populations to one another. If a genome is sampled at enough sites, as can now be done with gene chips, two very similar populations can be distinguished. For instance, Ashkenazis can be distinguished genetically from non-Jewish Europeans on a statistical basis if one grandparent is Jewish and with 100 percent accuracy if all four are.
One of the most interesting discoveries in Jewish genetics concerns the Kohanim, or priests, whose office has been transmitted from father to son since Aaron, the first high priest. Some 46 percent of all Kohanim carry on their Y chromosomes a specific genetic signature, one that does not appear …