The Secret Life of Jewish Genes
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. To 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 to exist in non-Jews. The first bearer of this signature is estimated, by a genetic calculation, to have lived some 3,100 years ago, a date that fits with archaeological evidence for the beginning of Judaism.
In The Wandering Gene and the Indian Princess, Jeff Wheelwright, former science editor at Life magazine, covers much of the same ground as Ostrer but presents the information in the anecdotal framework of a young woman from New Mexico, Shonnie Medina, who died in 1999 from breast cancer. She was of mixed Spanish and Native American heritage but surprisingly had been found to carry a distinctively Jewish genetic mutation known as BRCA1 185delAG. The BRCA1 gene, when mutated, can lead to breast cancer.
Wheelwright’s book goes in depth into Medina’s life and family history, suggesting that the mutation was introduced into the New World by Sephardi Jews who had been forced to convert to Catholicism. This leads Wheelwright into a survey of Jewish genetics, much of it told through discussions with Ostrer, who has surveyed Hispanic/Native American families like Shonnie Medina’s in search of Jewish mutations.
Wheelwright dwells on the practicalities of genetic counseling and discusses an interesting divergence within the Jewish community. Medical geneticists such as Ostrer believe in telling patients everything, a practice which enables them to abort an embryo that will likely be born with a lethal disease such as Tay-Sachs. To prevent possible stigma, Orthodox Jews test people without telling anyone the result; young couples are then advised not to marry if the central registry shows that both carry the mutation for a particular disease. This is the approach pioneered by Rabbi Yosef Ekstein of Brooklyn through his Dor Yeshorim program.
Both approaches have been effective at reducing the incidence of Tay-Sachs and other Mendelian diseases (those caused by mutation in a single gene) that affect Jews. “While other minorities, such as blacks and Native Americans, backed away from genetic probes, fearing that the results could be used to discriminate against them, Jews took custody of their DNA,” Wheelwright observes.
A disappointment in both Ostrer’s and Wheelwright’s books is the short shrift both authors give to what many readers will find the most interesting question of all: Whether there is any connection between Jewish genetics and the outsize contributions that Jews, particularly Ashkenazis, have made to Western science and culture.
Observers from Sir Francis Galton to Charles Murray have commented on the unusual frequency of Jewish intellectual distinction and suggested it has a hereditary basis. Ostrer downplays this idea, offering instead the implausible conjecture that Jewish accomplishment in recent history derived from “being in the right place at the right time.” Ostrer also mentions, but only to dismiss it, the intriguing hypothesis recently advanced by three authors at the University of Utah: Gregory Cochran, Henry Harpending and Jason Harding.
The Utah hypothesis argues against a central orthodoxy of the academic left by holding that there is a genetic basis to intelligence and that some populations are better endowed in this respect than others. Steven Pinker is the only scholar, so far as I know, with the courage to have said publicly that the Cochran-Harpending hypothesis should be seriously considered.
In brief, Cochran-Harpending argues that Ashkenazis were confined to professions, like money lending and tax collecting, which happen to be intellectually demanding, for a period of some 900 years—easily long enough, they calculate, for variant genes favoring improved cognitive ability to have become more common in the population. Thus Ashkenazi average IQ is around 115, the highest of any ethnic group, whereas Oriental Jews, who were assigned to menial occupations by their Muslim rulers, have no such elevation of IQ, the Utah team says.
The Utah researchers then identify the genes they think are likely to be involved: They are harmless in single doses but which, when inherited from both parents, cause certain Mendelian diseases found in high frequency in Jewish populations. Mendelian diseases should strike at random throughout the genome, but several Jewish Mendelian diseases, of which Tay-Sachs is one, occur in clusters that affect specific biochemical pathways.
The clustering is a strong sign of natural selection at work, as is the relatively high frequency of the diseases (though in this case genetic drift could be an alternative explanation). No one else has managed to explain what selective advantage might be conferred by the clustered mutations. The Cochran-Harpending hypothesis also has the distinct merit of being testable: The number of clustered mutations a person possesses should correlate with his IQ score. Such tests would be simple to conduct and yet, strangely, no one has published the results of any. The Cochran-Harpending hypothesis seems condemned to lie in conspicuous limbo, unconfirmed and unrefuted.
It would doubtless be invidious for researchers who are Jewish to spend a lot of time discussing whether and why Jews are smart. Perhaps Ostrer goes as far as he can by presenting the idea to readers but in a backhanded and dismissive way. But Wheelwright, who writes that he is not Jewish, had no such constraints and could have shown more curiosity about the idea.
The new genomic analyses, when applied on a worldwide basis, give no indication that Jews are special. On genomic maps, Middle Eastern and European populations lie close together, and Jewish communities are sandwiched between them, reflecting their origin in one and their admixture with the other. Genomically, Jews are extremely similar to their parent populations. But they have a special history, not free of suffering and natural selection, which is interested in survival, could well have enhanced the cognitive qualities that underlie intellectual achievement, either by the process Cochran and Harpending suggest or some other.
This probable gift, more precious than any, is surely the genetic legacy that Ostrer, despite his title, cannot quite bring himself to spell out, and that Wheelwright ignores. Nonetheless, both authors have written books that succeed on different levels. Legacy in particular is a careful and approachable survey of a complex field. And it establishes that as well as the religious and cultural answers, there is also a biological component, of a significance yet to be assessed, to the question of who is a Jew.
Nicholas Wade writes about biology for The New York Times and is working on a book about human variation. His most recent book, The Faith Instinct, explores the evolutionary basis of religion.
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