Genome study helps map Diaspora, highlights how heritages blended
Scientists taking a genomewide view of ancestry have traced the genetic roots of seven Jewish groups.
The study examines genetic markers spread across the entire genome — the complete set of genetic instructions for making a human — and shows that the Jewish groups share large swaths of DNA, indicating close relationships. Comparison with genetic data from non-Jewish groups indicates that all the Jewish groups originated in the Middle East. From there, groups of Jews moved to other parts of the world in migrations collectively known as the Diaspora.
Maps of those migrations were inscribed in the DNA in the form of genetic signatures of local people Jews interbred with as they moved. Today, contemporary Jews carry evidence of their Middle Eastern origin along with genetic heritage from European and North African ancestors.
“I like to think of relatedness as a tapestry, and these shared segments [of DNA] are threads in the tapestry,” says Harry Ostrer, a geneticist at New York University School of Medicine and the leader of the study, published online June 3 in the American Journal of Human Genetics.
Each of the Jewish groups (Iranian, Iraqi, Syrian, Italian, Turkish, Greek and Ashkenazi) has its own genetic signature but is more closely related to the other Jewish groups than to non-Jewish groups, the researchers found. Even though the researchers took care to exclude from the analysis people known to be directly related, any two Ashkenazi Jewish participants in the study shared about as much DNA as fourth or fifth cousins, Ostrer says.
Better understanding the genetic heritage of individuals may help researchers zero in on disease genes that are more common in certain ethnic groups, he says. For instance, many Ashkenazi Jewish women are at high risk of developing breast cancer, even though the women often don’t carry versions of genes known to be linked to breast cancer.
Previous studies of Jewish heritage using parts of the genome inherited only from the mother or father have reached similar conclusions, but the new study examines the entire set of genetic blueprints, allowing researchers to uncover contributions from many ancestors at once.
“This is an almost perfect confirmation of what we knew,” says Francesc Calafell, a human population geneticist at the Institute of Evolutionary Biology and Pompeu Fabra University in Barcelona. Although the new study doesn’t contain any real surprises, it gives a more complete picture of ancestral ties and leaves little room for doubt about the origin of different Jewish groups, he says.
About 2,500 years ago, Iranian and Iraqi Jews diverged from European and Syrian Jews into genetically distinct groups, the study shows.
Analyses of Ashkenazi Jews — a group that settled in the Rhine Valley sometime before the year 1000 and migrated into Eastern Europe between the 11th and 15th centuries — and of non-Jewish European groups show that southern Europeans and Ashkenazi Jews are closely related. The researchers found no evidence for a theory that Ashkenazi Jews are descendents of Khazars, a group of people who lived near the Black Sea, or of Slavic speakers in Eastern Europe who converted to Judaism.
Historical records suggest there were about 50,000 Ashkenazi Jews in 1400 and that the number expanded to 5 million by 1800, one of the largest population booms ever recorded, Ostrer says. That bottleneck and population explosion is also recorded in the DNA, his team found.
Companies specializing in DNA tests for genealogy are likely to use the new data to tell clients how much Jewish ancestry they have. But Calafell notes that there is more to being a Jew than genetic heritage.
“I don’t think Jewishness can be defined in the genome,” he says.
Ostrer says it is for clerics to decide who is a Jew.
“We’re not proposing to replace rabbinical law as a method for determining who is Jewish,” he says.
Atzmon, G. et al. 2010. Abraham’s children in the genome era: Major Jewish diaspora populations comprise distinct genetic clusters with shared Middle Eastern ancestry. American Journal of Human Genetics, published online June 3.