What does genetics say about being Jewish?

In 2010, human geneticist Harry Ostrer of the Albert Einstein College of Medicine in New York City and colleagues found that three of the major Jewish groups – the Middle Eastern, Sephardic, and Ashkenazi Jews – share a genetic connection going back more than 2,000 years, and are more closely related to each other than to nearby non-Jewish groups.

The study showed genetic ties within each of the groups were even closer, about the equivalent of fourth or fifth cousins. But that study didn’t include North African Jews, who represent the world’s second-largest Jewish population, or any groups whose claim to Jewishness has been controversial, such as Ethiopian Jews.

So Ostrer and his colleagues gathered new DNA samples from Jews living everywhere from Morocco to Yemen. Using three distinct strategies for identifying genetic similarities – including a method called identity by descent (IBD) that can determine how closely related two individuals are – the team compared these DNA samples to each other, to the samples from their 2010 study, and to samples from non-Jews.

Most of the sampled groups shared genetic features, indicating a common heritage dating to before Roman times, the team reported this month in the Proceedings of the National Academy of Sciences. North African Jews – Moroccan/Algerian Jews in particular – showed a close genetic connection to Ashkenazi and Sephardic Jews and little evidence of interbreeding with contemporary non-Jewish populations in North Africa. Georgian Jews shared genetic features with Middle Eastern Jews, instead. Yemenite Jews were distantly related to Middle Eastern Jews, while Ethiopian Jews formed their own cluster and shared little IDB with other Jewish populations.

Each group showed little interbreeding with local non-Jewish groups. Moroccan/Algerian Jews, for example, were about as close genetically as third or fourth cousins; Jews from the Tunisian island of Djerba were as close as first cousins once removed.

The team’s results suggest that while most Jewish groups are genetically related, some are not and instead arose from converts to Judaism. But regardless of their origins, Jewish groups remained genetically isolated once formed.

Identifying the genetic component of Jewishness is controversial because the Holocaust was predicated on the idea that Jewishness was a genetic trait that could be eliminated from the German population. But it could have medical as well as historical value.

Geneticist Sarah Tishkoff of the University of Pennsylvania, who was not involved in the work, notes that many Jewish populations have high incidences of genetic disease.