Albert Einstein was the most famous Jew of the 20th century. In 1921, he wrote a letter to the Berlin rabbis in which he observed, "I notice that the word Jew is ambiguous in that it refers (1) to nationality and origin, (2) to the faith." Population genetics research is adding new meaning to Einstein's view of being Jewish. Most Jews are familiar with the game of Jewish geography, the ability of two Jews to pick each other out from a crowd and to develop a narrative based on shared acquaintances. Mention genetics and most Jews will talk about the risk of Tay-Sachs disease, a devastating childhood condition that has been virtually eliminated by identifying the carriers of the deleterious gene prior to their having affected children. Or they will talk about the risks for breast and ovarian cancer from mutations in the BRCA 1 and 2 genes that are prevalent in many Jewish groups. They will also talk about the set of markers on the Y chromosome that are transmitted from father-to-son for men who claim the special privileges of the Cohen priestly lineage, one that is thought to date back to Moses' brother, Aaron.
The new genetics is demonstrating that most Jews share genetic links. Any two Ashkenazi Jews whose great grandparents lived in Central and Eastern Europe before the war share enough genes to qualify as fourth to fifth cousins. Any two Iranian Jews whose forebears lived in Iran from the time of Cyrus the Great to the fall of the Shah share enough genes to qualify as third cousins. The Jews of Djeba, who come from an island off the coast of Tunisia that once had a majority Jewish population, all qualify as first cousins once-removed. The links don't stop within each group. Rather, they cross most Jewish groups so any Ashkenazi and Iranian Jewish duo is more likely to share genes with each other than they are with their non-Jewish neighbors. Jews have characterized themselves as a people, but with these genetic links, they are more like an extended family.
The new genetics is also showing that the history of the Jews can be seen in their genes. As they migrated around the Mediterranean Basin in Greco-Roman times, the Jewish people converted members of local populations, who genes then became part of the local Jewish genetic pool. Indeed, Josephus, the well-known Roman Jewish historian, wrote about the six million Jews who lived across the Roman Empire. As a result, European Jews demonstrate greater European ancestry and North African Jews demonstrate greater North African ancestry than their Middle Eastern brethren. Besides showing shared links and ancestral origins, these studies also demonstrate that some Jewish populations expanded rapidly in recent times.
Jews can be said to be a people with a shared genetic legacy, but not all Jews share the same genes, nor is having part of that legacy a requirement for being Jewish. Jewish groups that lived in India or Ethiopia share strong genetic links within their communities, less so with the Jews who came from communities around the Mediterranean Basin. Jews by choice have converted under the direction of a rabbinical court may not share genes with their co-religionists.
Nonetheless, shared genetic legacy can be a factor in Jewish identity that takes its place alongside those factors identified by Einstein -- nationality (or group membership), the culture emanating from group membership, and shared religious belief. In the United States, religion remains a powerful force in Jewish identity, as do an inner commitment to being Jewish and maintaining significant Jewish friendship ties. Holocaust remembrance has become less of a feature of Jewish identity and more of a feature of national identity, as most states have mandated teaching in schools about Nazi intolerance. During Einstein's era, anti-Semitism was common and its own cohesive force among American Jews. Now most Jews report no personal experience with anti-Semitism.
Not all Jews agree on the issue of membership. Gorshem Gorenberg has written, "More than any other issue, the question of 'Who is a Jew?' has repeatedly roiled relations between Israel and American Jewry." Psychologically, it is an argument over who belongs to the family as the validity of religious conversions and personal choices have been questioned. Yet, genetic analysis can answer with a fair degree of accuracy who is my mother, my uncle, my fifth cousin. In the process, it can identify not only our relatedness, but also our ancestry. The notion is starting to stick outside the community of geneticists. Rabbi Yuval Cherlow has suggested that genetic analysis might serve as a consultant to Jewish law.
As a consultant, Jewish genetics is unlikely to replace the hegemony of Jewish law and Jewish culture, nor should it. But as population genetics gains a foothold in the community with Jews and non-Jews alike wanting to know about their origins, ancestors, and relatives it will take its place in the formation of group identity. In the process, observers can marvel at seeing their ancestral histories woven into their genomes.