Recent media reports that Israel’s Rabbinate has used DNA testing to prove Jewishness are alarming. Israel needs to consider the science and probability behind DNA testing before it allows such practices to continue.
Recent scientific studies have shown that the Jewish people are neither genetically homogenous, nor genetically unique. There is no specific gene or genetic marker that offers proof positive that one is Jewish.
While some specific genetic markers are statistically more prevalent among Ashkenazi Jews versus non-Jews, there are also many Jews who do not have these markers, making them inconsequential for proving Jewishness.
Despite what you may remember from the 19th and 20th centuries, race is an entirely social construct, with no basis in genetics or science. Attempts to use science to prove to what extent someone is or is not a Jew have failed.
Unlike belief and ideology, an individual’s DNA does not change due to conversion or assimilation. Throughout millennia, some non-Jews became Jews and some Jews became non-Jews, mixing and obscuring any semblance of Jewish DNA, which almost certainly never existed in the first place.
Genealogical DNA tests can only compare DNA from an individual to DNA samples in existing databases, in order to find matching DNA sequences.
If person X has a certain amount of DNA sequences that are identical to person Y, and person Y is Jewish, then there exists some probability that the two are related and have at least one Jewish ancestor in common. But it is also possible that person X and Y have a common ancestor who is not Jewish. There also remains the possibility that the sequences person X and Y share is just a random coincidence.
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Using DNA to prove ancestry is even more complicated, because DNA is inherited non-deterministically: You get half your DNA from each parent, but the half you receive is seemingly random, based on our current level of understanding.
A DNA test cannot reveal equal information about all of one’s ancestors as, inevitably, traces of some ancestors will be more dominant than others, and some ancestors may not be traceable at all. It is also possible that the majority of an individual’s ancestors have certain genetic markers, but those markers change at conception and become untraceable in that individual and their offspring.
Media reports indicate the tests that were performed for Israel's Rabbinate analyze mitochondrial DNA, which is inherited from one’s mother. Theoretically, if one keeps to a halakhic definition of who is a Jew – maternal descent – this is appears to be an ideal test to determine Jewishness. If there were a database of the mitochondrial DNA of every Jewish woman (including converts), then one could simply perform a DNA test and check for a match – and bingo, we have a Jew.
But no such database exists.
Even if it is true that 40% of Ashkenazim are descendants of four women, as one genetic study theorizes, since we have no way of knowing if those women were halakhically Jewish, being able to trace linage to them should be of no consequence to rabbinic authorities.
Consider a scenario in which two non-Jewish sisters marry, one to a Jew after a recognized halakhic conversion, and one to a Catholic. For successive generations, the Catholic sister’s descendants remain Catholic and the Jewish sister’s descendants remain Jewish.
After a few generations the Jewish sister’s daughter’s daughter’s daughter’s daughter (a great-great-granddaughter), who is now from a Jewish family no different from any other Jewish family, needs to do a mitochondrial DNA test to prove she is Jewish. Her test results would be practically identical to those of a fourth cousin who is a direct maternal descendent of the non-Jewish sister.
What would such a test prove? Nothing, except that these two individuals share a direct maternal ancestor. How is that information of use to the Rabbinate?
Since none of God’s revelations included DNA sequences from ancient Israelites, we have no primordial Jewish DNA to use as a reference point for determining genetic Jewishness. Genealogical DNA testing is good for assessing how closely individuals are related, but trying to determine ancestral heritage or ethnicity through DNA is an exercise in statistics.
If the Rabbinate is using DNA testing to prove Jewishness, what level of statistical confidence are they requiring? If test results indicate an 85% probability that someone is of Jewish heritage, is that sufficient evidence?
What DNA database is the rabbinate using to compare test results to? Does this database contain DNA samples from every Jewish community in the world, including those of India or Ethiopia, or is the database biased toward large Ashkenazi Jewish communities?
What portion of the genome (the totality of DNA found in one cell) is the Rabbinate sequencing in its tests, and how was that amount decided?
Such basic questions should be posed to the Rabbinate to assess the validity of their scientific method; test results will vary significantly depending on the methodology and sample.
If the Rabbinate is using DNA testing to verify biological relationships, for instance to answer the question: Is this person’s mother really her mother? – then the tests are drastically more reliable, but are no better at definitively proving Jewishness. Again, the tests can't determine whether or not the biological mother was halakhically Jewish.
In his day, Maimonides (Rambam) wrote a famous letter to Obadiah the Proselyte in response to the latter’s question: As a convert to Judaism, can he use the first person plurals "us," "we," and "our" in reference to the Jewish people in prayer, whether alone or in the synagogue?
Maimonides concluded that those who adopt Judaism and follow the laws of the Torah become the descendants of Abraham: "In the same way as he converted his contemporaries through his words and teaching, he converts future generations through the testament he left to his children and household after him."
If the Rabbinate and other Diaspora rabbinic authorities start relying on DNA testing for proof of Jewishness, it would be a major departure from the traditions of equality and community among the Jewish people.
In that dystopic future, your Jewishness and right to belong could end up being measured in a lab and calculated on a spreadsheet.
Noah Slepkov is a fellow at the Jewish People Policy Institute and the author of the 2014 JPPI study, "Crowd Sourced Genealogy and Direct-to-Consumer DNA Testing: Implications for the Jewish People." Twitter: @NZS81