According to Jewish law, tribal affiliation (including whether one is a kohen) follows the direct paternal line, while the question of Jewishness follows the maternal line. Does this mean that genetic testing is a valid way of ascertaining whether one is Jewish or a kohen?

First, some basics. Females have XX chromosomes and males have XY. All females carry one X chromosome from their mother and one X chromosome from their father. Males, on the other hand, get their X chromosome from their mother and their Y chromosome from father. Since these chromosomes are passed from one generation to the next, it is theoretically possible to identify one’s ancestors through genetic testing.

Jewish Ancestry and Mitochondrial DNA

As mentioned, Jewish identity follows the maternal line. If your mother is Jewish, you’re Jewish. However, there is no such thing as a “Jewish gene,” so genetic testing cannot conclusively state whether a person is Jewish.

However, there does seem to be at least one way in which genetics may be used to help determine a person's Jewishness. This involves using what is called mitochondrial DNA (or mtDNA), which is passed exclusively from the mother through the female line.

In a fascinating study published in 2006, it was shown that 40% of all Ashkenazi Jews are descended from just four Jewish women who lived more than 1,000 years ago. The study concluded that if someone bears specific mitochondrial DNA markers, there is a 90-99% chance that he or she is descended from one of these Jewish women.1

Of course, there are the other 60% of Ashkenazi Jews who do not come from these four women, as well as Sephardic Jews and converts.

Nevertheless, although still a matter of debate, there are some who hold that in a case where there is some evidence of Jewishness but no iron-clad proof, having this marker in conjunction with other supporting evidence can be used to conclude that the person is indeed Jewish.2

(As a disclaimer, this article is for informational purposes only. All practical questions regarding one’s Jewish identity should be directed to a qualified rabbi.)

The Kohen Gene

We can now turn to the question of kohanim (Jewish priests).

All kohanim are directly descended—on their father's side—from Aaron the High Priest (Moses’ brother). Knowing that a copy of the Y chromosome is passed from father to son, Dr. Karl Skorecki, together with other colleagues, conducted a study in the 1990s to analyze and compare the Y chromosomes of kohanim with those of the non-kohen Jewish population.

In addition to the genes in the Y chromosome that determine if a person is male, the chromosome mostly consists of non-coding DNA, which tends to accumulate mutations. Based on the fact that the Y chromosome is passed down the paternal line without recombination, the genetic information on a Y chromosome of a man living today is basically the same as that of his ancient male ancestors, except for the rare mutations that occur along the hereditary line. A combination of these neutral mutations, known as a haplotype, can serve as a genetic signature of a man’s male ancestry.

Looking at six kinds of the YAP haplotype of the Y chromosome and comparing their frequency in kohanim and Jewish non-kohanim, Dr. Skorecki found that the majority of self-identified kohanim, both those of Sephardic as well as Ashkenazi descent, are all descended from the same person who lived roughly 3,000 years ago.

It should be noted that this marker was found in a much lower frequency among Jews who had no tradition of being kohanim, and in an even lower rate among non-Jews (although interestingly, it was found in a higher rate among the Lemba tribe in Africa, who have a tradition of being descendants of Jews).3

However, kohen status is dependent not only upon being the biological descendant of Aaron, but upon numerous other factors as well. For example, if a kohen marries a divorcée (or certain other women), their offspring would not be kohanim. So if one carries the genetic marker of kohanim, then perhaps he had a kohen in his ancestry, but he himself may not be a kohen or even Jewish, since that is dependent upon the mother.

Our sages tell us that when Moshiach comes, he will clarify our lineage and determine who in fact is a kohen, Levite or Israelite.4 May we merit the messianic era speedily in our time!