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.
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.
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