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Y-chromosomal Aaron is the name given to the hypothesised ancestor of the Kohanim (singular Kohen), a patrilineal priestly caste in Judaism. In Scripture, this ancestor is identified as Aaron, the brother of Moses. This has come about since the appearance in the 1990s of some relevant publications, relevant to the patrilineal ancestor of all humans, Y-chromosomal Adam.
In genetics it is understood that every male inherits 23 chromosomes from each parent, but because the Y chromosome is only present in males, it is never inherited from the mother. Mitochondrial DNA (mtDNA), which is not on any chromosome but is instead passed from mother to child in the cytoplasm of the egg, always comes from the mother.
As membership of the Jewish priesthood caste is patrilineal, and male children always get their Y chromosome from their father, it can be hypothesised that all kohanim should share the same Y chromosome, neglecting any mutations that have occurred since Aaron. These mutations should occur at a relatively constant rate, allowing scientists to estimate the time since two lineages split by counting the number of mutations. See molecular clock.
Testing the hypothesis
This hypothesis was first tested by Prof. Karl Skorecki and collaborators from Haifa, Israel. In their study (1997) it was discovered that many Kohanim share certain Y-chromosome markers, which argues in favor of the hypothesis. Other studies (Thomas MG et al, 1998) have corroborated the findings. This led to the development of the Cohen Modal Haplotype, a set of Y-chromosomal markers that might have been shared by the Biblical Aaron.
One might argue that from the premise of the study, all Jews should share the same Y chromosome. After all, Aaron and Moses descended patrilineally from Jacob, the ancestor of all Jews. However religious conversion as a source of distinct genetic material and the special status of the Cohen family (Kohanim) among the Jews militate against the expectation that all Jews should share the same Y chromosome. Converts can never be part of the Kohanim, a status which is passed from father to son only, whereas status as a Jew is based upon the mother only. So all members of the Kohanim are Kohanim based upon the father and Jews based upon the mother.
- Skorecki K, Selig S, Blazer S, Bradman R, Bradman N, Waburton PJ, Ismajlowicz M, Hammer MF. Y chromosomes of Jewish priests. Nature 1997;385:32. PMID 8985243.
- Thomas MG, Skorecki K, Ben-Ami H, Parfitt T, Bradman N, Goldstein DB. Origins of Old Testament priests. Nature 1998;394:138-40. PMID 9671297.
- Thomas MG, Parfitt T, Weiss DA, Skorecki K, Wilson JF, le Roux M, Bradman N, Goldstein DB. Y chromosomes traveling south: the cohen modal haplotype and the origins of the Lemba--the "Black Jews of Southern Africa". Am J Hum Genet 2000;66:674-86. PMID 10677325.
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