Cohen Modal Haplotype

Y-chromosomal Aaron is the name given to the hypothesized most recent common ancestor of the majority of the patrilineal Jewish priestly caste known as Kohanim (singular "Kohen", also spelled "Cohen"). According to the Hebrew Bible, this ancestor was Aaron, the brother of Moses.

The original scientific research was based on the hypothesis that a majority of present-day Jewish Kohanim share a pattern of values for six Y-STR markers, which researchers named the Cohen Modal Haplotype (CMH).

Additional early research using twelve Y-STR markers indicated that about half of contemporary Jewish Kohanim shared Y-chromosomal J1 M267, (specifically haplogroup J-P58, also called J1c3), while other Kohanim share a different ancestry, including haplogroup J2a (J-M410).

For human beings, the normal number of chromosomes is 46, of which 23 are inherited from each parent. Two chromosomes, the X and Y, determine sex. Women have two X chromosomes, one inherited from each of their parents. Men have an X chromosome inherited from their mother, and a Y chromosome inherited from their father.

Males who share a common patrilineal ancestor also share a Y chromosome, diverging only with respect to accumulated mutations. Since Y-chromosomes are passed from father to son, all Kohanim men should theoretically have nearly identical Y chromosomes; this can be assessed with a genealogical DNA test. As the rate that mutations accumulate on the Y chromosome is relatively constant, scientists can estimate the elapsed time since two men had a common ancestor. (See molecular clock.)

Although Jewish identity has, since at least the second century CE, been passed by matrilineal descent according to Orthodox tradition (see: Who is a Jew?), membership in the group that comprises the Jewish priesthood (Kehuna), has been determined by patrilineal descent (see Presumption of priestly descent). Modern Kohanim claim descent from a biblical Aaron, a direct patrilineal descendant of Abraham, according to the tradition codified in the Tanakh (שמות / Sh'mot/Exodus 6).

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