Y-chromosomal Aaron Totally Explained

Everything about Y-chromosomal Aaron totally explained

Y-chromosomal Aaron is the name given to the hypothesised most recent common ancestor of many of the patrilineal Jewish priestly caste known as Kohanim (singular "Kohen", "Cohen", or Kohane). In the Hebrew Bible this ancestor is identified as Aaron, the brother of Moses. Research published in 1997 and thereafter has indicated that a large proportion of contemporary Jewish Kohanim share Y-chromosomal haplogroup J1 with a set of genetic markers, known as the Cohen Modal Haplotype, which may well derive from a single common ancestor.
This DNA among Jewish Kohanim, J1+CMH, also exists among Samaritans; however Samaritan Kohanim descend from a non-Aaron family line, having haplogroup E3b1a M78..

Background

Although membership in the Jewish community has, since the second century CE, been passed maternally (see: Who is a Jew?), membership in the group that originally comprised the Jewish priesthood ("Cohen" or "Kohen"; plural: Kohanim), is patrilineal. Modern Kohens claim descent from a biblical person, Aaron, brother of Moses, in the direct lineage from Levi, the patriarch of the Tribe of Levi, grandson of Abraham, according to the tradition codified in the Tanakh (שמות / Sh'mot/Exodus 6). DNA testing is aiding scholars to trace the lineages found among modern Jewish populations, including contemporary Cohen families, to decipher origins of the people groups that were joined to the ancient Israelites and to identify genetic admixture and genetic drift.

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

Males who share a common patrilineal ancestor should 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 almost identical Y chromosomes; this can be tested 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.)

Initial studies

The Cohen hypothesis was first tested by Prof. Karl Skorecki and collaborators from Haifa, Israel, in 1997. In their study, "Y chromosomes of Jewish priests," published in the journal Nature, they found that the Kohanim appeared to share a different probability distribution compared to the rest of the Jewish population for the two Y-chromosome markers they tested (YAP and DYS 19); and that furthermore the probabilities appeared to be shared by both Sephardi and Ashkenazi Cohens, pointing to a common Cohen population origin before the Jewish diaspora under the Roman empire.
   A subsequent study the next year (Thomas MG et al, 1998) increased the number of Y-STR markers tested to six, as well as testing more SNP markers. Again, they found that a clear difference was observable between the Cohanim population and the general Jewish population, with many of the Cohen STR results clustered around a single pattern they named the Cohen Modal Haplotype:
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In words, this says that the odds in favour of Cohen ancestry C (for example, the probability of having Cohen ancestry, divided by the probability of not having Cohen ancestry), having observed some piece of data D, is given by the odds one would assign given only one's initial information I, multiplied by the probability of having observed D if C is true, divided by the probability of having observed D if C is false.
   (In fact, for convenience we'll work with the reciprocal of this equation, ie work in terms of odds against, rather than odds on).
   The proportion of the whole male Jewish population that has Cohen ancestry has been estimated at 5%

   Note, some lineages of subgroup J2 also, coincidentally, developed a similar set of genetic markers corresponding to the Cohen Modal Haplotype. Low frequencies of individuals with these lineages even exist among Jewish Kohanim, albeit they don't descend from Y-Aaron.
   The subdivision of J2 which most closely matches the genetic signature of the J1 Cohens is subclade J2a1b, a large fraction of members of which will also have a 6/6 match for the 6-marker CMH. However, this is an example of haplotype convergence: Basically the haplotype "distribution" within one lineage (Haplogroup) overlaps with the haplotype "distribution" of another lineage - it's like overlapping branches from two different trees. The more likely reason for the match is convergence (coincidence) or sharing a common haplotype in the same lineage (Haplogroup). Convergence: Mutation is a random process and over thousands of years can occur in different lines so that by coincidence, different "lines" end up with "matching" haplotypes. This accidental agreement is called convergence of different genetic lines, which it's believed have been not been closely related for at least the last 10,000 years; the group in J2a1b who have the 6-marker CMH are devoid of any Cohen traditions in their families.
   On the other hand, there are families in Haplogroup J2 who do have a Cohen religious tradition and are proud of it (as there are in several other haplogroups, including Haplogroup R1b). The haplotypes of these Haplotype J2 Kohanim cluster in a unique, small offshoot of J2a1*, close to haplotypes of the J2a1k clade, not the J2a1b clade.. This appears to explain the finding of that paper that "typing a limited number of Italian Cohanim (A. Novelletto unpublished obs.) for the STRs used here, we determined that the Cohen Modal Haplotype ('an important component in the sharing of Ashkenazic and Sephardic Israelite Y chromosomes', Thomas et al. 2000) does indeed belong to network 1.2" (ie the population having DYS413a,b<=18, which is the signature of the J2a1 subclades).

More detailed Cohen haplotypes

In the table below, the first line gives the original 6 marker Cohen Modal Haplotype (CMH-6), which was the basis for the original published papers. The second gives an extended 12 marker haplotype (CMH-12) informally released by the private company FTDNA, based on further work by much of the same research team. It hasn't yet been peer group reviewed by other scientists or published in the open technical literature.
   The next sequence of rows identify other 6-marker haplotypes in haplogroup J found to occur more than once in the sample of 145 Cohanim tested in Behar et al (2003) || DYS
393 || DYS
390 || DYS
19 || DYS
391 || DYS
385a || DYS
385b || DYS
426 || DYS
388 || DYS
439 || DYS
389i || DYS
392 || DYS
389ii || || AC possible links were discussed between the Jews and the Kurds; and some suggested that 4/4 matches in non-Jewish Italians might be a genetic inheritance from Jewish slaves, deported by Emperor Titus in large numbers after the fall of the Temple in AD 70, some of whom were put to work building the Colosseum in Rome.
   Such speculation was to some extent tempered when it was realised that Haplogroups J1 and J2 represented at least two different lineages which could be associated with the CMH, (the Italians mostly belong to Haplogroup J2); and that individuals with at least 5/6 matches for the original 6 marker Cohen Modal Haplotype occur widely across the Middle East, with significant frequencies in various Arab populations mainly with J1 Haplogroup, "that are not traditionally considered admixed with mainstream Jewish populations" -- notably Yemen (34.2%), Oman (22.8%), Negev (21.9%), and Iraq (19.2%); and amongst Muslim Kurds (22.1%), Bedouins (21.9%), and Armenians (12.7%).
   On the other hand, Jewish populations were found to have a "markedly higher" proportion of full 6/6 matches, according to the same (2005) meta-analysis, The detailed data confirms that the main cluster of haplogroup J1 haplotypes from the Yemen appears to be some genetic distance different from the CMH-12 pattern typical of Eastern European Ashkenazi Cohens.

Y-chromosomal Levi?

A similar investigation was made with men who consider themselves Levites. Whereas the priestly Kohanim are considered descendants of Aaron, who in turn was a descendant of Levi, son of Jacob, the Levites (a lower rank of the Temple) are considered descendants of Levi through other lineages. Levites should also therefore share common Y-chromosomal DNA.
   The investigation of Levites found high frequencies of multiple distinct markers, suggestive of multiple origins for the majority of non-Aaronid Levite families. One marker, however, present in more than 50% of Eastern European (Ashkenazi) Jewish Levites points to a common male ancestor or very few male ancestors within the last 2000 years for many Levites of the Ashkenazi community. This common ancestor belonged to the haplogroup R1a1 which is typical of Eastern Europeans, rather than the haplogroup J of the Cohen modal haplotype, and most likely lived at the time of the Ashkenazi settlement in Eastern Europe. .
   The E1b1b1 haplogroup formely known as E3b1 has been observed in all Jewish groups world wide. It is considered to be the second most prevalent haplogroup among the Jewish population. According to one major paper it has also been observed in moderate numbers among individuals from Ashkenazi, Sephardic and Samaritan communities having traditions of descending from the tribe of Levi, suggesting that the E1b1b1 Levites may have existed in Israel before the Diaspora of 70 C.E.
   The Samaritan community is a small, isolated, and highly endogamous group today numbering some 650 members who have maintained extensive genealogical records for the past 13–15 generations. Since the Samaritans maintain extensive and detailed genealogical records, it's possible to construct accurate pedigrees and specific maternal and paternal lineages. The Samaritan community in the Middle East survives as a distinct religious and cultural sect and constitutes one of the oldest and smallest ethnic minorities in the world. Y-Chromosome studies have shown that the majority of Samaritans belong to haplogroups J1 and J2 while the Samaritan Cohanim belong to haplogroup E1b1b1a, formely E3b1a.. In 1623-1624 the last member of the High-Priestly family, which claimed descent from the Eleazar son of Aaron, died. The office was then given to a Levite branch, descended from Uzziel, the son of Kohath. Since that date the priest has called himself "Ha-Kohen Ha-Lewi", which means the Priest-Levite, instead of "Ha-Kohen Ha-Gadol", a title which referred to the High-Priest as in previous times. The approximately 650 individuals comprising the total group of present day Samaritans trace their ancestry over a period of more than 2,000 years to the Biblical Israelite tribes of Ephraim, Menashe and Levi. As a religious sect, the Samaritans broke away from the main stream of Judaism around the fifth century B.C.E.

   The biblical tradition of the origin of the Cohen family among the Samaritans is found in 2 Kings 17:27-28 where it indicates that only one Israelite Cohen was sent back from exile in Assyria, circa 722 BCE, by the King of Assyria to teach those living in the Northern Kingdom of Israel (Samaria); this suggests a strong association of haplogroup E3b1a with the biblical Cohenim who authored and compiled the biblical text of the Book of Kings. Those non-Israelites relocated to the region around Samaria by the Assyrians in the same period that the Northern Tribes were exiled to Assyria, later appointed other non-Israelite Cohenim from their own people who were performing non-Israelite authorized priestly functions as observed by the biblical, priestly author at the time of the composition of the Book of Second Kings (cf. 2 Kings 17:32-34); this may be an origin for some among the J1 and J2 Cohenim haplotypes observed among Jewish populations today.

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