questioning the "cohen gene" #dna


Joan Hartman <joanhartman@...>
 

Does anyone have any science-based response to the following paraphrase
from an anthropology blog, which challenges the conventional understanding
of the "cohen gene" (the blog is written anonymously so I cannot evaluate
the author's credentials):

"One of the first applications of Y-chromosome testing was the discovery
that Jewish Cohanim exhibited a particular Y-chromosome haplotype, called
the Cohen Modal Haplotype (CMH). The first letter announcing this discovery
(www.familytreedna.com/nature97385.html) showed a spectacular difference
between Cohanim and non-Cohanim. The Cohanim had trace frequencies of
haplogroup E3b, and a particular DYS19 allele at high frequency. While this
finding proved different histories for priests and non-priests, it did not
prove descent >from a single individual because the YAP- DYS19B combination
is not a monophyletic lineage. (If two men have the same haplotype, it
does not mean that they are descended >from the same ancestor. This is due
to the fact that microsatellites defining haplotypes mutate quite fast, so
two unrelated men may have the same haplotype by chance. The probability of
this increases as the number of microsatellites decreases.) In another
study
(www.journals.uchicago.edu/AJHG/journal/issues/v66n2/990488/990488.html),
the authors discovered a more extended version of the same haplotype
(DYS19-14, DYS388-16, DYS390-23, DYS391-10, DYS392-11, DYS393-12) at high
frequencies.

The Cohen modal haplotype belongs to a Y-chromosome haplogroup called J (or
HG-9). A haplogroup is defined by a unique event polymorphism, and men who
belong to the same haplogroup are indeed descended >from a single man. But,
in the case of J, that single man lived more than 10,000 years ago, long
before the time of Aaron. However, J is split into two lineages that are
also more than 10,000 years old: J2 (or Eu9) and J1 (or Eu10). If people
who have the CMH are always in just one of these groups -- either only in
J1 or only in J2 -- then the CMH could reflect descent >from a single
individual. But, if it is found in both, then by definition the CMH could
not reflect descent >from Aaron, because he lived approximately 3500 years
ago, much after the common ancestor of J1 and J2. Both groups can't be
descended >from Aaron. In this study
(bioanthropology.huji.ac.il/pdf/Nebel_2001b.pdf), 22 out of 25 individuals
with the CMH belonged to J1. But the appendix of the same study shows that
haplotype 108 is also part of the CMH, but haplotype 108 belongs to
haplogroup J2. In another study
(www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11592923&dopt=Abstract)
the authors state that: "By typing a limited number of Italian Cohanim for
the STRs used here, we determined" that the CMH "does indeed belong to
network 1.2." Network 1.2 falls under the J2 haplogroup. In the
supplementary materials of the following comprehensive article on Ashkenazi
Jewish Y-chromosome variation, 28 of the Jewish CMHs belong to J1 and 25
belong to J2.
(www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14740294&dopt=Abstract)

Note that in Anatolia, in region 6 (south), 12% of the population belongs
to the
J1-CMH. (hpgl.stanford.edu/publications/HG_2004_v114_p127-148.pdf). Among
Armenians in the West region, the CMH frequency is
4.4%. (www.ucl.ac.uk/tcga/tcgapdf/Weale-HG-01-Armenia.pdf).

Thus, several studies have now shown that the CMH occurs among Jews in both
J1 and J2 and does not represent a single lineage. More importantly, the
CMH was first identified because of its high frequency compared to other
haplotypes. The strength of this evidence is diminished by the finding that
CMH chromosomes belong to two unrelated lineages. Furthermore, the dating
of CMH chromosomes to Aaron's time should be reconsidered, and the
molecular variation within J1 and J2-background CMH and its neighbors
should be considered separately. In conclusion, the true genetic identity
of Aaron remains elusive."

Joan Hartman

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