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Archiver > GENEALOGY-DNA > 2006-08 > 1156622088


From: "Earl Beaty" <>
Subject: The Unreliable SNP
Date: Sat, 26 Aug 2006 13:54:48 -0600
References: <a78f6d2d0608252036icce286fodb054dea1883aa1b@mail.gmail.com> <003301c6c920$d69ba9a0$26c794d0@S0029789850>


A few weeks about I reported here an unusual result on SNP P25 for myself.
There are plenty of indications that I am derived for P25, but the test was
done anyway by Ethnoancestry. This test was done in a detailed way on me and
several other men. Some background information will be useful for those who
don't remember the previous post.

P25 has 3 alleles and as such it is said to really not be a SNP at all. The
three alleles are all of the C type in the ancestral state. The ordinary
derived state has one of the C alleles converted to an A type. In 2005 Adams
et al published a paper noting that in some men a second C gets convert to A
producing CAA, and sometimes the first A gets converted back to a C,
restoring the CCC. [Adams et al, The Case of the Unreliable SNP: Recurrent
Back-mutation of Y-chromosomal Marker P25 Through Gene Conversion.
(abstract) Forensic Science International 159:14-20, 2006] The derived state
has long been called R1b, but is now being called R1b1. The report on my
case was that I have no C type allele. A presumption was that I am AAA. I
requested that Ethnoancestry review the evidence to confirm or correct this
unusual state. I got a report from Jim Wilson saying that I definitely have
no C, but he can't tell about the number of As. There could be one, two or
three of the A type. The cause of this situation is unknown, but Jim seems
to think that it is probably as second gene conversion of a C to A just like
the first. That would mean that I am AAA. This kind of mutation is
apparently rare, and two of them would be extremely rare.

This is the second time I have been found to have a rare condition. The
first was ccgg at 464X. I consulted Mark Jobling and he could see no way for
these two rare events to have a common cause. He concludes that it probably
is just an accident. Both Jim and Mark note that it is possible that
somewhere in my ancestry a C got deleted (perhaps 2 of them).

We don't have very good information on how rare this condition is. FTDNA has
done many P25 tests so I consulted Bennett Greenspan. He says that they use
a testing method which does not reveal these details. Adams et al conclude
that P25 is too unreliable to be used in forensic work. I wonder if it is
too unreliable to use in genealogy work.

--Earl Beaty


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