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Archiver > GENEALOGY-DNA > 2009-12 > 1260862984


From: "Alister John Marsh" <>
Subject: Re: [DNA] R-U152 and R-L21 on the European Continent- back mutations
Date: Tue, 15 Dec 2009 20:43:04 +1300
References: <4B264DA0.4020206@san.rr.com><200912150636.nBF6aIcW020798@mail.rootsweb.com>
In-Reply-To: <200912150636.nBF6aIcW020798@mail.rootsweb.com>


Tim,

An example which I encountered today which demonstrates some difficulties in
TMRCA calculations....

I have a person of different surname to me who is a 61/67 match to me, and
is 70/76 match on 76 markers. This match includes 11 quite distinctive off
R1b modal marker scores. FTDNA Tip estimates over 96% chance of us sharing
a male line common ancestor within the past 24 generations, say 750 years
ago. My feeling based on DNA information that we are definitely not related
in the past 400 years, and probably not related in the past 550 years. But
there is a scenario where we could be related between 700 and 900 years ago.

CDY is as you know one of the faster mutating markers. I am CDYb=38, and
the other person CDYb=37. One mutation step difference, at least as counted
by FTDNA when doing the Tip calculation. However, by testing a number of
distantly related lines, I now know that in addition to the obvious one step
difference, my own CDYb has mutated to =39, then mutated again back to =38.
This means that although only 1 step mutation is visible at first glance, it
is apparent that there have been at least 3 mutations difference between us
on that marker in the past 750 years, and for all we know there could be 5
or 7 mutation steps difference between us on that marker. If 5 or 7
mutation steps are being concealed by the obvious 1 step difference, the
TMRCA may be much further back than we or Tip think.

The faster markers are more vulnerable to these back mutations which
considerably distort apparent mutations, particularly over longer time
periods. FTDNA count me as 6 mutation steps difference on 67 markers, but
in fact I am a minimum of 8 steps difference, and possibly 10, 12, or more.

I know you are aware of this, but this example today was a very graphic
demonstration of the effect "in action".

John.






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