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Archiver > GENEALOGY-DNA > 2010-02 > 1265835788


From: "Alister John Marsh" <>
Subject: Re: [DNA] MRCA Estimates
Date: Thu, 11 Feb 2010 10:03:08 +1300
References: <4B02064423F54F2889C553CF681379FA@ibm5ye12amji2d>
In-Reply-To: <4B02064423F54F2889C553CF681379FA@ibm5ye12amji2d>


Charles,

Testing more markers, including faster mutating markers, can increase the
resolution of TMRCA estimates.

A 2 mutation step difference between uncle and nephew on fast mutating
markers is not all that improbable, as are no mutations in 8 generations, so
I don't think TMRCA estimates in your case are too misleading. (I have at
one mutation between myself and my son on 46 markers.)

TMRCA being "statistical estimates", they are not going to give the exact
generation every time. But when the paper trail is unknown, they are
something to start with.

Better to make an assumption (even a wrong assumption) based on TMRCA, and
find a way to test the validity of the assumption, than to do nothing. You
don't learn anything by doing nothing.

In most cases, people know by paper trail or hearsay that relatives as close
as uncle/ nephew are related. Many of us can get 6 or 8 generations before
the paper trail runs out, so the TMRCA estimates are more often looked at
for guidance for the period 6 to 20+ generations, than 1 or 2 generations.
As you approach 20 generations, the TMRCA calculations probably have
bypassed statistical anomalies which sometimes turn up in very short time
spans.

In my case, I have tested more than 130 markers. In 20 generations, that is
2,600 individual mutation opportunities. Between me and a relative related
20 generations back, there would be 5,200 mutation opportunities to smooth
out the statistical anomalies. Between an uncle and nephew tested on 46
markers, there would 148 mutation opportunities, less than 3% of 5,200.

Something which I think is overlooked, is the fact that mutations can be
either up or down. So instead of 5,200 mutation opportunities in the above
example, there could be considered to be 10,400 "different" mutation
opportunities. A reasonable number to smooth out statistical anomalies
sometimes found in very short timescales.

John.

-----Original Message-----
From:
[mailto:] On Behalf Of Charles Acree
Sent: Thursday, February 11, 2010 6:53 AM
To:
Subject: [DNA] MRCA Estimates

Spirited discussion of the value and methodology of Most Recent Common
Ancestor (MRCA) statistical inference is worthwhile because those estimates
can be helpful, particularly with regard to pre-historic times. However, as
we all know, MRCA predictions can often mislead when applied to the
genealogical time-frame (roughly the past millennium).

In our Acree surname project, for instance, MRCA statistics have proven
totally inapplicable. Of the nineteen of us (mostly and impressively,
4th-8th cousins) who descend from a common Acree ancestor born 300 years ago
(affirmed by genealogical documentation), none of us has a genetic distance
(GD) from the mode greater than two, and all of us share the distinguishing,
invaluable partial-repeat at DYS385b that I've mentioned. Seven of us match
46/46 (by Ancestry.com's count); five match 45/46; and six match 44/46. (The
nineteenth matches 12/12.)

Two participants have an uncle/nephew relationship. Guess what their match
is. No, you got it wrong. The uncle is one of those who matches the mode
44/46 - differing from his nephew in two (fast-mutating) markers (GD=2, one
up, one down).

Charles Acree

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