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Archiver > GENEALOGY-DNA > 2010-06 > 1277716800


From: Robert Stafford <>
Subject: Re: [DNA] Genetic Distance and Probability
Date: Mon, 28 Jun 2010 04:20:00 -0500
References: <000301cb165f$8b7b9f30$a272dd90$@net>
In-Reply-To: <000301cb165f$8b7b9f30$a272dd90$@net>


In the TRMCA methodology, you are supposed to use the probability to
determine whether further research is likely to be productive. A 50/50
probability of finding the common ancestor in a genealogical time frame is
the usual cut off, but each can select his own. For many Americans, the time
frame is effectively about 15 generations, since it is very rare to "cross
the pond" for a Colonial immigrant.

Unfortunately, use of the method is largely futile for the simple reason
that, in most cases, both have hit brick walls and cannot do further
research. To find the common ancestor bottom-up you have to connect one
generation at a time. (except in the cases where a grandson is identified).
Knowing that the lineages will eventually intersect does not help.

The exceptions involve a little bit of serendipity. You might happen to
match with someone who has an older genealogy that contains your ancestor.
You can then work top-down from possible candidates. The BYU methodology
seeks to eliminate the serendipity by collecting or compiling as many
top-down genealogies as possible. Researchers deduce the haplotype of each
progenitor by triangulation, i.e., testing descendants of 2 or more of the
progenitor's sons until a match is found on all markers. Those from "orphan
lines" can then eliminate non-matching families from their research and look
for possible ancestors in matching genealogies.

One popular misconception is that testing will provide a time range where
you can find the ancestor and work top-down. However, the ranges are much
too large for this. Besides, you also need the location.

Bob Stafford

On Sun, Jun 27, 2010 at 8:16 PM, Lee K. Ramsey <> wrote:

> The closet surname match I have with FTDNA is the 67 marker test with a
> genetic distance of three, showing a 99.90% probability we share a Common
> Ancestor at 24 generations. (600 yrs = c.1410)
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