GENEALOGY-DNA-L Archives

Archiver > GENEALOGY-DNA > 2011-08 > 1313114420


From: "Belinda Dettmann" <>
Subject: Re: [DNA] exact match at 67
Date: Fri, 12 Aug 2011 12:00:20 +1000
References: <005801cc574b$96bb1af0$c43150d0$@dgmweb.net> <CAKWx04SbrJFe_UDuKKBnSY2wxTX2b6BE6KVJmjhjb27M5gHD-g@mail.gmail.com> <950D783B573A4FE78DD7CD26F8C8DB53@jimpc> <CAKWx04SAWri+7oaHsDsp3dUfjY10AY3He7gCgbiNOmdY-o0K1A@mail.gmail.com> <4e43c01b.46b0340a.182f.4774@mx.google.com> <009e01cc5853$e3c54d30$ab4fe790$@dgmweb.net> <4E442386.3030500@gmail.com><00f201cc587a$cf36c7f0$6da457d0$@dgmweb.net>
In-Reply-To: <00f201cc587a$cf36c7f0$6da457d0$@dgmweb.net>


Diana wrote:
>In other words, for a single family in genealogical time, the GD is
*not* an accurate indicator of relative relatedness. I wish it were
because then my job would be easier, but it's not. And it's not rare
that it's not, it's routine that it's not. And if you truly
understand the statistics here, you understand why it is not.>

And of course she is right!

Before you all start screaming about me not understanding statistics, I am a
statistician, and have been for my entire professional career. And one of
the first things I noticed in my field of biological statistics was just how
unusual most small samples are. They hardly ever follow the exact
configuration you can expect from theory, or from large samples, and
predicting an exact result for an individual from a known distribution is a
mug's game.

I'm also co-admin for the Stewart DNA project, where we have a situation
where one particular group of DNA testers (most with surname Stewart/Stuart)
almost certainly descend from a known individual named Alexander, 4th High
Steward of Scotland, who was born about 1214. There are about 40 men in this
R1b group who match each other at a GD of 7 or less (ie 67/60 or closer),
plus they all carry the Ancient Stewart signature of GATAH4=10, 406S1=11,
565=11. Their relative configuration for 67 markers shows the classical
ring-structure of descendants from a single common ancestor, in which their
DNA signatures radiate out from a single value in many directions. We
actually have 6 testers who all have the same DNA result as the central
marker in the ring, ie they all match each other perfectly with a GD of 0.
Two of these can actually trace their lineage back to two different sons of
Alexander. With an exact match at 67 markers, plus a common surname, you
might expect them to be related within the last two or three generations,
but you would be wrong, as their most recent common ancestor was born 800
years ago. We can't go back that far for the other four perfect matches but
they can all trace their descent back for at least 10 generations without
any sign of relationships, so although they must descend from Alexander at
some point, they are not nearly such close relatives as you might expect
from their GD.

I use the FTDNA distance calculation as a tool for grouping men into
possible family groups, but I am very reluctant to use it as a measure of
closeness of a particular relationship. I believe that blind reliance on
TMRCA calculations can easily lead you astray.

Belinda





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