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Archiver > DNA-R1B1C7 > 2007-08 > 1187015537


From: "David Ewing" <>
Subject: Re: [DNA-R1B1C7] Network.exe
Date: Mon, 13 Aug 2007 08:32:17 -0600


Ken Nordtvedt said, "Fast markers actually should have better statistics
connected with their variance than slow markers. This is so because there
are more mutational events. The superiority of fast markers in this respect
is reflected in the recommended way to combine the variances or mutational
counts of the whole set of markers employed."

Interesting. I'm not clear about what "better statistics connected with
their variance" means, though. When I did the little exercise of estimating
TMRCA for McLaughlin v. Ewing with progressively larger sets of markers
having progressively higher average mutation rates (per Chandler), the most
likely TMRCA in generations declined at each step: 85, 48, 40, 34 (for 15,
23, 32 and 37 markers respectively). Why should this be so? Is it because of
the number of markers, the average mutation rate, or just coincidental? And
does this mean that we might get more fruitful (not "prefered," but rather
more believable) results by considering only the 15 fastest mutating markers
rather than the 15 slowest?

Here is a another pair of questions for you, Ken. I commented before that it
would be silly to calculate an "average" mutation rate based on a single
observation of a mutation distinguishing father and son. It wouldn't be much
less silly if you had two such. How many observations would you need in
order for it not to be silly? And I think maybe my next question is just the
same question turned on its head. How many surname project participants does
one need to begin meaningful calculations of TMRCA?

Take the Ewing project as an example. We have 42 R1b1c7 Ewings that are
within genetic distance 4 of their own 37-marker modal. Is that a large
enough sample to expect meaningful results? Twelve of these men are known on
the basis of conventional genealogy to be in one kindred, having a common
ancestor 7.4 generations ago on average; another kindred has five men who
have common ancestor 8 generations ago on average, and another kindred has
four men who have a common ancestor 8.25 generations ago on average. Does
this change anything about how many men we need in the project to do
meaningful TMRCA calculations?

David Ewing

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