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Archiver > GENEALOGY-DNA > 2005-07 > 1120254746


From: (John Chandler)
Subject: Re: [DNA] Back Mutation and Parallel Mutation
Date: Fri, 1 Jul 2005 17:52:26 -0400 (EDT)
References: <1c5.2b1208c7.2ff6fdfc@aol.com>
In-Reply-To: <1c5.2b1208c7.2ff6fdfc@aol.com> (Mikenhumphrey@aol.com)


Mike wrote:
> There's evidence to support the notion that back-mutation rates are higher
> when the measured allele length is near the lower or upper limit of the
> generally observed interval of lengths. E.G. a paper by Dupuy on Norwegian
> father/son pairs (several refs in List archives over the past 6 months).

No. I think you may be confusing the absolute difference in up versus
down rates with the forward/backward question. In other words, I
think you're just saying that there is a tendency toward down
mutations at the upper end of the allele range and a tendency toward
up mutations at the lower end. There is indeed evidence that the
up-down bias changes from "up" to "down" between the lower and upper
excursions of a marker, just as you say. However, "forward" is
defined simply by the first of two mutations on the same marker. It
is not the same as "up" at the upper end (the bias is actually
"down"). As long as there is no sudden, dramatic shift in the bias
from one length to the next, the back-mutation rate is always close to
50%, as I showed with the numerical example. In any case, no one has
ever found evidence of a sudden shift in the bias.

John Chandler

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