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Archiver > GENEALOGY-DNA > 2002-07 > 1025632800


From: "John F. Chandler" <>
Subject: Re: [DNA] Re: MRCA and FTDNA
Date: Tue, 2 Jul 2002 14:00 EDT
In-Reply-To: LEllis148@aol.com message <11f.12f8c6cf.2a530cc1@aol.com> of Tue, 2 Jul 2002 08:03:58 -0600


Lloyd wrote:
> For a 12/12 match (.002) at 95 % FTDNA shows 62 generations while our
> MRCA site shows 76.9 generations. A substantial difference in years !

"Our" MRCA site? Perhaps you mean Ann's MRCA calculator? As I
understand it, her algorithm assumes a Poisson distribution for the
number of mutations. Basically, this means that the time line is
treated as a continuum, and a mutation is deemed equally likely to
occur in any time segment of the same length as another segment.
This is a good model for many physical processes. Unfortunately,
the unit of time in this implementation is the "generation" and is
not constant, so the model suffers from a statistical distortion.
In addition, it is not at all clear whether the Poisson model is
applicable to mutations between generations, since the production
of sperm cells is from many parallel lines of source cells, not just
one.

An altermative approach is to treat time as discrete, with indivisible
generation units. The relevant model is the binomial distribution.
It's not clear that this is the right approach either, but it's the one
I use, and I can confirm that 62.4 is the 95% confidence result for a
12/12 match using this model. I wouldn't worry too much about the
difference between the two approaches, since the mutation rate is
poorly determined and enters directly in each.

John Chandler

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