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Archiver > GENEALOGY-DNA > 2002-11 > 1037681040


From: "John F. Chandler" <>
Subject: Re: [DNA] MRCA calculations
Date: Mon, 18 Nov 2002 23:44 EST
In-Reply-To: orinwells@wells.org message <5.1.1.6.0.20021118190410.03bfcd88@wells.org> of Mon, 18 Nov 2002 21:29:30 -0700


Orin wrote:
> OK, humor me here. I have two subjects. They match at all of the 26
> markers sampled except the following two (DYS389II is an artifact of DYS389
> so it does not count):
>
> Subject DYS389 DYS389II DYS392
> A 14 30 11
> B 15 31 12
>
> What does your calculator say about their most recent common ancestor?

Your tabulation didn't line up on my screen, but you seem to have two
one-step differences. That means D=2. You have L=26. If you are
willing to use p=0.002, then the median time to MRCA is 2.69/(2x26x.002)
= 25.9. This is different from what Bruce Walsh's table would give if
he had a table for n=26 because his table doesn't distinguish between a
one-step difference and a two-step difference. If there is only one
locus that is different, a good case can be made for assuming a two-step
difference at that locus is a single mutation (with a much lower average
rate, of course!). In this example, if B had had DYS392=13, I would say
it should be treated as two separate one-step mutations that happen to
line up, in addition to the one-step difference at DYS389i, which means
that D is 1 + 2^2 = 5, so the median jumps to 54.7.

John Chandler

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