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Archiver > GENEALOGY-DNA > 2002-08 > 1028389842
From:
Subject: Re: [DNA] Test Variability
Date: Sat, 3 Aug 2002 11:50:42 EDT
In a message dated 08/02/02 10:04:22 AM Pacific Daylight Time,
writes:
> Thanks Ann for the instructive calculations. Please more: Ken and I match
> counts at 21 of 23 loci except:
> Counts Me Ken
> DYS 390 23 24
> DYS 392 11 13
>
> From these RG estimated our common ancestor lived around 1690. Which factors
> are instrumental? The two loci that count different or the two count
> difference in the one locus or are both equally important? What would
> conclusions have been if Ken's DYS 392 count had been 12 or for that matter
> if his counts for both loci were identical to mine. Is there reading or
> examples on these calculations?
>
Most mutations are one-step, but there is debate on how to handle two-step
mutations. The mechanism of mutation is called "replication slippage," which
doesn't really explain it, just describes it. The idea is that the enzymes
responsible for replicating a strand of DNA lose track of where they are in
the process, and gain or lose a repeat section (typically 3-5 bases in
length). Two-step mutations have been observed to occur in father-son pairs
(incidence 1/14), so in that case, it would count as one mutation. However,
it's also possible that two separate mutations occurred at different times,
adding up to two steps.
Bruce Walsh, the population geneticist who consults with Family Tree DNA,
says you need to take the context into account. If the two-step mutation is
the only one, then it should probably be counted as one. If there are many
other mutations, then you should count it as two -- that is, the common
ancestor lived so long ago that there's been plenty of time to have two
mutations at the same locus. He has created some tables at
http://nitro.biosci.arizona.edu/ftdna/TMRCA.html -- you could probably use
the table for 25 markers to get an approximation of your RG test (23 or 24
markers?).
You will notice that the results are given in generations, not years. That is
another point of controversy -- how many years in a generation? Some authors
use 20 years, which we feel is far too short for the average length in the
past few hundred years. There is a research paper by Heyer which supports a
longer generation length of 30 or so years -- search the archives for Heyer
if you want the exact numbers.
But any way you count it, you'll notice that FTDNA gives a longer time to the
MRCA than RG. We have concluded that RG is using a higher mutation rate, but
we don't have the details on how they are calculating things.
Ann Turner
GENEALOGY-DNA List Administrator
http://lists.rootsweb.com/index/other/Miscellaneous/GENEALOGY-DNA.html
DNA preservation kits: http://www.dnafiler.com
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