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Archiver > GENEALOGY-DNA > 2002-08 > 1028256338


From:
Subject: Re: [DNA] Test Variability
Date: Thu, 1 Aug 2002 22:45:42 EDT


In a message dated 08/01/02 1:23:31 PM Pacific Daylight Time,
writes:


> My brother and I were one count different at Y-GATA-A10. Thirteen for me, 14
> for him. All the other loci counts were identical. The testing company just
> remarked that it was unusual for brothers to not have identical counts at
> all loci.

One of you has a mutation at that locus compared to your father. Mutations
are random events and can occur at any time. You can figure out the
probability of two brothers matching if you know the average mutation rate.
We generally use .002 per locus as the rate, although Relative Genetics
appears to be using a higher number.

If you are looking at one locus, the "expectation" of matching your father is
.998. If you are looking at two loci, the expectation is .998 x .998, three
loci is .998 x .998 x .998, etc. The expectation of matching at all 24 loci
is .998 ^ 24, or .953. The expectation that you will match your brother is
.953 x .953 = .908. So the probability of a mismatch is .092, which probably
qualifies as "unusual" but not "astonishing." RG probably doesn't test many
brothers -- it's more common to look for distant cousins to confirm their
common ancestor.

A number of people have had tests performed by different laboratories, and I
don't recall any recent cases of discrepant results. (There were some
calibration questions early on.) Of course any laboratory procedure is
subject to error, including "clerical mutations" where results are simply
copied to the wrong column. But I'd feel comfortable saying that you and your
brother now have distinctive haplotypes -- which may prove useful to your
future descendants!

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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