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Archiver > GENEALOGY-DNA > 2007-02 > 1171516223


From:
Subject: Re: [DNA] How do you do an estimated Common ancestor
Date: Thu, 15 Feb 2007 00:10:23 EST


In a message dated 2/14/2007 2:23:49 PM Pacific Standard Time,
writes:

> It CAN be expected that sons will differ from their fathers. It had
> to happen somewhere for a mutation to be evident. What is VERY
> unexpected is for two brothers to have haplotypes that not only
> differ from each other but in each case differs from their father in
> a different manner.

I think you were using my MRCA calculator in your examples (not quoted here).
That was written a long time ago, when 12 markers were the most you could
order, and I only did calculations for a maximum of two mutations. The numbers
are still correct, but there are other calculators out there now that will let
you explore a wider range of possibilities. They would illustrate my "triage"
statement better -- as the number of differences grows, there comes a point
when you can pretty much rule out the two men being brothers. But when does that
point come?

I have another calculator,

http://members.aol.com/dnafiler/MutationCalculator.exe

which you can use to find the probability of observing two mutations in two
brothers. I don't know what mutation rate RG is using for their set of 43
markers, but let's say it is .003. Plugging in 43 for the number of markers, .003
for the mutation rate, and 2 for the number of transmission events, the
probabilities of observing a certain number of mutations are

0 .772
1 .199
2 .026
3 .002

and so forth, with rapidly diminishing probabilities for 4, 5, 6+ mutations.

So most brothers (77.2%) will match, but we would expect to see two mutations
in brothers in about 2.6% of cases -- not "typical," but not what I'd call
"VERY" unexpected either. In fact, as databases grow, we are bound to encounter
such cases now and then.

And if two mutations have indeed occurred in two brothers, it's far more
likely that they would fall on different markers than on the same marker --
there's only one chance in 43 that the second mutation will fall on the same marker,
and 42/43 chance that it will fall on a different marker.

No one is saying that TMRCA calculators help your target your research to a
particular generation -- as you say, the 95% confidence intervals are very wide
(and the last 4.9% is very very wide). But if you learn that there's only a
.0001 chance of finding a common ancestor within 1000 years, you wouldn't want
to invest much effort in that line of research (the triage concept). I'm sure
you have a mental triage system in your project -- above a certain threshold
of genetic distance for two people who claim to have a common ancestor, you're
going to start suspecting that a non-paternity event or a laboratory error is
coming into play. You're using the TMRCA concept qualitatively, even if you
aren't using it quantitatively.

Ann Turner




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