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Archiver > GENEALOGY-DNA > 2007-07 > 1184270539


From: James Heald <>
Subject: Re: [DNA] Bottlenecks - R1b1c7 as a calibration tool - The " final"version
Date: Thu, 12 Jul 2007 21:02:19 +0100
References: <KHEKIJEABJGJEKDPFEDMAECEDPAA.Elizabethod@eircom.net>
In-Reply-To: <KHEKIJEABJGJEKDPFEDMAECEDPAA.Elizabethod@eircom.net>


Elizabeth O'Donoghue wrote:

> Thank you all for your informative responses.
>
> If I could recap, to see if I absorbed the jest of the comments:

Or even the gist of them :-)

> The possibility of bottlenecks, unless they could be identified and measured
> against the rest of the population being studied, particularly for a very
> young age group, are not a quantitative issue when calculating TMRCA for
> that population.

That's Ken's view; but I am not sure I am so sanguine.

Remember that each new mutation goes through its own private population
bottleneck, which the naive TMRCA calculation doesn't allow for.

Also, I suspect the tendency to large families to run in families is
systemic.

Both of these effects I think may alter the calculation - and not just
for young populations.

There was a paper a few years ago when some of Z's associates ran a
whole batch of monte carlo population simulations under various
scenarios to see what sort of correction factors came out. I haven't
seen the paper, but I don't believe it was only young populations that
they considered.


> The only issue I see as relevant when attempting the calculations is the
> consensus that if you go back beyond a genealogical time frame (would that
> be 600 years or so?), the mutation rates will inevitably underestimate the
> age to the MRCA. But since 'There are no fundamental grounds for applying a
> common scale factor to such age estimates,' (KN) there is no real way to
> know - and perhaps never will be - whether your calculations are at all
> accurate.

Note that it's /not/ the mutation rates that are wrong. The mutation
rates, applied to a pair of haplotypes, probably give a pretty good idea
of the spread of possible TMRCAs.

The systemic uncertianties come in with the methods for estimating
TMRCAs for whole populations.

>
> Maybe in a few millennia, with historical evidence to support backward
> calculations a few thousand years, we (not us, unfortunately) will have a
> better idea if we're accurate. :-(
>
> Elizabeth
>



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