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Archiver > GENEALOGY-DNA > 2008-01 > 1200174011


From: Vincent Vizachero <>
Subject: Re: [DNA] Markers - How many and which ones?
Date: Sat, 12 Jan 2008 16:40:11 -0500
References: <2dff56a0801121109k2ae8145ai6ca2b79ec374e52f@mail.gmail.com>
In-Reply-To: <2dff56a0801121109k2ae8145ai6ca2b79ec374e52f@mail.gmail.com>


Those who have problems accepting the utility of TMRCA estimates
often fail to accept them for what they are, which to say "estimates".

Also, though we often adopt the sloppy habit of quoting point
estimates (usually the 50% probability point, but sometimes the MLE)
we should always keep in mind that the number we are using is
implicitly a confidence interval.

If a 50% success rate is good for you, then just refer to the 50%
confidence interval.

For example, assume you have two haplotypes with GD of 3 for 37
tested markers with an average mutation rate of 0.0049.

The most likely estimate of generations seperating the two births is
17 generations (i.e. TMRCA is 8.5 generations). But the likelihood
of the common ancestor having lived precisely this generation is low
(less than 4%).

The 50% probability point is 21 generations (TMRCA is 10.5 gens).
Half the time, the real MRCA will be within this period, and half the
time outside of it.

The 50% confidence interval is 14 to 29 generations (TMRCA is 7 to
14.5 generations).

The 95% interval is 3 to 50 generations (TMRCA is 1.5 to 25
generations).

Identify your necessary confidence, then find your result. It's
pretty easy.l

You can add more markers to get narrower intervals.

Vince


On Jan 12, 2008, at 2:09 PM, Colin Ferguson wrote:

> Diana is correct, one cannot expect to ever calculate the TMRCA that
> matches an individual pedigree 100% or even 95% of the time. I'd
> settle for 50%.

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