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From: Dirk Schweitzer <>
Subject: Re: [DNA] Cruciani and 2007 TMRCA estimates.... a Question aboutthe units of mutation rates.
Date: Fri, 23 Mar 2007 17:29:17 -0700 (PDT)
In-Reply-To: <mailman.16213.1174660416.2838.genealogy-dna@rootsweb.com>


Hello All,

I have a general question about the units of mutation rates. For example, someone (Zhivotovsky, Am. J. Hum. Genet. 74:50–61, 2004) writes (a) "we suggest the following estimate of the effective mutation rate at the average Y chromosome locus: (6.9 +/- 1.3) x 10-4 per locus per 25 years" and then later (b) "Mutation rates estimated from pedigree and familial studies are substantially higher—three times higher— than our “evolutionary” estimate: an average of 18 events in 8,659 meioses, or 0.0021 per generation."

I understand that (b) means actually 18/8659 = 0.0021, i.e., on average there are 0.0021 mutations per locus (STR) per reproductive event (meiosis).

However, each "generation" or "25 years (per generation)" corresponds to TWO reproductive events.

For example, the most recent common ancestor of me and my cousin was two generations ago, our common grandfather. However, there are four reproductive events between me and my cousin.

Thus, if I compare the 67-marker haplotypes of two cousins with each other, I am expecting to find on average 0.0021 (marker)-1 (reproductive-event)-1 x 67 markers x 4 reproductive events = 0.56 mutations between them.

However, using the wrong units of (a) and (b), one arrives at the wrong numbers of (a) 0.00069 (locus)-1 (25 years)-1 x 67 markers x 2 generations = on average 0.09 mutations between them and (b) 0.0021 (locus)-1 (generation)-1 x 67 loci x 2 generations = on average 0.28 mutations between them.

Do people in TMRCA calculations consider this factor of two difference between "the mutation rate per marker per reproductive event" versus the "the mutation rate per marker per generation / 25 years ago"?

Do people use the correct units in their TMRCA calculations?

Dirk



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