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From: Bruce Walsh <>
Subject: [DNA] Measuring Genetic Distances
Date: 25 Nov 03 10:13:42 -0700


Greetings.
Given all of the discussion about the appropriate method for genetic distances (squaring versus simply summing the total number of differences), I've posted some material on this at the following website:

http://nitro.biosci.arizona.edu/ftDNA/Distance.html

I have heard that this is an on-going topic (I've just signed up for a couple of weeks), with lots of excellent discussion, motivated by the correct idea that the actual number of changes underestimates the observed number of changes and hence some correction is appropriate.

Bottom lines:
• A genetic distance attempts to estimate the actual number of underlying mutations that have occurred from some observed difference, here (Y chromosome markers) in the allele length (or state)

• The expressions for translating from an observed difference to an estimate of the actual number of mutations are complex and are also a function of the time to MRCA

• The simple sums of difference method works for fairly recent sets of ancestors (up to 200 generations)

• The sums of square differences method is inappropriate.

I've yet to post the technical details (i.e., the actual equations), but will do so over the next few days. I have to testify at a Frye hearing within the next few hours to determine how the State of Arizona will use Y chromosome markers in Forensic tests. Mike Hammer, another ftDNA advisor, will also testify. Frye hearings are for the admissibility new scientific approaches. Mike will address the molecular genetic aspects, I'll be addressing the statistical and population genetic aspects. Interesting, Arizona has not yet used Y chromosome markers and Mike and I will serve as experts for the State in this case. (Note that ftDNA has no vested interest, one way or the other, in forensic applications, and is no way involved in any aspect of this trail other than having two of its scientific advisors testifying as members of the Faculty of the University of Arizona).

Others hold differing views on genetic distance and I look forward to an interesting, engaging (and fun) discussion

Cheers

Bruce


Bruce Walsh
Associate Professor and Associate Department Head
(Associate Editor, Genetics)
Department of Ecology and Evolutionary Biology
Biosciences West
University of Arizona
Tucson, AZ 85721 USA

email:
Office: 520 621-1915
Fax (Departmental) 520 621 9190

Joint Appointments:
Department of Plant Sciences, Department of Animal Sciences, Department of Molecular and Cellular Biology, Department of Epidemiology
and Biostatistics

Graduate Interdisciplinary Programs Memberships:
Center for Insect Sciences, Genetics, Applied Mathematics

home page: "http://nitro.biosci.arizona.edu";

Quantitative Genetics page: "http://nitro.biosci.arizona.edu/zbook/book.html";





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