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Archiver > GENEALOGY-DNA > 2002-06 > 1023220920


From: "John F. Chandler" <>
Subject: Re: [DNA] clarify terminology
Date: Tue, 4 Jun 2002 16:02 EDT
In-Reply-To: GWJCAL@aol.com message <17d.9485c7b.2a2e74eb@aol.com> of Tue, 4 Jun 2002 13:54:40 -0600


Grant wrote:
> What exactly is a one step or two step
> mutation?

For STRs, a one-step mutation is a change in the allele length by one
repeat. A two-step mutation is a change by two repeats. As such, it
is impossible to tell the difference between a two-step mutation and a
successive pair of one-step mutations that happen to be in the same
direction at the same locus. The latter is exceedingly rare, but
anything that CAN happen DOES happen from time to time. There is
some spotty evidence that two-step mutations occur much more often
than pairs of reinforcing one-steppers (but still much less often
than individual one-step mutations). However, I couldn't begin to guess
exactly how frequent such things are. Back in the days when RelGen
first started, they gave out results as arbitrary "scores" instead of
repeat counts, and, in particular, I saw a score of "3" for locus F1
come up as a mutation relative to a score of "1" (and Allan Gleason
had the same experience). At the time, I treated it mathematically
as two separate one-step mutations and fretted about its low
probability, but then it turned out that those scores were adjacent
in terms of repeat counts, so the problem went away.

John Chandler

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