Archiver > GENEALOGY-DNA > 2011-10 > 1317679021

From: (John Chandler)
Subject: Re: [DNA] 67/67 match
Date: Mon, 3 Oct 2011 17:57:01 -0400
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Nora wrote:
> I'm curious as to how common is this to happen, a known first cousin is off
> by one marker and an unknown probable cousin is a perfect 67/67 match?

Let's take those two events separately. The probability that two
first cousins will differ by at least one step is about 2/3. The
probability that an unknown cousin who comes to your attention
BECAUSE he is an exact match will, indeed, be an exact match is
about 100%. If he hadn't come to your attention, you wouldn't be

> why are there
> so few men who are positive for the SNPs that were found in my cousin even
> though their GD was as close as 4?

It sounds as if you're asking why "private" Y SNPs exist. The reason
is that every new baby boy can expect to have about 1 brand-new SNP of
his own -- or at least it would be solely his own, except that dozens
of other living males have already independently come up with the
exact same mutation by pure coincidence. Leaving aside those
coincidental duplicates, the new SNP is intensely private at first,
and it may be thousands of years later (or maybe never) that the
male-line progeny of any particular baby boy become a detectable

John Chandler

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