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From: John Lerch <>
Subject: [DNA] re Megan's husband's kinship
Date: Tue, 17 Nov 2009 14:14:58 -0800


When I said "exotic allele", I didn't mean that she and a relative were some of the very few to have them. All I meant was: the alleles were not part of the 99.8% of the genome for which there is only one known possibility. So out of the 3 billion pairs, that mean there might be 6 million things that are variable. So when we multiply that by 1 out of 2*10^6, we find there are almost no matches due to being 10th cousins; there are only matches due to being genetically on average ~6th cousins with everyone on earth (including ourselves).
Ann t wrote

I'll just address the "exotic allele" comment. None of the alleles in the
23andMe test are particularly rare -- in fact, the SNPs are deliberately
chosen to have some minimum frequency in the population. It wouldn't be worth
testing everyone to find a rare allele.

What makes a segment special in determining ancestry is the fact that there
are long consecutive runs of matching SNPs. This happens only when a
segment is identical by descent (IBD). It's sort of like two people tossing
coins
and getting the same result (whether it be heads or tails) hundreds of times
in a row. That just doesn't happen by chance.

I've recommended this before, but I think the animation for autosomal
inheritance at SMGF illustrates this quite nicely.

http://www.smgf.org/education/animations/autosomal.jspx

Ann Turner




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