GENEALOGY-DNA-L Archives

Archiver > GENEALOGY-DNA > 2010-03 > 1268658416


From: Steven Bird <>
Subject: Re: [DNA] Clades, Definitions, Discoveries, FTDNA
Date: Mon, 15 Mar 2010 09:06:56 -0400
References: <270548.86246.qm@web111316.mail.gq1.yahoo.com>, ,<SNT115-W100A5F4E30C0231B9E890ACC2E0@phx.gbl>, ,<016301cac3e3$fe0b33e0$5e82af48@Ken1>,<SNT115-W196CFB24CCDD653D7C3D50CC2E0@phx.gbl>,<01ba01cac3fb$87b368d0$5e82af48@Ken1>
In-Reply-To: <01ba01cac3fb$87b368d0$5e82af48@Ken1>


The comment was made that the same SNPs are occurring in different clades of the Y haplotree. I expressed the opinion that this was very unlikely. That was my point, because:



If we have an average of about 50000 SNPs on the Y chromosome (that's 51 million BP divided by an average of 1 SNP every 1000 bp - I've rounded a bit for convenience) then the odds of any one SNP changing on any one site is .00005 per transmission, given a mutation rate of 1 mutation in 1 billion transmissions. The odds of this happening twice, in two different men but in the same location, is the square of that number: 2.5^10-9. Multiplying by 67 million births per year (in the last few decades, down somewhat from the peak in 1962) we obtain a probability of any two random individuals born last year having the same SNP occur: .1675 per transmission.



However, that is the probability for any one of about 50000 random Y SNPs having occurred in two different men born last year. The probability of a PARTICULAR Y SNP OF INTEREST (P2+ for example) occurring twice is .1675 divided by the 50000 SNP locations on the Y; so the overall probability of any Y SNP that is phylogenetically significant occurring twice in two individuals is .00000335 per year at present. In the past, this number would have been even lower, because the number of births per year was much smaller.



The entire reason that SNPs are useful is that they are stable. If they weren't, the entire humane genome would never have been sequenced. If each and every SNP site mutated every 15 years, then they would not be useful at all either for aligning shotgun DNA libraries or for identifying human Y clades.



>--- so in half a generation
> EACH and every practical snp site on the y chromosome has on average mutated
> once.>
>


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