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Archiver > GENEALOGY-DNA > 2009-12 > 1260121899


From: David Faux <>
Subject: Re: [DNA] real relatedness or fluff and fairytales?
Date: Sun, 6 Dec 2009 09:51:39 -0800
References: <587770.27617.qm@web24502.mail.ird.yahoo.com><953242.47354.qm@web24503.mail.ird.yahoo.com><91E96C076B6F44FD94D81A3E9CFFB6A3@RESTES2007>
In-Reply-To: <91E96C076B6F44FD94D81A3E9CFFB6A3@RESTES2007>


Roberta,

For most purposes we can consider one cM = one Mb. There could be a segment
that is 10 Mb but is a virtualy SNP desert with say only 300 SNPs. Here the
size of the block gives the illusion that a great deal of DNA is shared.
However if one has a 7 Mb haplotype block (meaning there are 7 million
nucleotide bases) and it contains 700 SNPs (single nucleotide polymorphisms
or variants on what every human and most chimps would have in a stringed
sequence), this would be a far more compelling scenario for identical by
descent. Since Anders uses a "pruned" dataset of SNPs it is not possible to
directly compare his work with PLINK and deCODEme to the algorithm and
procedures used by 23andMe.

David K. Faux.

On Sun, Dec 6, 2009 at 8:49 AM, Roberta Estes <> wrote:

> I find this just a hair confusing, so I'll ask what is probably a stupid
> question.
>
> Since there are two criteria to be fulfilled, both the 7cM and the 700
> SNPS,
> I'm presuming that SNPs take up a different amount of space, or there
> wouldn't be two criteria, just one. But my understanding of a SNP is that
> it is just one occurrence of a, t, c or g. So unless there is an insertion
> or a deletion, how can different groupings of SNPs be of different lengths.
> Don't a, c, t and g take up the same amount of space?
>
> Roberta
>
>
>

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