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Subject: [DNA] phasing [was Re: Question about matches on Chromo #7 at23andme RF
Date: Fri, 12 Mar 2010 13:21:52 EST


That's correct. The testing techniques aren't looking at whole chromsomes,
but tiny little snippets of DNA floating in a soup. The only thing we can
detect is the presence or absence of the variants (alleles). The two alleles
in the genotype are always given in alphabetical order, e.g. rs9442372 = AG,
and it's not possible to tell which allele came from which parent.

If you have tested both parents, you can figure out quite a bit of your two
"haplotypes" (the alleles occurring on one copy of the chromosome). This is
called "phasing." The word haplotype was originally coined for autosomal
markers, but we've applied it to the Y, which is automatically a chromosome
inherited from one parent and thus automatically a haplotype.

I wrote a 'Satiable Curiosity column about phasing, which has some examples
about the complexity of this task:

http://www.jogg.info/42/files/turner.pdf

This website has some diagrams and tools:


http://www.chromosomechronicles.com/2009/09/30/use-family-snp-data-to-phase-your-own-genome/

Ann Turner

In a message dated 3/12/2010 9:35:28 AM Pacific Standard Time,
writes:

> Thanks for the responses. I have asked for clarification on the way
> 23andme is reporting the pair of chromosomes a few times and haven't gotten any
> answers. So, am I correct in my understanding that 23andme is not
> distinguishing between the pair of chromosomes in their reporting? Wouldn't it make
> more sense to report them in a way that differentiates between them - like
> Chr #1A and Chr #1B?


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