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Archiver > GENEALOGY-DNA > 2010-03 > 1270032513


From: "Sandy Paterson" <>
Subject: Re: [DNA] Questions about alternate means of searching DNA Genealogy
Date: Wed, 31 Mar 2010 11:54:50 +0100
References: <209bca921003291224k5f0b3ae2gef0ece518f0327f1@mail.gmail.com> <REME20100329212910@alum.mit.edu> <000001cacfe2$817a39b0$846ead10$@com> <REME20100330160616@alum.mit.edu> <000001cad0ac$a0be9f80$e23bde80$@com><0794A073-8DE6-4F21-B95E-8CB1F20F2ED3@vizachero.com>
In-Reply-To: <0794A073-8DE6-4F21-B95E-8CB1F20F2ED3@vizachero.com>


Hi Vince

I'm sure this is just a question of interpretation.

You wrote

"In the R1b project at FTDNA, DYS393=12 is about 5.4% of the project.
DYS426=11 is about 1.7% of the project. You might assume the joint
probability is 5.4%*1.7%=0.09%, but you'd be wrong. The joint
probability is actually 0.7%, nearly eight times greater."

No, I would not assume that the joint probability is .09%. But I'd assume
that it would be lower than the rarest of the two, namely 1.7%, which it is.

In other words, if 3 alleles are present with individual frequencies f1,f2
and f3 then the frequency with which all three are present at the same time
will be lower than the lowest of f1,f2 and f3.

And this makes it a good idea to concentrate searches on the rare alleles,
which is all Jonathan Clayborn implied in the first place.

As I said, this is just interpretation/pedantics and does nothing to detract
from the logical conclusion that it remains a good idea to concentrate on
the rare alleles.


Sandy




-----Original Message-----
From:
[mailto:] On Behalf Of Vincent Vizachero
Sent: 31 March 2010 10:10
To:
Subject: Re: [DNA] Questions about alternate means of searching DNA
Genealogy

No, not in the context of the Y-chromosome.

As John said, the Y does not split and recombine: it is handed down
pretty much identically from father to son. If a father has two rare
alleles, his son will almost surely have the same two rare alleles.

Autosomal DNA does recombine, so you can assume that allele
frequencies are somewhat independent. But in Y-DNA, you can't assume
it: you'd have to investigate it. It becomes an empirical question,
not a theoretical one.

Take an example in haplogroup R1b1b2. DYS393=12 is fairly rare, and
DYS426=11 is fairly rare. However, those two alleles tend to co-
occur: nearly half the people with DYS426=11 also have DYS393=12.

In the R1b project at FTDNA, DYS393=12 is about 5.4% of the project.
DYS426=11 is about 1.7% of the project. You might assume the joint
probability is 5.4%*1.7%=0.09%, but you'd be wrong. The joint
probability is actually 0.7%, nearly eight times greater.

VV



On Mar 31, 2010, at 4:31 AM, Sandy Paterson wrote:

> Surely, if we examined the haplotypes of the sons of a large number
> of men
> with two rare alleles, although we'd find that in most cases sons
> would have
> both rare values, we'd also find that some sons would have only 1? And
> (albeit very rarely), some brothers would have none?


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