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From: (John Chandler)
Subject: Re: [DNA] A Genealogical Goldmine - the X Chromosome
Date: Tue, 16 Dec 2008 22:02:26 -0500
References: <ea3bd9560812161609v3ee9e6a6g11bffb65f603f2@mail.gmail.com>
In-Reply-To: <ea3bd9560812161609v3ee9e6a6g11bffb65f603f2@mail.gmail.com>(fauxdk@gmail.com)


David wrote:
> It is necessary to realize that in the X line, of your 64 gggg grandparents,
> only 13 contributed to your X.

Yes, this is a topic that has been discussed here before. The number of
X contributors follows the Fibonacci sequence 1,2,3,5,8,13,21,... This
sequence approaches exponential growth with an asymptotic ratio of 1.6
-- more precisely, 0.5+sqrt(5)/2. In the long run, then, the X haploblocks
are subject to the same diffuseness of source as autosomal haploblocks,
since exponential growth implies the same "paradox" of more X contributors
than there were humans at some point in the past. (The only difference
is that the crossover point is about 50% further back in time.) More
importantly, there is no large extinction rate for X haploblocks, as there
is for Y or mtDNA in a static population, and so there should be lots of
immensely old bits of X chromosome floating around out there -- no
coalescence to an X-Eve later than the first mammals.

> However in my case, the ancestor gave a Mohawk X to her son who gave
> it to his daughter (my gggg grandmother) who gave it to her son who gave it
> to his daughter and so on to my maternal grandmother.

You seem to be implying that there was no recombination at any of the
above heterozygous females. Or did you mean to say that your gggggm
gave PART of it to her son, and so on? Ok, so your expected heritage
from the Mohawk gggggggm is 1/16 (6%, not 8%) of your X chromosome,
but, just as in autosomal inheritance, there is a wide variance of
outcome about that expectation. Indeed, if you figure an average of
two crossovers per mother-to-child transmission, you would expect that
original Mohawk X to be split into about 9 pieces by now, as seen from
your end, not all of equal size. Thus, your actual share of the
Mohawk X could be as much as, say, 1/2, or (easily) as little as zero.

Now, how about genealogical applications? In order to show kinship,
you need to be able to compare markers from one person to another, but
you have only on the order of 1/16 of a Mohawk X, and the person you
want to compare against may not have that same portion of Mohawk X,
especially if he or she is also well admixed.

Now let's compare what the X can do with what can be done for Mohawks
(and has been done) using the Y chromosome. In the Rice Y-DNA
project, we have some participants who happen to be Mohawks (as it
happens, the surname Rice is relatively common among the Mohawks).
One of them has been shown to match Group 1 in the Rice project, which
is linked to early-Colonial-immigrant Edmund Rice. This constitutes
very close to a proof that our participant is a male-line descendant
of that immigrant. With that as inspiration, and the good fortune of
having available the Jesuit mission records, we now have an unbroken
line traced from the testee back to a young Rice boy who was captured
in 1704 from what is now Westborough, Massachusetts, by a band of
Kahnewake Mohawk on a "mourning raid"; and from the young boy back to
the immigrant via Colonial records.

The "gold mine" here is the amplification of the signal from about
0.2% in the autosomal DNA to 100% in the Y.

John Chandler


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