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Archiver > GENEALOGY-DNA > 2005-06 > 1118106257


From: (John Chandler)
Subject: Re: [DNA] Likelihood of Descent from Common Ancestor
Date: Mon, 6 Jun 2005 21:04:17 -0400 (EDT)
References: <20050606182301.19514.qmail@web81806.mail.mud.yahoo.com> <001c01c56ac8$94cfd8a0$6501a8c0@KEN>
In-Reply-To: <001c01c56ac8$94cfd8a0$6501a8c0@KEN> (ken.graves@gravesfa.org)


Ken wrote:
> One of the major Graves families I am researching is one in Virginia in the
> early 1600's. It was believed that all the lines were from a common
> immigrant ancestor. However, it appears from our Y-DNA results that there
> may be as many as 4 different immigrant ancestors represented. But I want to
> be very certain before I publish this conclusion as fact.

I'm obviously missing something. The table shows "generations from
common ancestor" with values of 4, 4, 5, 4, and 8. Since all of the
testees were presumably alive within the past three years, how can any
of them be only four generations away from the common 1600s immigrant
ancestor? On the other hand, how can one of them be four generations
further removed from the common ancestor than the others in just four
elapsed generations?

> I have determined a haplotype for each of the four branches of this family.

Your table shows five different haplotypes and a kit number for each.
Are these really just representatives of their respective branches,
and one branch was two evenly divided to call all 25 markers? Or
do you really have only five test subjects from this major family?
If these are just representatives, does that mean the generation
count is from the immigrant to the common ancestor of each branch?

> What I would appreciate help with is a variety of ways of assessing the
> probability that these 4 branches are and are not descended from a common
> immigrant ancestor.

Several ways. First, if you know the relationships asserted in the
traditional genealogy for all of these test subjects, you can count
up the number of mutation opportunities and use Ann's calculator to
tell you the likelihood of 13 or more mutations in that amount of
time. Second, you can compute the average squared difference over
all markers for all pairs of haplotypes and divide by twice the
mutation rate to calculate the expected TMRCA of the group. Third,
you can obtain the 95% confidence interval of TMRCA for pairs of
haplotypes and decide which pairs are -- or are not -- compatible
with the believe common ancestor. Each of these ways will give you
a slightly different perspective on the question.

John Chandler


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