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Archiver > GENEALOGY-DNA > 2008-02 > 1203363440


From: Jonathan Day <>
Subject: [DNA] MRCA question
Date: Mon, 18 Feb 2008 11:37:20 -0800 (PST)


Hi,

Thought someone might be able to answer this one for
me. The first part may well be a misunderstanding on
my part. Is it is, the second part won't apply.

The first part of the question is this. As best as I
can tell, most calculators for most recent common
ancestor are "open-tailed". They will approach 100%
but can never reach it. Would I be correct in this?

Now for the second part. In "reality" (or what passes
for it on a good day), the test must actually be
closed-tailed. Ultimately, everyone is a descendent of
y-Chromosome Adam, which means that the cumulative
probability is absolutely guaranteed to be 100% and no
less (no matter how very slightly).

However, there have been plenty of other bottlenecks
within societies, obviously though, on a far smaller
scale. These bottlenecks should disrupt the "pure"
mathematical model, although it is not obvious how
you'd include or describe them.

Herein lies the second part to the question: Let us
say, for the sake of argument, that two people A and B
do not know when their most recent common ancestor
was, but DO know this person must have lived after
such-and-such a date, the adjusted probability must
become 100% at this time and the remaining
probabilities should surely be adusted accordingly.

Are there any such caculators? Is the (probably
miniscule) shift in proabilities worth the effort of
determining an ICA (Intermediate Common Ancestor)?



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