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Archiver > GENEALOGY-DNA > 2006-02 > 1139860144

From: "Sue Waite-Langley" <>
Subject: RE: [DNA] Earliest common ancestors for 37/37 and 43/43 matches
Date: Mon, 13 Feb 2006 13:49:04 -0600

BUT...unlike a birthday...which we all must have...there is nothing that
says ANY person MUST have a mutation. Mutations are random
occurrences...just as we can flip a coin fifty times and will likely get
an equal number of heads and tails based on the probability of the
situation...there is also a probability that we will get fifty tails or
fifty heads. Even if we assume that the probability of a mutation
occurring is fixed (and it isn't...there are fast and slow markers) a
database of samples large enough to "assure" ANY mutation will occur
would have to be very large...certainly not just 74 samples. Sue

-----Original Message-----
From: John Lerch [mailto:]
Sent: Monday, February 13, 2006 1:35 PM
To:
Subject: RE: [DNA] Earliest common ancestors for 37/37 and 43/43 matches

I've gotten kind of tired of this thread. Let us take a worst case
scenario at the party and ask the people what their birthdays are. Say
there are 367 people (leap year + 1). What is the probability that two
people share the same birthday. Everyone (including everyone who has no

math skill at all) can immediately say "Why it's absolutely certain.
Since there are more people than there are days in the year."
Similarly consider how many 1 step mutations must occur in a data base
(in this case a surname project) for there to be the same guarantee that

two people have the same 1 step mutation. Well if there are 37 markers,

there are 74 1 step mutations possible so if there are just 75 people
in the data base, YOU ARE GUARANTEED THAT 2 OF THEM HAVE THE SAME 1 STEP

MUTATION. And although 75 people in a surname data base is a nice size
data base, it's not astronomical. And it's not of a different order of
magnitude from the answer that one needs only ~2*sqrt(37) to have a
better than 50/50 shot of having a parallel mutation.
So please. No more assertions that the odds are astonomical.
JAL

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