GENEALOGY-DNA-L ArchivesArchiver > GENEALOGY-DNA > 2002-11 > 1037723217
From: OrinWells <>
Subject: Re: [DNA] MRCA calculations
Date: Tue, 19 Nov 2002 08:26:57 -0800
References: <email@example.com message <firstname.lastname@example.org>
At 11:44 PM 11/18/2002 -0500, John F. Chandler wrote:
>Your tabulation didn't line up on my screen, but you seem to have two
>one-step differences. That means D=2. You have L=26. If you are
>willing to use p=0.002, then the median time to MRCA is 2.69/(2x26x.002) =
OK, but here are the facts which just don't line up with the common
thinking on this sort of thing.
Subject A is the brother of Subject B. Their father differed from both of
them by one mutation, each on a different marker. The subjects passed
these mutations on to their sons. What are the odds that two sons of a
single individual would each have a mutation on different markers? One of
the above also had a mutation at another site, Y-GATA-A10, where they
picked up a double reading (13/14). This mutation/anomaly was NOT passed
on to his sons. This sort of thing is not supposed to happen according to
the tidy rules.
I have a major discomfort with the "standard" assumptions being made as
though mutation rates will always play by some nice tidy
scientific/mathematical rules. They don't seem to.
In one of our baseline families we have what amounts to 43
transmissions. According to the tidy theory that should work out to
.002x43x26 = 2.23 mutations. We experienced exactly 1.
In another family with 37 transmissions theory says .002x37x26 = 1.94. We
had no mutations.
In other families we did experience more mutations. But the point is I
question that you can apply the "average" theoretical mutation rate against
any two individuals and really hope that it is going to give you accurate
results. It may be a good "rule of thumb" but in any given instance it
probably is not really correct.
I have not had time to really take all the 160 samples we have (discarding
those who just don't fit anywhere) and have a look at how many mutations
were experienced over the lot versus unique transmissions. Eventually I
will do this just for the curiosity factor. But I already know it will
skew one way or another by the families where no mutations were encountered
and those where several were encountered. To lump a family with a
relatively high rate of mutation with one with a low rate of mutation seems
to me to be wrong science. I believe there may be some other
influences. They may be genetic or environmental but seem to impact one
family over another and thus cause the "average" mutation rate to be
overstated in one case and understated in another.
Also, on an unrelated but interesting subject. We have encountered
possibly a rate of 5% of our participants where alleged descendants of
specific families revealed a probable non-paternal event or adoption. We
have not been able to trace any of these to their origins yet so we don't
know which we are looking at. But I would guess if the average number of
generations involved is actually 10 we may be looking at a 0.5% rate rather
than 5%. In most lines it is clear there were no instances of this over
the time period under consideration. In other cases there may well have
been more than one - no way to tell about this. In other instances, we
already knew from the genealogy that an out of wedlock birth took place.
Orin R. Wells
Wells Family Research Association
P. O. Box 5427
Kent, Washington 98064-5427
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