Archiver > GENEALOGY-DNA > 2010-02 > 1266045472

From: "Alister John Marsh" <>
Subject: [DNA] TMRCA assessments
Date: Sat, 13 Feb 2010 20:17:52 +1300


Thanks for your further comments. I now better understand your system.

BACK MUTATIONS: In "most cases" in the genealogical time frame, back
mutations and parallel mutations may (gut feeling plus a little basic maths)
have less than 10% impact on TMRCA estimates. Less than 10% impact is not
significant, unless there are several other factors which might be adding
10% errors.

Given how you have explained you "calibrated" your average generation time/
mutation rate, I don't believe your choice of 25 years for generation time
has caused any significant errors. Given this, back mutations alone are not
typically a serious problem in the "genealogical time frame" if they are
mostly have less than 10% impact.

bears no relationship to father/ son mutation study data, perhaps I did not
word that very well. What I meant was that you "did not use" father son
study data to arrive at your calibration, and that appears to be correct.

However, it appears that your calibration of mutation rates/ generation
times is probably coming up with similar average results to father son
studies. So although your process is different and does not rely on father/
son studies, your mutation rate would be similar (f adjusted for a 30 year
generation time). I have not checked them in detail, but they do look
generally similar.

If you used 700 year pedigrees like the MacDonald one to calibrate your
system, on the one hand you might have some advantages in that if there are
any non random aspects to occurrence and survival of mutations, your system
would automatically make allowance for them. But on the other hand, perhaps
we don't know for sure if all of the haplotypes in the MacDonald project
"descend from" the clan founder.

It is interesting to have your different approach to compare to other

TIME SINCE BIRTH OF TEST SUBJECTS: I was not referring to the time from
"birth to death of the common ancestor", but the average time "since the
birth of the test subjects to the present". If the test were of a 4,000
year old archaeological sample, that time is 4,000 years, and clearly would
have to be allowed for. If 90 year olds are commonly tested in surname
projects, people are able to allow for that if they wish.

You don't regard that as a significant matter, so it is your choice not to
allow for it. Others if they wish can allow for it. So it is not an issue
as long as people decide for themselves to allow for it or not.

Previously you were encouraging people to think narrow margins of error are
more realistic, you are now talking of 500 year margins of error. As I
understand the Margins of error of +/-500 years you quote are for
statistical uncertainties, do not to allow for the DNA test subjects being
4,000 or 100 years old.

In the example of mine which you looked at, you said you counted a 4 step
difference on one haplotype at DYS607 as a single 4 step mutation. However,
given that I think this person may be the most distantly related Marsh to
all of the other Marshes, it is not impossible that his 4 step difference on
the very fast mutating marker is the result of either 1, 2, 3, 4, 6, or more
separate mutations. Hypothetically, this arbitrary decision to count it as
one could contribute to a margin of error. You arbitrarily decided to read
a 4 step difference as one mutation, when it could possibly be 4 individual
steps, or even 6. You are probably right, it may be one single 4 step
mutation, but if you are wrong, then you may have underestimated the
mutation count by up to 3 or 5 mutation events.

In the case of back mutations, you say you do not count them because you
can't see if they occurred or not. In the case of the 4 step mutation, you
counted it as one, although it is 4 units difference.

Regarding back mutations, if you study a cluster of related haplotypes
closely, you can sometime prove that a back mutation or parallel mutation
has taken place. So it is not quite as you said in a previous post that a
back mutation can't be counted because you can't see evidence it occurred.

If I could prove to you, as I may eventually be able to do, that 2 mutations
on the example I gave you to analyze were back mutations, would you still
refuse to count them, even if I could prove they had taken place?


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