GENEALOGY-DNA-L Archives

Archiver > GENEALOGY-DNA > 2006-05 > 1146544492

From: "Ken Nordtvedt" <>
Subject: Re: TMRCA interpretation, was Re: [DNA] scots versus sc2
Date: Mon, 1 May 2006 22:34:52 -0600
References: <200605020339.XAA24521@alexander.xo.com>

Repose the question or calculational goal. On face value there is no number
of generations between a person's haplotype and a "modal". You can ask for
the TMRCA estimate for a particular haplotype and another which happens to
be equal to the modal of a population, but that won't mean much which is
significant beyond its explicit description. A modal comes into definition
by examining a present population, and it is a property of that contemporary
population. Now it also happens to be a likely candidate for the haplotype
of the founder for that whole population --- the MRCA *for the whole
present-day population*.

The MRCA for a random pair taken from the contemporary population will
generally be *less* than the time back to the founder. The random pair
could happen to be close cousins whose MRCA is just a few generations back,
or the pair could be two guys whose MRCA was the founder himself (the other
extreme) Most of the random pairs taken from the population will have a
MRCA between those extremes.

I am not concerned at the moment with Doug's mutation rates, *effective*
rates, "my" rates, or whether you are doing "the calculation" correctly; we
should just first understand what is being calculated. I'd like us to pose
the question so we all understand what is being sought, then we can discuss
how to best get the answer.

Ken

----- Original Message -----
From: "Mark MacDonald" <>
To: <>
Sent: Monday, May 01, 2006 9:39 PM
Subject: Re: TMRCA interpretation, was Re: [DNA] scots versus sc2

> Ken
>
> OK If you don't think I'm doing the calculation correctly, what do you
> contend is the correct answer for the most probable number of generations
> between a modal for sc2 and scots modal or between the McCutcheon portion
> of sc2 and the scots modal if you use Doug's mutation speeds or if you
> pick your stated average mutation speed. Is it more than 46 generations?
> If so, why.
>
> Mark MacDonald
>>
>>
>> ----- Original Message -----
>> From: "Doug McDonald" <>
>> To: <>
>> Sent: Monday, May 01, 2006 6:01 PM
>> Subject: TMRCA interpretation, was Re: [DNA] scots versus sc2
>>
>>
>> > Mark MacDonald wrote:
>> >> Using Doug McDonald's mutation speed calculator which he has prepared
>> >> for
>> >> our to be published Clan Donald website, there are probably only 23
>> >> generations between a signature which is 36/37 to the scot modal to
>> >> the
>> >> McCutcheons. 515 to 690 years is not very much time.
>>
>>
>> Every haplotype today which descends from the founder of the Scot variety
>> (and whose haplotype is good candidate for today's modal haplotype) is
>> the
>> same number of generations from the founder (give or take the slop due to
>> lines having longer or shorter average generation length) So each person
>> trying to separately determine how far back the one founder was makes no
>> sense. You will have a distribution of pseudo generational estimates to
>> time back to the founder from zero to quite a long time. They obviously
>> can't all be accurate. Somehow they need to be combined to yield a
>> single
>> estimate. Just averaging the individual estimates is not the best way to
>> go. That's what the average squared difference or variance methods do.
>>
>> Or put another way: consider two presentday Irish men whose haplotypes
>> descend from the founder of North Irish R1b. In truth both guys are
>> the same number of generations from the founder.
>>
>> But one guy has a haplotype which by chance mutations ends up agreeing
>> totally with the North Irish R1b modal haplotype. Is his estimate of
>> zero
>> generations back to the founder to be valid? The other guy has a
>> haplotype
>> which by chance mutations is out by the fringe of the distribution of
>> descendant haplotypes. His estimate of generations back to the founding
>> will be greater than the true distance. The truth only emerges with
>> using a
>> sufficient population of descendant haplotypes, and using the whole
>> population by an intelligent process to arrive at a single estimate of
>> the
>> distance back to the founder (together with a confidence interval for
>> that
>> single estimate).
>>
>> Ken
>>
>>
>>
>>
>>
>>
>> ==============================
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>>
>
>
> ==============================
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