GENEALOGY-DNA-L Archives

From:
Subject: Re: [DNA] calulating ancestor relationships & DNA probabability
Date: Tue, 1 Jun 2004 09:34:13 EDT

In a message dated 5/30/04 4:12:40 PM Pacific Daylight Time,
writes:

> >Terry wrote:
> >> The picture looks like this:
> >> our common male ancestor
> >> +wife #1 +wife #2
> >> / \
> >> his g-grandfather my ggg-grandfather
> >> his grandfather my gg-grandfather
> >> his father my g-grandfather
> >> him my grandfather
> >> my father
> >> me
> >>
> >>
> > >Does this mean that we are 5 generations or 7 generations or
> >> something in between apart?
> >
> >For the purposes of TMRCA, we ordinarily just calculate the
> >up-and-then-back-down total separation and divide by two. If
> >the two testees are in the same generation, then this result is
> >the same as the generation number. In your picture, they are
> >in different generations, so you take (5+7)/2=6.
> >
> > John Chandler
> >
>
> Now I'm really confused. I thought one used the number of
> transmission events to calculate TMRCA, not the average number of
> generations. BTW, I'd have said that "him" was 4 generations from
> ocma, and "me" was 6 generations, not 5 and 7, so I had everything
> wrong.

I agree with you on the generation count.

We do some calculations in one direction (two people back to a common
ancestor) and some in the other (from the common ancestor down to any number of
descendants).

The TMRCA calculations (time to the most recent COMMON ancestor, given a
certain number of mutations) do use the average and thus already take into account
that we are considering two lines. If we say the TMRCA is 5 generations, the
actual configuration might be 4 generations on one line and 6 on the other, or
3 and 7, or anything that adds up to 10 transmission events.

The Mutation Calculator I've mentioned from time to time goes in the other
direction. It asks how many mutations might you expect, given a certain number
of transmission events. This could be configured in many different ways: one
man with 10 sons, or an ancestor 10 generations ago and his living descendant,
or two or more lines tracing their ancestry back. This is illustrated in the
Example button of the Mutation Calculator program:

http://members.aol.com/dnafiler/MutationCalculator.exe

Ann Turner - GENEALOGY-DNA List Administrator
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