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Archiver > GENEALOGY-DNA > 2003-12 > 1070736368


From:
Subject: Re: [DNA] Query on Haplogroups
Date: Sat, 6 Dec 2003 13:46:08 EST


In a message dated 12/06/03 8:44:46 AM Pacific Standard Time,
writes:

> <<The more recent the
> SNP, the more likely you are to find the same haplotype in different
> haplogroups.>>
> Ann Turner - GENEALOGY-DNA List Administrator
>
> Question: If genetic DNA change only occurs in one person and is then
> transmitted through inheritance to others (as DNA scientists seem to say)
> should not all haplotypes be in the same haplogroup?
> The reality is (as far as I understand) that most haplotypes are mostly in
> the same haplogroup but there are numerous exceptions as the above
> sentence suggests.

A SNP (Single Nucleotide Polymorphism) is a one-time event (or Unique Event
Polymorphism) that defines the beginning of a new haplogroup. All the
descendants of the person with the SNP will pass it on to the next generations.

The person who has the SNP also has a haplotype, as defined by the Short
Tandem Repeats that the genealogical companies test. This haplotype is the
starting point for 100% of the descendants in the new group. STRs mutate more
frequently, and the number of repeats can go up or down. The longer ago a SNP
occurred, the greater the variety of haplotypes in the haplogroup.

But the founding father of the new haplogroup probably had brothers and
cousins of various degrees who had the same haplotype. Their descendants don't have
the new SNP, so they stay in the old haplogroup, but they are starting with
the same haplotype. Their descendants start accumulating variations in the
haplotypes, too. The longer ago a SNP occurred, the more likely that the
haplotypes have been diverging along different random pathways. The more recent the
SNP, the more likely you are to find that some people have preserved the original
haplotype. If the haplotype has been preserved, then all those people are
"Identical by Descent" (IBD).

But people can also be "Identical by State" (IBS). That can happen when they
start out with different haplotypes, but the mutations just happen to be in
directions that make them more similar. Say for example, two people living a few
hundred years ago had these haplotypes:

11-11-11-11-12-13-11-11
11-11-11-11-13-12-11-11

Their common ancestor probably lived several hundred years or more earlier.
If a descendant of person #1 has a mutation in the fifth marker that changes it
to a 13, and a descendant of person #2 has a mutation that changes the sixth
marker to a 13, then both people end up

11-11-11-11-13-13-11-11

That's called convergence. They are IBS, and it's just accidental that they
end up with the same haplotype. That's not a practical concern over a
genealogical time frame, however, since the mutations are rare to begin with, and the
mutations would have to fall on exactly the right markers.

Ann Turner - GENEALOGY-DNA List Administrator
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