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Archiver > APG > 2006-04 > 1146065193


From: Ruy Cardoso <>
Subject: Re: [APG] Mills DNA
Date: Wed, 26 Apr 2006 08:26:33 -0700 (PDT)
In-Reply-To: <200604260658.k3Q6w3Y9005282@lists2.rootsweb.com>


There have been a few recent posts on Y-DNA testing,
excerpted below.

> wrote:
>
> I would also agree with Ms. Lustenberger's
> assessment, and with the excellent explanation
> added by Adam Bradford. There are estimates that
> 12/12 matches are unreliable in up to 21% of such
> matches,
>
> wrote:
>
> It's not that 12 markers isn't sufficient to
> identify the individual testee's haplotype - a
> haplotype is simply the values a person holds
> on those 12 markers.
>
> Helen S. Ullmann wrote:
> She was emphatic that a 12-marker test
> really isn't sufficient to identify the haplotype.

I have a few clarifying points to mention in response
to the excerpts above, and I hope that they will not
confuse things further. But let me give it a go.

First, there is nothing that says a haplotype has to
be defined in terms of 12 markers, though it is common
to speak of them that way since 12-marker tests have
been popular to date; in fact, they are still being
used in the Genographic Project run by National
Geographic. But you can just as easily speak in terms
of a 27-marker haplotype or a 33-marker haplotype or
whatever other number you like. Helen's comment
referred to *the* haplotype, but such a concept
doesn't really exist except in terms of a defined set
of markers.

Second, the more markers you consider, then the more
haplotypes there can be. Taking things to an extreme,
you might imagine a 1-marker haplotype. Depending on
the marker being used, there might only be eight or
nine different values that such a 1-marker haplotype
could take on. But look at, say, 50 markers, and
there could be millions or billions of combinations.
In theory, there are far more combinations possible
than there are people on Earth, but of course not
every combination can show up in the population.

Now brothers, for example, would likely have pretty
similar 50-marker haplotypes, but even they might not
be identical -- most of the markers would match but
not necessarily all -- and the more markers being
examined, the less likely you are to find a perfect
match. This is actually a good thing, because if
mutations *never* occurred, then there would be only
one haplotype out there for any number of markers, and
we would be deprived of a useful tool for sorting
people into families.

Third, the conventional wisdom is that a 12-marker
test is not really useful to support a supposed
genealogical relationship, but it may certainly be
enough to disprove one. So the test continues to have
some genealogical value, just not in the direction
that people would prefer. Don't we all prefer to
support hypothetical relationships than to disprove
them?

Finally, it is easy to confuse two similar terms,
haplotype and haplogroup. A haplotype, as already
noted above, is simply a combination of particular
marker values, and of a particular type (STRs, or
short tandem repeats). A haplogroup, on the other
hand, is defined by a different type of mutation
altogether (SNPs, or single nucleotide polymorphisms).
While there is some dispute about the frequency with
which the different types of mutations occur, there is
no question that STR mutations, the kind most commonly
discussed when talking about Y-DNA tests, happen far
more often than SNP mutuations.

In fact, SNP mutations are so rare that they aren't
really useful for traditional genealogy at all (i.e.,
for relationships over the past three or four hundred
years). But they are very useful for determining more
ancient origins. Now while one can test for SNP
mutations directly, a lot of people try to determine
possible relationships between particular STR marker
values (or combinations of them) and particular SNP
mutations; in other words, they want to use haplotypes
as a clue in determining haplogroups. If you read the
postings on the Genealogy-DNA mailing list at
Rootsweb, I would say that the majority of posts are
on this very topic.

I hope that all helps people's understanding of Y-DNA
testing, at least a little.

Ruy Cardoso

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