by Megan Smolenyak Smolenyak
When it comes to genetic genealogy (which I often refer to as
genetealogy), Y-DNA is by far the most popular type of testing--and
understandably so. Since Y-DNA is passed from father to son down
through the generations (just like surnames) its application is
fairly obvious. But over time, mitochondrial DNA (usually shortened
to mtDNA) testing has been gaining in popularity.
How mtDNA Travels
Many folks regard mtDNA as the equivalent of a maternal version of Y-DNA testing and while there are some parallels, there are also some
differences, and that creates a lot of confusion.
For instance, mtDNA is passed down through maternal lines, but
mothers pass it on to both their sons and their daughters. The sons,
however, become mtDNA dead ends and do not pass it on. This means
that a brother and sister (who share the same mother) can both get
tested for mtDNA, and that they both can serve as living
representatives of their mother, their mother's mother, their
mother's mother's mother, and so on. (Think of the bottom line of a
pedigree chart). But when this brother and sister pass on, her
children will continue to sport the same mtDNA, while his will have
his wife's mtDNA.
Primarily a Deep Ancestry Tool
Perhaps the most important aspect of mtDNA to grasp is that it's
essentially a deep ancestry test, and is not as genealogically useful
as Y-DNA. If you're familiar with Bryan Sykes's best-seller, The
Seven Daughters of Eve, you may recall that the underlying premise
is that 95 percent of people of European origin can trace their
maternal roots to one of seven women who lived in Europe
approximately 10,000-45,000 years ago.
I, for example, am from haplogroup H (haplogroups might be thought of
as branches of the world's family tree--in this case, the world's
maternal family tree), which Sykes dubbed Helena. Her descendants
were the most successful in reproducing. Consequently, roughly 30-40%
of those of European origin are also H. And this, in turns, means
that I have millions of maternal cousins. That's not tremendously
helpful to know when it comes to tackling my family tree.
Are You My Cousin?
Unfortunately, a lot of genetealogy newbies fail to understand this,
so once they get their results, they tend to play the matchmaking
game. By this, I mean that they e-mail everyone who matches their
mtDNA, share the usual name/place/date details of their direct
maternal line back to their earliest-known ancestor, and then cross
their fingers hoping for someone to report back having found some
overlap. But because it's such a massive fishing expedition, this
hardly ever happens. (I know of one success story, and in my mind,
this person hit the mtDNA lotto!)
Given its limited utility, why would folks even be interested in
taking mtDNA tests? One reason is simple curiosity. A lot of people
are genuinely interested in knowing about their deep ancestry. So if
you want to know roughly when and how your direct maternal line
migrated out of Africa, you might take this test to find out. For
most folks, this is all they will learn, but for many, it's
sufficient.
Three Exceptions and...
Having just said that mtDNA tests are not very helpful for
genealogical purposes, I'd like to point out a few important
exceptions:
- First, you might be lucky enough to have rare mtDNA (taking the
tree analogy a little further, you could think of this as coming from
a branch or even a twig with very few leaves on it). This was true of
Ann Turner, who co-authored Trace Your Roots with DNA: Using Genetic
Tests to Explore Your Family Tree with me. Some people like Ann have
very few, if any, mtDNA matches. Even though she is also in
haplogroup H, the most common European one, she has a couple of
additional mutations that reduce the number of exact matches all the
way down to one. In cases like this, it makes sense to play the
matchmaking game and compare notes.
- Second, you might be dealing with degraded remains, as is often
the case with history mysteries. For instance, I recently worked on a
BBC show to try to identify the two skeletons found in the turret of
the USS Monitor a few years ago. In cases like this, scientists would
love to use Y-DNA, but it's too fragile and doesn't survive. MtDNA,
by contrast, is plentiful, so it is more resilient. It's not as
precise, but it's usually all scientists have to work with. So in
this case, I took the maternal lines of the sailors who lost their
lives in the USS Monitor, and traced them forward in time to find
living maternal relatives to serve as a basis for comparison to the
DNA extracted from the remains. This is also what I routinely do on
my cases for the U.S. Army's repatriation project (to help identify
remains of servicemen still unaccounted for from WWII, Korea, and
Southeast Asia).
- Third, you may have a specific, maternally-oriented genealogical
conundrum--and if you're strategic about it, you just might be able
to come up with a clever way to resolve it using mtDNA testing. There
are a few examples on pages 69-73 of "Trace Your Roots with DNA."
(Those who are registered at Amazon can use the "search inside this
book" feature to view these pages or you can click here for a detailed version of one of the examples given).
...A Couple of Caveats
To this list of exceptions, I now need to add a pair of qualifiers.
The good news is that testing companies and avid genetealogists are
working together in an attempt to refine haplogroups. For instance,
when I first got my mtDNA tested several years ago, I learned that I
was H. Now I can find out that I'm H1*. Returning to the tree
analogy, it's somewhat akin to learning which twig off of the H
branch my maternal line comes from.
Also, mtDNA is finite. It has 16,569 base pairs, which sounds like a
lot, but is nothing compared to the billions of bases found in
nuclear DNA that's used for Y-DNA tests. So it's in the realm of the
possible to test for your entire mtDNA sequence, which translates
into very precise results.
The catch? Well, there are two. Once you have your sequence, you can
theoretically play the matchmaking game again, but at prices
currently in the $795-895 range, you probably won't have too many to
compare against until prices come down. And I need to point out that
a full-sequence mtDNA test is the first genealogical one that could
conceivably give away medical secrets. Some conditions are passed
through maternal lines (e.g., some kinds of muscle disorders), so
your results could potentially reveal more than you'd care to know.
Of course, you'd have to take your results (which are well-protected)
and do some fairly intensive research or consult a genetic counselor,
but in the interest of full disclosure, I feel obligated to mention
this possibility.
Hop in the mtDNA Pool!
If you're new to genetic genealogy, I know this is a lot to absorb,
so I'd suggest that you start with more conventional Y-DNA testing,
perhaps joining a surname project. But if you're ready to take the
next step or are just plain curious, I hope this primer will help you
understand just what you can and can't learn from mtDNA testing.
About the Author
Megan Smolenyak Smolenyak, co-author (with Ann Turner) of Trace Your
Roots with DNA: Using Genetic Tests to Explore Your Family Tree (as
well as In Search of Our Ancestors, Honoring Our Ancestors and They
Came to America), can be contacted through her websites Genetealogy.com, Honoring Our Ancestors, and
Megan's RootsWorld.
Upcoming Appearances by Megan
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