The state of the universe.
From
www.slate.com on March 15, 2006
How African Are You?
What genealogical testing can't tell you.
By John Hawks
Updated Wednesday, March 15, 2006, at 1:43 PM ET
Recently on PBS's "African American Lives," host Henry Louis Gates had
his
DNA tested to learn about his ancestry. Gates' family suspected its
paternal
ancestry could be traced to a white slave owner. But DNA testing showed
that
his Y chromosome did not match the man's white descendants. A second,
newer
test gave Gates another result he didn't expect: His DNA showed that
only
half of Gates' ancestry was African. The rest were apparently European.
DNA testing for genealogy has become increasingly popular, as a Newsweek
cover story in February attests. Especially attention-getting have been
efforts to trace genetic relationships along the male lineage. In
January
the New York Times wrote up attempts to trace Irish genealogy through
the
male line to Niall of the Nine Hostages, a fifth-century Irish warlord.
Other tests have also shown that as many as 14 million men may share the
Y
chromosome of Genghis Khan. But tests that seek a single, Y-chromosome
male
lineage are limited: They leave out the vast majority of ancestors.
Newer
tests can survey all the DNA that can be inherited from either parent,
but
at a cost of precision: They don't tell which ancestors lived where, and
they can't detect traces of ancestry.
The newer "genetic admixture tests" examine DNA from genes inherited
from
all of a person's grandparents, great-grandparents, and so on. A few of
these genes reflect the part of the world where those ancestors lived.
Like
postcards, they track the movement of people from the lands of their
ancestors to their current address. Scientists studying these genetic
variations now focus on sites that vary between people by one chemical
letter. They're called "single nucleotide polymorphisms," or SNPs. Some
of
these SNPs are important: They may contribute to traits like skin color
or
resistance to regional diseases like malaria. Others vary among
populations
just because of chance.
For geneticists, finding the SNPs that mark populations is a challenge.
For
the most part, the same SNP might be found in Africans, Europeans, and
people from every other part of the world. It's now possible to test
quickly
for hundreds of SNPs by using special microchips that bind to the
distinctive DNA sequences. These tests examine hundreds of SNPs at once;
if
among these a person has many that are common in Africa, it is likely
that
she has some African ancestors.
Admixture testing works best in groups like African-Americans, whose
ancestors in Africa and Europe lived far from each other. Most of the
ancestry of today's African-Americans can be traced to West or Central
Africa, with a minority from other parts of the continent. (Gates'
family is
a bit exceptional in terms of origin.)
But for other groups things can get a lot more complicated. Many amateur
genealogists are interested in whether they might have a Cherokee
ancestor,
for example. And for some people, admixture tests can give a relatively
accurate answer about Native-American ancestry. But other people,
including
Greeks and Ashkenazi Jews, may have "Native American affinity,"
according to
the tests, even if they and their ancestors have never been to America.
As
far as anthropologists know, there were no lost tribes connecting
Greeks,
Jews, and ancient Americans. So, maybe this "Native American affinity"
reflects the scattering of alleles by prehistoric Asian nomads to the
ancestors of Greeks and Jews as well as to American Indians. Maybe the
SNPs
that they share gave these groups a leg up in fighting diseases.
All we know for sure is that such genetic similarities can make ancestry
testing very confusing. Suppose a person of mostly German ancestry
discovers
that his DNA has 6 percent Native-American affinity. Does he have a
Native-American ancestor, or a Greek or Jewish ancestor, or all three?
There's only one way the 6 percenter can know for sure: He has to know
most
of his genealogy already.
From a practical point of view, that is the biggest problem with
today's
genetic genealogy tests. In many cases, they can't tell you what you
don't
already know. And unlike DNA fingerprinting tests with error rates of
one in
a billion or less, the chance of misidentifying ancestral groups in
these
genealogy tests may be 5 percent or higher. With this chance of error,
the
test won't be wrong about a full Native-American grandparent, but it
might
be wrong about a great-great grandparent. In addition, SNPs that
separate
central Africans from northern Europeans aren't nearly as good at
separating
Ethiopians from Arabs. So, in the test results of some
African-Americans,
European means Europe, while in others, it may mean East African, or
Arab,
or Indian. Depending on where his African ancestors came from, Gates'
apparently European origins might lie somewhere else entirely.
A deeper problem with admixture testing is its claim to identify the
"ancestral components" of different populations. For example, admixture
testing considers people from India to be a mixture of "Indo-European"
and
"East Asian" ancestors. And indeed, Indians have some alleles otherwise
common in Europe, and some otherwise common in China. But Indian
populations
have been on their subcontinent for tens of thousands of years, and they
have many alleles that don't come from anywhere else. Anthropologists
studying genetic variation have always found complexity rather than
simple
one-plus-one racial mixtures. SNP-testing companies don't seem to have
gotten that news.
SNP-based tests can help you find out where your great-grandfather came
from. But his distant ancestors ultimately came from other places. So,
most
of your genetic ancestry will still be a question mark, no matter how
many
tests you shell out for.
_____
John Hawks is an anthropologist at the University of Wisconsin-Madison
who
specializes in human evolution and genetics. He maintains an
anthropology
weblog.