Determining ancestry was once the realm of genealogists who followed
paper trails and traveled the world, picking up clues. But now,
researchers can provide a picture about a person's past with a
DNA kit and a swab of his or her cheek.
Genetically speaking, humans are all one race: Homo sapiens sapiens.
The Human Genome Project, an international research collaboration
that mapped human DNA in 2001, discovered that 99.9 percent of
people's DNA, the cell's genetic blueprint, is identical to that
of everyone else on the planet, whether a family member or a stranger.
doesn't know the political, religious or cultural distinctions
on which people have built concepts of race, but in the 0.1 percent
of the genetic code that makes each person unique is inscribed
a chronological and geographic history.
The history reveals itself in a set of DNA markers -- usually
harmless genetic mutations passed on over generations -- that
have accumulated in populations over Homo sapiens sapiens' 150,000-
to 200,000-year history.
When humans first migrated out of Africa and into the Middle
East about 60,000 years ago, new DNA markers appeared that were
unique to the relocated groups. As populations continued to migrate,
to Asia and Europe and then to the Americas, the new groups developed
their own patterns of markers, defining their genetic ancestral
families, called "haplogroups." By collecting haplogroup
information from indigenous communities that are thought to have
remained geographically isolated over their histories, scientists
have constructed detailed human migration maps.
Map of human migration
Now, some two dozen companies offer "genetic genealogy"
tests that, for a few hundred dollars, can match these haplogroups
to the DNA markers in anyone who has a desire to dig into his
or her own heritage.
The testing process
The tests begin by scraping the inner cheek with a toothbrush-like
tool. This swab loosens cheek cells that, like almost every other
cell in the body, contain a person's entire genetic code. Most
of human DNA is found in the nucleus, the cell's control center,
in the form of a double-helix, a twisted ladder that has some
3 billion steps. Each step holds a pair of matching building blocks,
either an adenine and tyrosine or a guanine and cytosine, represented
by the letters A, T, G and C.
DNA kit flowchart
Some DNA tests measure single typographical errors in the genetic
code, called single nucleotide polymorphisms, or SNPs. Other tests
measure changes in microsatellites -- small sequences of repeated
letters that shrink or grow in length when they mutate. Either
of these two marker types can be used to define a person's genetic
signature and place the person in one or more ancestry groups.
The National Human Genome Research Institute says there are about
10 million common SNPs in the human genome, but 97 percent of
these markers are redundant; genetic heritage companies examine
between as few as 10 and as many as hundreds, depending on the
ancestral region and the geographic accuracy their tests are expected
DNA from the cheek cells can be replicated and chopped into strands
that contain a variety of SNPs and microsatellites. In a popular
style of test, the chopped DNA strands of the cheek cells are
placed on a gene chip -- a small device originally developed for
drug research that has a grid of SNP or microsatellite markers
corresponding to known ancestry groups. The chopped strands stick
to the markers on the chip if there is an ancestral group match.
A computer can then tally the matches and compare them to DNA
databases to determine a person's overall ancestry. Heritage is
usually ascribed to four main ancestral groups, corresponding
to major branches on the human migration tree -- African, Asian,
European, or Indigenous American -- though uncommon ancestral
groups can be traced with higher geographic accuracy by testing
more markers, assuming those groups are well represented in the
genetic ancestry company's databases.
These types of mapping tests tell individuals about their overall
genetic composition -- their deep ancestry -- but people often
want to know about closer relatives, such as black Americans whose
family's roots aren't easily traced before the trans-Atlantic
Two other DNA tests can answer lineage questions by analyzing
two branches of the family tree.
SNPs or microsatellites in the Y chromosome, the gender-determining
DNA sequences found only in males, can trace a paternal genetic
lineage. Because the Y chromosome is passed unaltered -- save
mutations -- from fathers to sons, the test becomes a powerful
analogue to the traditional genealogist's last name analysis.
In 2003, researchers reported in the American Journal of Human
Genetic that 16 million people share a 1,000-year-old haplogroup
with a single Central Asian patriarch, possibly a near ancestor
of Mongol emperor Genghis Khan.
tests examine SNPs in DNA from mitochondria, the power plants
of human cells. Mitochondrial DNA, which is distinct from the
DNA in the nucleus, is passed from mothers to all children and
can determine a maternal lineage. Using these methods, scientists
have traced all humans back to a common haplogroup belonging to
a "mitochondrial Eve" who could have lived in Africa
about 150,000 years ago.
The Y chromosome and mitochondrial DNA tests don't draw family
trees -- these two tests give information on only two ancestors
that lived at any one time -- but the tests can be used to prove
or disprove a family connection found by a genealogist, or give
some information when there was none.
As tests become cheaper and DNA databases grow, scientists expect
genetic genealogy results to improve. And though these tests attempt
to explain what makes us different, they also remind us, as deep
down as our DNA, how much we are the same.
DNA: The chemical that carries the genetic instructions
for making living organisms
Gene chip: Sets of miniaturized chemical reaction areas
used to test DNA fragments
Genome: All the DNA contained in an organism, including
the DNA within the cell's nucleus and the mitochondria
Haplogroup: A collection of similar DNA markers that mark
branches in genetic evolution and can indicate common ancestry
Microsatellites: Genome regions with repeating sections
of DNA sequences; used as markers
Mitochondria: The parts of the cells that generate energy.
Mitochondria have their own DNA which is passed from mothers to
Nucleus: The central cell structure that holds DNA
Single Nucleotide Polymorphisms: Genome variations that
occur when a single DNA nucleotide (A, T, C, or G) is altered
Y chromosome: One of the two chromosomes that specify
gender. Humans have two kinds of sex chromosomes: X and Y. Females
have two X chromosomes and males have one X and one Y.