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.

DNA 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 to fulfill.

The 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.

Lineage tests
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 slave trade.

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.

Other 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.

Genetics Glossary
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 offspring.

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.

-- By Adnaan Wasey, Online NewsHour