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Genetic ancestry tests are all the rage these days.

Dozens of companies now offer tests that are supposed to help people chart the geographic origins of their ancestors. Some firms even imply they can tell you whether you're related to the leading lights of antiquity, ranging from the Mongolian warlord Genghis Khan to the last of the Russian czars.

And the test kits, which are readily available through slick Internet sites, don't even require you to draw blood. With just a scrape from the inside of your cheek, you can mail in the sample to a lab for the genetic analysis.

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But in today's edition of the journal Science, 14 leading U.S. researchers warn that the popular tests, which range in price from $100 (U.S.) to $900, can produce incomplete, misleading and erroneous results.

"I would caution people against spending money on these tests unless they really understand what they can and cannot learn from them," said the lead author, Deborah Bolnick, an assistant professor of anthropology at the University of Texas in Austin.

The two most common tests are based on an examination of either the Y-chromosome, which is passed down from father to son, or mitochondrial DNA, which is passed down from mother to child in both males and females.

As part of the analysis, the DNA is compared with the company's own database of samples from around the world to identify others with similar DNA sequences.

"These comparisons can identify related individuals who share a common maternal or paternal ancestor, as well as locations where the test-taker's haplotype [genetic bits linked together and usually inherited as a unit]is found today," the researchers write.

"However," they go on to say, "each test examines less than 1 per cent of the test-taker's DNA and sheds light on only one ancestor each generation."

For example, a mitochondrial DNA assessment will tell you about only one of your 16 great-great-grandparents, Dr. Bolnick noted.

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That means the tests can identify some of the groups and locations around world where people with similar genetic traits are found. But they are unlikely to pick them all.

Furthermore, the genetic comparisons are only as good as the company's database, Dr. Bolnick said. "Some companies have 10,000 to 20,000 samples ... and that may not actually be a really thorough sampling of any particular location."

Even if your genetic test matches a person living in some distant part of the world today, there is no guarantee that your ancestor ever visited that place. "People have been moving around from one place to another for generations," she explained. "So it may be that your shared ancestor lived in a different part of the world."

The concept of race also throws a spanner in the works. Generally, people are lumped into very wide groupings such as Africans, East Asians, Europeans or Native Americans. "There is actually a huge amount of genetic variation within each of those groups," Dr. Bolnick said. Overall, racial groups are more similar to one another than they are different.

But the gene-testing companies often make big and sometimes faulty assumptions in their comparisons. Dr. Bolnick points out that certain DNA sequences common in Native Americans are also routinely found in Asians. As a result, some people are informed they have Native American origins when their ancestors never migrated beyond Asia.

There is also a significant risk that some people will suffer an identity crisis after their long-held family histories have been shattered by incomplete or faulty results.

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Dr. Bolnick knows of people who were firmly convinced they had First Nation blood somewhere in their distance past. "They take the test and the results come back and say they don't have Native American mitochondrial DNA," Dr. Bolnick said. "But that test is only telling them about one out of many ancestors," she said. "Your mother's, mother's mother was not Native American, but your mother's father might have been."

Not all the gene-testing companies are alike.

National Geographic and IBM, for example, have established the Genographic Project, which is using all the samples it receives from the public for research. Led by population geneticist Spencer Wells, "the project seeks to more fully chart the genetic migratory history of the human species and answer age-old questions about the diversity of humans," the website says. Many of the other companies are purely commercial ventures.

"I want to state clearly that Genographic is not a 'company' selling genetic ancestry tests - it is a non-profit research and educational project," Dr. Wells wrote in an e-mail interview.

"With that said, it is the case that Genographic has sold more participation kits than any of the companies out there - over 225,000 at last count. We are careful to explain, in advance, before any testing is conducted, what the project is about. We explain the limitations of genetic testing, most importantly the fact that Y-chromosomal and mtDNA [mitochondrial]analysis only tell you about a small fraction of your genetic background."

Even so, the public may still not appreciate the stated limitations of Genographic Project, said one of the authors of the Science article, Kimberly TallBear, assistant professor of American Indian Studies at Arizona State University in Tempe.

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"I think they [Genographic]make claims that are similarly problematic [as some other companies] but they are more careful and more eloquent in the way that they do it," Dr. TallBear said.

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Genetic ancestry tests

Benefits

Can identify related individuals.

May support or refute suspected connections between families.

Can potentially support existing genealogical records.

May provide locations for further genealogical research.

Can trace your direct maternal or paternal linage.

Drawbacks

Tests trace only a few of your ancestors and a small portion of your DNA.

May report false negatives or positives.

Limited sample databases mean results are subject to misinterpretations.

You likely have relatives in many unstudied groups.

There are no clear connections between your DNA and racial/ethnic identity.

Cannot determine exactly where your ancestors lived or what ethnic identity they held.

Source: The University of Texas at Austin website.

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