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.
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.
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.
