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"Genography" Puts European Ancestry on the Map
2 September 2008 (All day)
What do you get when you plot the genetic fingerprints of more than 1000 Europeans on a grid? An image that looks surprisingly like a map of Europe. The findings reveal that our DNA contains a sort of global positioning system, which researchers can use to pinpoint where in the world both we and our relatives came from.
Humans have been obsessed with their ancestry for millennia, yet it wasn't until the advent of DNA sequencing that scientists had the hard data to begin tracing origins to various parts of the globe. Merely comparing a few sections of DNA gives only a rough, continent-scale idea of a person's heritage, however, and scientists have struggled to glean a more detailed genetic profile.
Enter single nucleotide polymorphisms (SNPs). Each genome contains millions of these tiny variations, and because they differ from person to person, they provide scientists with a way to tell one person's DNA from another's. Over the past decade, SNPs have shed new light on the genetic risk factors for common illnesses such as diabetes and heart disease--and they've even helped federal investigators hone in on a suspect in the anthrax case (ScienceNOW, 12 August). Population geneticist John Novembre of the University of California, Los Angeles, had a hunch that SNPs would be valuable for tracing ancestry as well.
He and colleagues compared 500,000 SNP differences among 1387 Europeans. To ensure that the people they examined had their roots firmly set in a certain region, the team looked at individuals whose grandparents hailed from the same region as they did. The scientists then employed a statistical technique known as principle component analysis, which allows large amounts of data with multiple variables to be condensed onto a two-dimensional space. Individual points were marked as two-letter abbreviations corresponding to the region of Europe to which each subject claimed ancestry (see image).
The result was a map of Europe-- fuzzy, but unmistakable. "I couldn't believe the picture was so clear," says Carlos Bustamante, senior author and statistical geneticist at Cornell University. "I, for one, fell off my chair." Italy and Spain clearly had their own cluster of genetically similar individuals, for example, and there were even distinctions between French-, German-, and Italian- speaking populations within Switzerland.
The results make sense, says Bustamante. Because people in a region are more likely to marry and mate with each other--a factor that may be largely due to shared language--that gene pool will evolve as a separate cluster that corresponds to a place on the globe, he explains. "You don't randomly mate within Europe. ... If you live in Strait of Gibraltar, you're more likely to marry someone in Spain versus someone in Moscow."
The findings, published online this week in Nature, have practical uses. The researchers believe that with more SNP data, they'll be able to create an even more detailed "genography" map; that will allow individuals armed with their own genetic fingerprint to pinpoint exactly where their families originated. Rather than just knowing that your family came from Italy, Bustamante notes, you could hone in on the specific county or village your kin hailed from. Forensics could get a boost from the findings as well: DNA from hair or blood could be used to reveal the precise ethnic background of a victim or suspect, for example.
Experts praise what they call a visually stunning map. Scientists have suspected that people's genes were linked to their geography, says Mattias Jakobsson, a population geneticist at Uppsala University in Sweden, but "this level of detail has not been demonstrated before--this paper shows some nice, strong results." Still, although the data shows clear genetic distinctions between Europeans, Paolo Menozzi, an evolutionary geneticist at the University of Parma in Italy, cautions that, overall, Europeans' genetics do not vary vastly from country to country. On the whole, their genes are much more similar than they are different, he notes.