Mapping Genes
Similarities in DNA unite people all over the world
By Gia Scafidi, USC News Service
Human populations from different parts of the world are genetically more similar to each other than previously thought, according to research by a USC scientist and his colleagues. The genetic study, one of the largest of its kind, demonstrates that individuals’ geographic ancestry can be accurately inferred from DNA. The results also provide implications for an ongoing debate about the usefulness of ancestry information in medical research.
The findings appeared in the Dec. 20 issue of the journal Science.
“It was surprising to see just how similar different populations were,” says Noah Rosenberg, a research scientist in the molecular and computational biology program of USC College and one of the study’s contributing researchers.
“The easiest groups to identify as distinctive were those that were either extremely small in number (several thousand or less) or those that had been extremely isolated for long periods of time,” he says.
In contrast, “individuals from intermediate regions—such as Central Asia and North Africa—tended to have intermediate genes between those of neighboring regions,” he adds.
The group, led by Marcus Feldman of Stanford University, examined nearly 1,100 DNA samples from 52 populations in Africa, Europe, Asia, the Middle East, America and Oceania. Populations were defined by geography, language and culture. Participating individuals were also well rooted in their region, with several generations of ancestors known to have lived in the same locale.
The team focused on 377 segments of the human genome—the same DNA sections commonly used as inheritance markers in medical and evolutionary studies. Each of the 377 sections contained 4-32 distinct genotypes (types of DNA sequence).
“Most of the genotypes were found in people from several continents,” says Rosenberg. This suggests that only a small fraction of genetic traits are unique to specific groups.
“By sampling genotypes from people from all parts of the world, geneticists have reconstructed the major features of our history: our ancient African origin, migrations out of Africa, movements and settlements throughout Eurasia and Oceania, and [the] peopling of the Americas,” wrote Mary-Claire King and Arno Motulsky, both of the University of Washington, in a commentary accompanying the Science paper.
Uncovering humans’ genetic history is anything but a small task. Diversified DNA samples take years to assemble, and the human genome varies only slightly.
“The challenging aspect of trying to infer history of migration is that there is so little to work with in the human genome,” says Rosenberg. “Because people are so similar, there’s only a small amount of information you can use to determine how different groups are related.” The genomes of all humans are more than 99 percent identical.
In the less than 1 percent of the genome where genetic differences do exist, it would seem likely that two people from different regions would have many more differences than two people from the same region.
On the contrary. Rosenberg and his colleagues found that 94 percent of genetic differences were among individuals from the same populations, an estimate considerably exceeding previous ones of about 85 percent, based on studies with less data.
Despite humans’ genetic similarities, DNA can play a vital role in identifying the geographic region from which one’s ancestors came. “While most genetic types are widely distributed geographically, the frequencies of these types vary around the world,” Rosenberg explains. “Combinations of types across many parts of the genome may be frequent in one group but rare in most others.”
To demonstrate this, Rosenberg applied a powerful statistical technique that uses many independent genes to detect geographical patterns of ancestry in samples from any species. In this approach, the geographic labels of the individuals are removed, and individuals are placed into clusters using only their DNA genotypes.
The team found that people from Eurasia, the geographic region including Europe, the Middle East, and Central and South Asia, were among the most difficult populations to assign ancestries. “This is most likely due to a complex history of migrations, conquests and trade over the past few thousand years,” says Rosenberg.
Because different populations experience varying disease rates, recent studies have begun to question how ancestry information might be useful in disease research.
The question at hand: Is information obtained by grouping subjects by their responses to ancestry questions more useful than information obtained by grouping them according to genetic similarities?
The researchers suggest that self-reported and genetic ancestry are equally sound gauges, but Rosenberg points out the importance of both sets of information. “On the one hand,” he says, “grouping patients by genetic similarities will benefit forthcoming studies that will scan the entire human genome for potential genetic causes of disease.
“On the other hand, when you ask someone about his ancestry, you also get information about cultural differences and behaviors, which may be associated with risk factors for certain diseases,” he says. “They’re both important.”
Meanwhile, the debate continues, the research moves forward and the complete tale of humans’ genetic history remains to be told, says Rosenberg.
“Next, we hope to increase our collection of individual DNA samples and examine a number of regions not yet studied,” he says. “It’s going to take much more data to figure out how we’re all related.”
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