Geography determines similarities, differences in DNA, studies find
THURSDAY, Feb. 21 (HealthDay News) -- In the largest such examination of human genetic diversity yet conducted, an international team of geneticists has used clues in DNA to track differences and similarities between people around the globe.
Reporting in the Feb. 22 issue of Science, a team led by Richard Myers, of Stanford University, looked at more than 650,000 variants in individual genes, called single-nucleotide polymorphisms (SNPs) found in 938 unrelated volunteers in countries around the world.
Among other findings, the new data support the notion that "a small group of individuals migrated out of eastern Africa and their descendants subsequently expanded into most of today's populations," the researchers wrote.
That expansion occurred mainly over the past 100,000 years, the team says. Using high-tech DNA analysis of the SNPs, they were able to determine the distinct genetic identities of eight European groups of modern humans, as well as four in the Middle East. They also believe that shared genes found among Native Americans and certain groups in Russia "reflects shared ancestry before the predecessors of the Native Americans crossed the Bering Strait."
The report in Science follows on the heels of similar international efforts by other researchers, reported in two studies published in the Feb. 21 issue of Nature.
Noah Rosenberg, co-senior author of one of two Nature studies, explained that his group has "investigated genetic similarities of populations across a very large number of sites in the human genome. We found that it's possible, with a high degree of accuracy, [to tell] which continent, in which population, the individual comes from."
In addition, the primary determinant of genetic similarities and differences seems to be geographic location, said Rosenberg, who is an assistant professor of human genetics at the University of Michigan. "The closer populations are, the greater the degree of similarity between the populations," he noted.
Rosenberg's team also found that the number of distinct genetic variants declined as the distance from Africa increased. "This suggests that a genetic history reflects a history in which populations migrated out of Africa, and along the way only a portion of the genetic diversity available migrated to the next location," he said.
There is still a great deal of genetic diversity in each population, Rosenberg pointed out.
In the study, Rosenberg's team looked at more than 500,000 DNA markers. The markers came from 485 volunteers in the Human Genome Diversity Project. Rosenberg's team looks at genetic variations in 29 different groups from Africa, Europe, the Middle East, Southeast Asia, the Pacific Ocean islands, and the Americas.
The implication of what Rosenberg's group found is that scientists now have a finer understanding of human evolutionary history.
"We can infer how our ancestors migrated across continents and became successful at living in a very diverse range of environments," Rosenberg said. "In addition, the genes are [a] database [that] can be used to search for disease genes in the human genome."
In the second Nature report, Carlos Bustamante, an assistant professor of biological statistics and computational biology at Cornell University, looked at DNA from European Americans and African Americans.
The researchers looked at 10,000 genes in 15 African Americans and 20 European Americans, all of whom were healthy.
"Across all the individuals, we found almost 40,000 DNA sites that varied. The African-American sample [had] more variations than the European-American sample, which is consistent with previous work showing higher levels of overall genetic diversity in African-Americans," Bustamante said.
This finding suggests that only a subset of diversity was present in the founding populations of Europe, Bustamante said, adding, "We refer to this as a population bottleneck."
Bustamante's team found that the proportion of mutations that are associated with risk for disease is higher in the European-American population. "This is consistent with evolutionary theory that mutations may undergo slightly relaxed natural selection in bottleneck populations," he said.
All the individuals in the study had about 400 mutations that may be linked to disease, Bustamante said.
How these mutations affect human health isn't known, Bustamante added. "Efforts to do sequencing to look at individuals [with] and without disease will likely find rare mutations that may be contributing to disease," he said.
Last month, it was announced that the genomes of 1,000 people worldwide will be mapped in what scientists are calling the most detailed and medically relevant look at human genetic variation ever conducted.
The 1,000 Genomes Project will receive major support from the U.S. National Human Genome Research Institute (NHGRI), the Wellcome Trust Sanger Institute in England, and the Beijing Genomics Institute in China.
"This new project will increase the sensitivity of disease discovery efforts across the genome fivefold and within gene regions at least 10-fold," NHGRI director Dr. Francis S. Collins said in a prepared statement.
For more information on understanding the human genome, visit the Human Genome Project .
SOURCES: Noah Rosenberg, Ph.D., assistant professor, human genetics, University of Michigan, Ann Arbor; Carlos D. Bustamante, Ph.D., assistant professor, biological statistics and computational biology, Cornell University, Ithaca, N.Y.; Feb. 22, 2008, Science, Feb. 21, 2008, Nature
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