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DNA Findings Reveal Genetic History of Humans

Geography determines similarities, differences in DNA, studies find

WEDNESDAY, Feb. 20 (HealthDay News) -- Scientists have compiled a mass of detailed DNA information that could help reconstruct the genetic history of humans.

"We have investigated genetic similarities of populations across a very large number of sites in the human genome," said Noah Rosenberg, co-senior author of one of two studies on the findings in the Feb. 21 issue of Nature. "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 report, Carlos Bustamante, an assistant professor of biological statistics and computational biology at Cornell University, looked at DNA from European-Americans and black Americans.

The researchers looked at 10,000 genes in 15 black 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.

More information

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. 21, 2008, Nature

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