"Although this research probably won't allow doctors to identify individual risk factors, knowledge of a large fraction of all the major genetic risk factors contributing to a treatment response or common disease could have immediate utility, allowing existing treatment options to be matched to individual patients."

The work was unveiled Thursday at the Annual Meeting of the American Association for the Advancement of Science (AAAS), which publishes Science.

The mapping effort describes 1.58 million single-letter DNA variations across 71 individuals of European American, African American and Han Chinese American ancestry. Although the human genome contains millions more single-letter variations, or single-nucleotide polymorphisms (SNPs), they seem to occur within patterns that have been preserved for thousands of years -- despite the DNA reshuffling that happens from generation to generation. The new mapping effort therefore appears to capture most common human genetic variation, researchers said.

The work is believed to have significant implications for the study of cardiovascular disease, mental illness, and many other conditions thought to result from a complex interplay of multiple genetic and environmental factors.

Controversy over Races Unresolved

Researchers made use of the fact that two genes located closer together are far less likely to be reshuffled over generations by the biological process known as "recombination." As a result, certain patterns of variation, or "haplotypes," have been preserved across human history. The presence of these patterns, known as "linkage disequilibrium," allowed the Science authors to create a first picture of the structure of human genetic variation based on short- and long-range clustering of single-letter variations (SNPs).

Most common DNA variations are found across all populations and likely date back to the exodus of modern humans out of Africa. But, other genetic differences may be specific to certain populations, explained David R. Cox of Perlegen Sciences, Inc., in Mountain View, California.

The research thus should "provide a tool for exploring many questions remaining regarding the causal role of common human DNA variation in complex human traits and for investigating the nature of genetic variation within and between human populations," the Science paper concludes.

But, the findings, based on publicly accessible data, won't resolve ongoing debate over whether distinct races of people exist, explained Cox, corresponding author of the study, with lead author David A. Hinds, also of Perlegen Sciences. "Our study was really designed to help us understand patterns of variation that are common and cut across populations," said Hinds.

"People cannot use our data to say, 'See, I told you there are races,' or, 'See, I told you there aren't races,'" said Cox. "Nor can they say, 'See, the differences are more important than the similarities. But, these data will be useful for starting to address such questions as which kinds of medical treatments should be used, based on physiological differences caused by genetic variations."

'Major Step Forward'

In a related Science Policy Forum article, Troy Duster of New York University describes the new research as "well-intentioned, well-crafted, and designed to help better understand the molecular basis of disease." But, he also urges caution, noting that biomedical researchers must "climb back on the tightrope to address racial disparities in health" to avoid "reification" or reinforcement of outdated concepts of race.

The new data "represent a major step forward," says David Altshuler of the Broad Institute at Harvard and Massachusetts Institute of Technology and the Massachusetts General Hospital, Boston, and author of a related Science Perspectives essay. "It is exciting to note that Perlegen and the public HapMap Project are now working together to generate an even denser map," says Altshuler.

The more detailed description of genetic variations is expected later this year from the international HapMap Project, directed by government agencies from Japan, China and Canada, as well as The Wellcome Trust of London and the U.S. National Institutes of Health. That mapping effort will describe variation across individuals of Japanese, Chinese, Nigerian and European ancestry, said Cox, who also works with the public group.