STANFORD - Checking a box next to a racial/ethnic category gives several pieces of information about people - the continent where their ancestors were born, the possible color of their skin and perhaps something about their risk of different diseases. But a new study by researchers at the Stanford University School of Medicine finds that the checked box also says something about a person's genetic background.

This work comes on the heels of several contradictory studies about the genetic basis of race. Some found that race is a social construct with no genetic basis while others suggested that clear genetic differences exist between people of different races.

What makes the current study, published in the February issue of the American Journal of Human Genetics, more conclusive is its size. The study is by far the largest, consisting of 3,636 people who all identified themselves as either white, African-American, East Asian or Hispanic. Of these, only five individuals had DNA that matched an ethnic group different than the box they checked at the beginning of the study. That's an error rate of 0.14 percent.

According to Neil Risch, PhD, a UCSF professor who led the study while he was professor of genetics at Stanford, the findings are particularly surprising given that people in both African-American and Hispanic ethnic groups often have a mixed background. 'We might expect these individuals to cross several different genetic clusters,' Risch said. This is especially true for Hispanics who are often a mix of Native American, white and African-American ancestry. But that's not what the study found. Instead, each self-identified racial/ethnic group clumped into the same genetic cluster.

The people in this research were all part of a study on the genetics of hypertension, recruited at 15 locations within the United States and in Taiwan. This broad distribution is important because it means that the results are representative of racial/ethnic groups throughout the United States rather than a small region that might not reflect the population nationwide.

For each person in the study, the researchers examined 326 DNA regions that tend to vary between people. These regions are not necessarily within genes, but are simply genetic signposts on chromosomes that come in a variety of different forms at the same location.

Without knowing how the participants had identified themselves, Risch and his team ran the results through a computer program that grouped individuals according to patterns of the 326 signposts. This analysis could have resulted in any number of different clusters, but only four clear groups turned up. And in each case the individuals within those clusters all fell within the same self-identified racial group.

'This shows that people's self-identified race/ethnicity is a nearly perfect indicator of their genetic background,' Risch said.

When the team further analyzed each of the four clusters, they found two distinct sub-groups within the East Asian genetic cluster. These two groups correlated with people who identified themselves as Chinese and Japanese. None of the other genetic groups could be broken down into smaller sub-sections. This suggests that there isn't enough genetic difference to distinguish between people who have ancestry from northern Europe versus southern Europe, for example. Risch admitted that few people in this study were of recent mixed ancestry, who might not fall into such neat genetic categories.

This work could influence how medical research is carried out. Often researchers ask study participants to identify their race and ethnicity at the beginning of a clinical trial. The researchers can then follow people of different racial/ethnic groups to see which group is more likely to get a particular disease or respond well to a new treatment. This information can help future doctors know which patients may need additional disease screening or should receive one treatment over another.

But recently some researchers have moved to examining genetic differences between participants rather than relying on race and ethnicity. Their reasoning is that genetic differences may be a more precise tool for tracking groups of patients. Risch points out that this genetic analysis is costly. If people fall into the same groups using self-identified race as using genetics, then that could bring down the expanding cost of medical research.

Other Stanford researchers who participated in this work include Hua Tang, a graduate student now at the Fred Hutchinson Cancer Research Center, and Tom Quertermous, MD, the William G. Irwin Professor in Cardiovascular Medicine.