Now, however, researchers are exploring the remaining 1 percent to explain differences among people of different continental originsand that's proving to be a lot trickier.
For example, scientists have recently identified small differences in DNA that account for the pale skin of Europeans, the tendency of Asians to sweat less, and West Africans' resistance to certain diseases.
At the same time, genetic information is slipping out of the lab and into everyday life: Ancestry tests tell people what percentage of their genes are from Asia, Europe, Africa, and the Americas. The heart-disease drug BiDil is marketed only to African-Americans, who seem genetically predisposed to respond to it; Jews are offered prenatal tests for genetic disorders rarely found in other ethnic groups. In the courts, DNA evidence has resulted in the exoneration of more than 200 wrongly convicted inmates.
But while these early benefits of the genetic revolution are undeniable, some critics fear this new genetic knowledge may also be giving long-discredited racial prejudices a new potency. The notion that race is more than skin-deep could undermine principles of equal treatment and opportunity that have relied on the presumption that we are all fundamentally equal.
"We are living through an era of the ascendance of biology, and we have to be very careful," says Henry Louis Gates Jr., director of the W. E. B. Du Bois Institute for African and African American Research at Harvard University. "We will all be walking a fine line between using biology and allowing it to be abused."
Certain superficial traits like skin pigmentation have long been presumed to be genetic. But the ability to pinpoint their DNA source makes the link between genes and race more palpable. On mainstream blogs and in college classrooms, the topic is prompting questions of whether more profound differences may also be attributed to DNA.
Scientists say that studying these differences is crucial to mapping the genetic basis for disease, but concerns remain. Renata McGriff, 52, a health care consultant in New York who had been encouraging black clients to volunteer genetic information to scientists, says she and other blacks have lately been discussing "opting out of genetic research until it's clear we're not going to use science to validate prejudices."
Such discussions are among thousands that followed the geneticist James D. Watson's suggestion in October that Africans are innately less intelligent than other races. Watsonwho won a Nobel Prize in 1962 as co-discoverer of the DNA double-helixsubsequently apologized. His remarks were sharply denounced by the scientific community.
There is widespread agreement that not only genetics, but also life experienceseven in the wombcan influence brain structure. Furthermore, scientists are divided over what defines intelligence and whether it can be accurately measured. For example, Steven Jay Gould, the late evolutionary biologist, dismissed "the I.Q. industry" as mostly an effort by people of European descent to maintain their prominence in the world.
Some scientists believe that the authority that DNA has earnedthrough its use in solving crimes, preventing disease, and reconstructing family tiesmay lead people to wrongly elevate genetics over other explanations for differences.
"I've spent the last 10 years of my life researching how much genetic variability there is between populations," says David Altshuler, a geneticist and director of the Program in Medical and Population Genetics at the Broad Institute in Cambridge, Mass. "But living in America, it is so clear that the economic and social and educational differences have so much more influence than genes. People just somehow fixate on genetics, even if the influence is very small."