Few issues in science have stirred more social controversy than race.

Race has a deceptively simple definition to a professional biologist like myself, referring to groups of individuals related by ancestry that differ from other groups, but not enough to constitute separate species. The controversy arises because of the way people have used the concept of race to justify the abuse of humans.

The African slave trade is but one obvious example. Perhaps in some measure responding to these sorts of injustice, scientists have largely abandoned the concept of race except as a social construct. This re-evaluation had its beginnings in 1972, when geneticists pointed out that if one looked at genes rather than faces, the differences between the genes of an African and a European would be hardly greater than the difference between those of any two Europeans. For 30 years, gene data has continually reinforced the validity of this observation, and the concept of genetic races has been largely abandoned.

festival of nations

Rachel Heidenry | Beacon intern

The annual Festival of Nations is a way to celebrate cultural differences. DNA analysis is showing that people evolved in five regions in isolation from on another.

 This stance is now being challenged. Detailed comparisons of the genomes of people lead to a different conclusion. The problem is not with the observation noted above, which is clearly accurate -- all human races have about the same collection of variant alleles (versions of genes). The problem is with looking at genes one-at-a-time when making the comparison of human races.

It turns out that particular alleles defining human features often occur in clusters on chromosomes. Because they are close together, the genes experience little recombination over the centuries. The descendants of a person with a particular combination of alleles will also have that same combination. The set of alleles, technically called a "haplotype," reflects the common ancestry of these descendants from that ancestor.

The decoding of the human genome has for the first time allowed a large number of genes to be compared at the same time. Ignoring skin color and eye shape, and instead comparing the DNA sequences of hundreds of regions of the human genome, investigators at the University of Southern California showed in 2002 that when a large sample of people from around the world are compared, the computer sorts them into five large groups containing similar clusters of variation: Europe, East Asia, Africa, America and Australasia.

That these are more or less the major races of traditional anthropology is not the point. The point is that humanity evolved in these five regions in isolation from one another, that today each of these groups is composed of individuals with a shared ancestry, and that by analyzing the DNA of an individual we can deduce that ancestry.

Why bother making that rather arcane and socially controversial point?

Because analysis of the human genome is revealing that many diseases are influenced by alleles that have arisen since the five major branches of the human family tree separated from one another, and are much more common in the human ancestral group within which the DNA mutation causing the disease first occurred.

For example, a mutation causing hemochromatosis, a disorder of iron metabolism, is rare or absent among Indians or Chinese, but very common among northern Europeans (it occurs in 7.5 percent of Swedes), who also commonly possess an allele leading to adult lactose intolerance (inability to digest lactose) not common in many other groups.

Similarly, the Hb s mutation causing sickle cell anemia is common among Africans of Bantu ancestry, but seems to have arisen only there.

This is a pattern we see again and again as we compare genomes of people living in different parts of the world -- and it has a very important consequence. Because of common ancestry, genetic disease (disorders that are inherited) has a lot to do with geography. The risk that an African-American man will be afflicted with hypertensive heart disease or prostate cancer is nearly three times greater than that for a European American man, while the European American is far more likely to develop cystic fibrosis or multiple sclerosis.

These differences in medically important DNA variants carry over to genetic differences in how individuals respond to treatment. African Americans, for example, respond poorly to some of the main drugs used to treat heart conditions, such as beta-blockers and angiotensin enzyme inhibitors.

Scientists and doctors who recognize this unfortunate fact are not racists. They fully agree that using the races of traditional anthropology to pin-point which therapeutic treatment to recommend, while an improvement over treating all patients alike, still is a very poor way of getting at these differences. It would be far better to simply ignore skin color and other single-gene differences and instead perform for each patient a broad "gene variation" analysis. Hopelessly difficult only a few years ago, this now seems an attractive avenue to improve medical treatment for all of us.

It is important to keep clearly in mind the goal of sorting out individual human ancestry, which is to identify common lines of descent that share common response to potential therapies. It is NOT to assign people to over-arching racial categories.

This point is being made with great clarity in a course being taught at Pennsylvania State University, where about 90 students had their DNA sampled and compared with four of the five major human groups. Many of these students had thought of themselves as "100 percent white," but only a few were. One "white" student learned that 14 percent of his DNA came from Africa, and 6 percent from East Asia. Similarly, "black" students found that as much as half of their genetic material came from Europe, and a significant amount from Asia as well.

The point is, rigid ideas about the biological basis of identity are wrong. Humanity is much more complex than indicated by a few genes affecting skin color and eye shape. The more clearly we can understand that complexity, the better we can deal with the medical consequences and the great potential that human diversity provides all of us.

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