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Academic If races are defined as genetically discrete groups, no. But researchers can use some genetic information to group individuals into clusters with medical relevance By Michael J Bamshad and Steve E Olson Look around on the streets of any major city, and you will see a sampling of the outward variety of humanity: skin tones ranging from milk-white to dark brown; hair textures running the gamut from fine and stick-straight to thick and wiry. People often use physical characteristics such as these--along with area of geographic origin and shared culture--to group themselves and others into "races." But how valid is the concept of race from a biological standpoint? Do physical features reliably say anything informative about a person's genetic makeup beyond indicating that the individual has genes for blue eyes or curly hair? The problem is hard in part because the implicit definition of what makes a person a member of a particular race differs from region to region across the globe. Yet common definitions of race do sometimes work well to divide groups according to genetically determined propensities for certain diseases. Sickle cell disease is usually found among people of largely African or Mediterranean descent, for instance, whereas cystic fibrosis is far more common among those of European ancestry. In addition, although the results have been controversial, a handful of studies have suggested that African-Americans are more likely to respond poorly to some drugs for cardiac disease than are members of other groups. Over the past few years, scientists have collected data about the genetic constitution of populations around the world in an effort to probe the link between ancestry and patterns of disease. These data are now providing answers to several highly emotional and contentious questions: Can genetic information be used to distinguish human groups having a common heritage and to assign individuals to particular ones? Do such groups correspond well to predefined descriptions now widely used to specify race? And, more practically, does dividing people by familiar racial definitions or by genetic similarities say anything useful about how members of those groups experience disease or respond to drug treatment? In general, we would answer the first question yes, the second no, and offer a qualified yes to the third. Our answers rest on several generalizations about race and genetics. Some groups do differ genetically from others, but how groups are divided depends on which genes are examined; simplistically put, you might fit into one group based on your skin-color genes but another based on a different characteristic. Many studies have demonstrated that roughly 90 percent of human genetic variation occurs within a population living on a given continent, whereas about 10 percent of the variation distinguishes continental populations. In other words, individuals from different populations are, on average, just slightly more different from one another than are individuals from the same population. Human populations are very similar, but they often can be distinguished. Classifying Humans As a first step to identifying links between social definitions of race and genetic heritage, scientists need a way to divide groups reliably according to their ancestry. Over the past 100,000 years or so, anatomically modern humans have migrated from Africa to other parts of the world, and members of our species have increased dramatically in number. To determine the degree of relatedness among groups, geneticists rely on tiny variations, or polymorphisms, in the DNA--specifically in the sequence of base pairs, the building blocks of DNA. Most of these polymorphisms do not occur within genes, the stretches of DNA that encode the information for making proteins (the molecules that constitute much of our bodies and carry out the chemical reactions of life). Accordingly, these common variations are neutral, in that they do not directly affect a particular trait. these can contribute to individual variation in traits and to genetic diseases. As scientists have sequenced the human genome (the full set of nuclear DNA), they have also identified millions of polymorphisms. The distribution of these polymorphisms across populations reflects the history of those populations and the effects of natural selection. To distinguish among groups, the ideal genetic polymorphism would be one that is present in all the members of one group and absent in the members of all other groups. But the major human groups have separated from one another too recently and have mixed too much for such differences to exist. Polymorphisms that occur at different frequencies around the world can, however, be used to sort people roughly into groups. One useful class of polymorphisms consists of the Alus, short pieces of DNA that are similar in sequence to one another. Alus replicate occasionally, and the resulting copy splices itself at random into a new position on the original chromosome or on another chromosome, usually in a location that has no effect on the functioning of nearby genes. Once an Alu sequence inserts itself, it can remain in place for eons, getting passed from one person to his or her descendants. Therefore, if two people have the same Alu sequence at the same spot in their genome, they must be descended from a common ancestor who gave them that specific segment of DNA. One of us (Bamshad), working with University of Utah scientists Lynn B Jorde, Stephen Wooding and W Scott Watkins and with Mark A Batzer of Louisiana State University, examined 100 different Alu polymorphisms in 565 people born in sub-Saharan Africa, Asia and Europe. First we determined the presence or absence of the 100 Alus in each of the 565 people. Next we removed all the identifying labels (such as place of origin and ethnic group) from the data and sorted the people into groups using only their genetic information. When we added the labels back to see whether each individual's group assignment correlated to common, predefined labels for race or ethnicity, we saw that two of the groups consisted only of individuals from sub-Saharan Africa, with one of those two made up almost entirely of Mbuti Pygmies. The other two groups consisted only of individuals from Europe and East Asia, respectively. We found that we needed 60 Alu polymorphisms to assign individuals to their continent of origin with 90 percent accuracy. To achieve nearly 100 percent accuracy, however, we needed to use about 100 Alus. Noah A Rosenberg and Jonathan K Pritchard, geneticists formerly in the laboratory of Marcus W Feldman of Stanford University, assayed approximately 375 polymorphisms called short tandem repeats in more than 1,000 people from 52 ethnic groups in Africa, Asia, Europe and the Americas. By looking at the varying frequencies of these polymorphisms, they were able to distinguish five different groups of people whose ancestors were typically isolated by oceans, deserts or mountains: sub-Saharan Africans; They were also able to identify subgroups within each region that usually corresponded with each member's self-reported ethnicity. The results of these studies indicate that genetic analyses can distinguish groups of people according to their geographic origin. The groups easiest to resolve were those that were widely separated from one another geographically. Such samples maximize the genetic variation among groups. When Bamshad and his co-workers used their 100 Alu polymorphisms to try to classify a sample of individuals from southern India into a separate group, the Indians instead had more in common with either Europeans or Asians. In other words, because India has been subject to many genetic influences from Europe and Asia, people on the subcontinent did not group into a unique cluster. We concluded that many hundreds--or perhaps thousands--of polymorphisms might have to be examined to distinguish between groups whose ancestors have historically interbred with multiple populations. The Human Race Given that people can be sorted broadly into groups using genetic data, do common notions of race corresp...
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