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Thursday, October 24, 2013

Does Race Exist?

Does Race Exist? (Scientific American)

"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. Each insertion is a unique event. 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.

Our analysis yielded four different groups. 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.

Other studies have produced comparable results. 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; Europeans and Asians west of the Himalayas; East Asians; inhabitants of New Guinea and Melanesia; and Native Americans. 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. But caution is warranted. 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...

Because traits such as skin color have been strongly affected by natural selection, they do not necessarily reflect the population processes that have shaped the distribution of neutral polymorphisms such as Alus or short tandem repeats. Therefore, traits or polymorphisms affected by natural selection may be poor predictors of group membership and may imply genetic relatedness where, in fact, little exists...

According to Shriver's analyses, approximately 30 percent of Americans who consider themselves "white" have less than 90 percent European ancestry. Thus, self-reported ancestry is not necessarily a good predictor of the genetic composition of a large number of Americans. Accordingly, common notions of race do not always reflect a person's genetic background...

Several polymorphisms in CCR5 do not prevent infection but instead influence the rate at which HIV-1 infection leads to AIDS and death. Some of these polymorphisms have similar effects in different populations; others only alter the speed of disease progression in selected groups. One polymorphism, for example, is associated with delayed disease progression in European-Americans but accelerated disease in African-Americans. Researchers can only study such population-specific effects--and use that knowledge to direct therapy--if they can sort people into groups.

In these examples--and others like them--a polymorphism has a relatively large effect in a given disease. If genetic screening were inexpensive and efficient, all individuals could be screened for all such disease-related gene variants. But genetic testing remains costly. Perhaps more significantly, genetic screening raises concerns about privacy and consent: some people might not want to know about genetic factors that could increase their risk of developing a particular disease. Until these issues are resolved further, self-reported ancestry will continue to be a potentially useful diagnostic tool for physicians.

Ancestry may also be relevant for some diseases that are widespread in particular populations"
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