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The Good and the Bad

A new study published in Genome Research by a team at the Chinese Academy of Sciences in Shanghai shows that disease-causing variants may be more common in African Americans because they conferred some benefit on their ancestors when they were first brought to the New World, reports The New York Times' Nicholas Wade. For example, the gene for sickle cell hemoglobin, though it has negative effects, protects against malaria. "The Shanghai team suggests the gene has become less common in African Americans because malaria is much less of a threat," Wade adds.

In addition, the researchers studied admixture in African-American genomes, and found that about 22 percent of the DNA in their samples came from Europeans, while the rest came from African ancestors. In one arm of the study, the team looked at all the African DNA segments from African Americans and compared them to DNA from Nigerian Yoruba participants to chart how the genomes had changed after Africans came to the US, Wade says. "They found that versions of some genes had become more common and others less so. The less common genes included several known to be involved in protection against malaria," he adds.

Critics, however, say that the study comes short of proving that selection occurred after Africans were brought to the US. Harvard School of Public Health geneticist Alkes Price tells Wade that there might be other explanations for what the Shanghai team found: The decrease in gene frequency could be explained by the fact that resistance to malaria varies in different regions of West Africa, and while the Shanghai team looked at the difference between today's African-Americans and today's Yorubans, it may not be seeing the actual differences between today's African Americans and their African ancestors, Price says.

The Chinese team tells Wade that their study should be repeated with many thousands of genomes to ascertain the validity of its conclusions.

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