The scientists, led by Li Jin of the Chinese Academy of Sciences, Shanghai, report in the journal Genome Research that some genes variant pathogens becomes more common in African-Americans after their ancestors reached American shores - perhaps because they have given a larger, offsetting the benefits. Other variants of the gene became less common, the researchers said, as the sickle cell hemoglobin gene, which, in its most common form single dose, protects against malaria. The Shanghai team suggests that the gene became less common in African Americans, because malaria is much less of a threat.
The purpose of the study of the genomes African-American is largely medical. Most searches variant genes for diseases occur in people of European descent, and doctors want to if ensure that they have missed step variants that may be more common in African Americans and useful for the development of treatment or diagnosis.
These searches often reveal events in history a population of the through the genes that have changed under the pressure of natural selection.
Exceptionally common variants identified by team of Shanghai are associated with a higher risk of hypertension, prostate cancer, cancer of the bladder and multiple sclerosis.
"Most of the genes associated with ethnic diseases African-American," they write, "" may have played an important role in the adaptation of African-Americans to the local environment. "" But the authors have not yet been able to identify the benefits that they believe such conferred genes.
Mark d. Shriver, a geneticist at Penn State, said it is plausible that some versions of a gene become more frequent than African-Americans to a new environment. "It is very valid expect that it will be subject to genetic adaptation factors and who are now most common in contemporary African-Americans in the ancestral group", he said.
But Alkes l. Price, a geneticist at the Harvard School of Public Health, said that the results of the Shanghai team, even if it is plausible to prove. "This article provides no evidence of selection held in post-Africa," he said.
Shanghai researchers used a method for the study of mixture, geneticist term for when two populations or races are married; China has several of these populations, perhaps accounting for the interests of the team. Using gene chips that analyze common variations in the human genome, researchers can deconstruct the chromosomes of an African-American, say, assigning each segment of DNA to a European or African origin.
Scientists have found that African-American genomes in their sample, 22% of the DNA came from European, on the average and the rest of the African ancestors, a figure with other estimates.
They have sought sites along the genome where the European or African ancestry was statistically significant at levels higher than the average, find four areas with a very common European ancestry, and two of African descent very common. Most of these sites sheltered genes of unknown function, but one, of European origin, is the holder of a gene that helps fight influenza, suggesting that it has become more common among African-Americans in conferring protection against disease.
Mr. Price, however, said that two other research teams had applied the same method of African-American genomes without finding any statistically significant excess of European or African ancestry. The Chinese team, in his view, should have applied a correction factor for their statistics and, if they did, would have obtained the same result.
In another approach, the Shanghai team focused on all segments of DNA of African origin in the African-American genomes, throw all European DNA. They then compared the African component of African-American genomes with DNA of the Yoruba of Nigeria, a well-studied population which is be genetically very close to the people of West Africa which many slaves have been taken.
The Shanghai team asked how African genome had changed after that African arrived at the United States. They found that the versions of some genes were become more common and other less. Less common genes included many involved in the protection against malaria.
Mr. Price, however, said the decrease in the frequency of the gene might have another explanation - the fact that resistance to malaria varies in force in different regions of West Africa. The Shanghai team can be watching the difference in resistance of malaria between the Yoruba and other African populations, not the difference between African-Americans today and their African ancestors, he said.
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