The scientists found that of the African-American genomes in their sample, 22 percent of the DNA came from Europeans, on average, and the rest from African ancestors, a figure in line with other estimates.

They then looked for sites along the genome where either European or African ancestry was present at statistically significant levels above the average, finding four regions with very common European ancestry and two with very common African ancestry. Most of these sites harbored genes of unknown function, but one, of European origin, holds a gene that combats influenza, suggesting it has become more common in African-Americans by conferring protection from the disease.

Dr. Price, however, said that two other research teams had applied the same method to 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 to their statistics and, had they done so, would have obtained the same result.

In another approach, the Shanghai team focused on all the DNA segments of the African origin in the African-American genomes, discarding all the European DNA. They then compared the African component of African-American genomes with the DNA of the Yoruba of Nigeria, a well-studied population that happens to be genetically very close to the West African population from which many slaves were taken.

The Shanghai team then asked how the African genome had changed after Africans arrived in the United States. 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.

Dr. Price, however, said the decrease in gene frequency might have another explanation — the fact that resistance to malaria varies in strength in different regions of West Africa. The Shanghai team may be looking at the difference in malaria resistance between the Yoruba and other African populations, not the difference between today’s African-Americans and their African ancestors, he said.