By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - A trawl through the genes of white people in Utah and Yoruba people in Nigeria shows a significant number of differences that can explain why some groups respond differently to drugs than others.

The findings also suggest that genes underlie some susceptibility to diseases in a general population, the researchers report in the American Journal of Human Genetics.

What the study does not show, the researchers stress, is that any of these differences are necessarily racial. But they are a first step toward a day when medical care may be tailored not only for individuals, but for entire groups.

"The idea of the study is just to get a sense of how populations may be different in their response to drugs and may be different in their disease susceptibility," Eileen Dolan of the University of Chicago, who led the study, said in a telephone interview.

Her team looked at batches of cells donated by people living in Ibadan, Nigeria, and in Utah for a gene-sequencing project. These cells have also been studied by other teams of researchers.

Dolan's team looked for genes that are active, or expressed, in immune cells known as lymphoblastoid cells grown from the blood from 180 healthy individuals.

"We found about 4.5 percent of all the genes that are present, expressed in these cell lines ... are significantly different," she said.

While the differences are not dramatic, some are clear enough to explain different reactions to drugs and differing susceptibility to disease.