Researchers may now reliably use genetics data to tell a person's ancestry -- what percent of an African-American's genome stems from Europe, for example, and what percent comes from Africa.

The technique detailed in a new study not only allows researchers to compare 300,000 common DNA landmarks in the diverse genetic heritage of humans, but also provides insights into "the personal genetic history of individuals and the journey individual segments of their genomes have taken, particularly those individuals with rich and diverse genetic heritage, such as African-Americans," said Carlos Bustamante, a Cornell population geneticist who co-led the study, which is published in the Dec. 21 issue of Proceedings of the National Academy of Sciences.

The study focused primarily on the genetic structure of West African populations, as previous genetic and historical studies suggested that the region was the source for most of the ancestry of present-day African-Americans. The results suggested clear and discernible genetic differences among some of the West African populations, whereas others appear to be nearly indistinguishable even when comparing more than 300,000 genetic markers. The researchers note that a larger sample size would likely reveal further diversity among these populations.

Of the 365 African-Americans in the study, the researchers found that the degree of West African ancestry in the individuals ranged from 1 to 99 percent. This wide variation has significant implications for using genetic information to personalize drug treatments and assessing disease risk. The median proportion of European ancestry in the African-Americans was 18.5 percent, with large variation among individuals.

The research, whose first author is Katarzyna Bryc, a graduate student in Bustamante's lab, could have implications for personalized ancestry reconstructions, personalized medicine, more effective drug treatments and for developing more effective methods to map genetic risk factors for such common diseases as hypertension, diabetes and prostate cancer.

The research team, led by Bustamante and Sarah Tishkoff, a geneticist at the University of Pennsylvania, collected and analyzed genotype data from the 365 African-Americans as well as 203 individuals from 12 West African populations and 400 Europeans from 42 countries.

The data revealed complex genomic diversity among African and African-American populations and deep historical, cultural, geographic and linguistic impacts on gene flow among populations, Bustamante said.

For example, among its many findings, the researchers found that the X-chromosomes of African-Americans had a higher proportion of African ancestry than the autosomes, consistent with the pattern of gene flow where mothers were more frequently of African ancestry than fathers, who were either of African or European ancestry.

Analyzing patterns of population structure and individual ancestry in Africans and African-Americans is critical for undertaking medical genomic studies on a global scale, the researchers noted. There is also strong reason to believe that high-density genotype data from African and African-American populations may pinpoint more precisely the geographic origin of African ancestry in African Americans, the researchers said.

The study was funded by the National Institutes of Health, the National Science Foundation and the David and Lucile Packard and Burroughs Wellcome Foundation.