Mitochondria (and thus mitochondrial DNA) are inherited only from the mother. Mitochondrial DNA is passed from mother to the children with few, if any, changes occurring over many generations. So mitochondrial DNA could trace maternal lineage.

More than 11 million Africans were forcibly shipped to the Americas during the slave trade that peaked in the 17th and 18th centuries. In the last years, many companies hired by African Americans use mitochondrial DNA to trace maternal ancestry in Africa. However, mitochondrial DNA proved to be almost useless in determining the ancestry of African Americans. Less than 10% of the analyzed African-American mitochondrial DNA led to a single African ethnic group. "At first glance, it seems that this DNA test would be a good place to start," said Dr. Bert Ely, a biology professor in the university's College of Arts and Sciences at University of South Carolina.

"It is just that some companies will over-interpret the data and give you the most likely result instead of all of the matches."

"The task is particularly difficult in Africa because there is more genetic diversity among Africans than among people from any other continent and because humanity has been in Africa longer than anywhere else," Ely said.

"That would be true if everybody stayed put, but they have a lot of history of moving around. DNA is spread geographically as people migrate. Thus, identical mitochondrial DNA can be found in people throughout sub-Saharan Africa," he said.

Ely's team relied on a database of more than 3,700 mitochondrial DNA sequences from people in sub-Saharan Africa. They sampled African-Americans, including people who identify themselves as "Gullah/Geechee" and live along the islands off the coasts of South Carolina and Georgia. These groups have close cultural ties to Sierra Leone, including language, stories and crafts. Contrary to what could be thought, the Gullahs/Geechees did not present a more pure genetic link than other African American groups.

More than half of the analyzed African-American DNA sequences were found in many different sub-Saharan ethnic groups. But 40% of the African-American DNA did not fit with any sequences in the database, and less than 10% were an exact match to a single African ethnic group. "It was unlikely because slaves were brought into Charleston from all over the western part of Africa," Ely added.

"Historical accounts indicate that virtually all enslaved Africans brought to North America came from either West or West Central Africa".

"The truth was we know perfectly well who the ancestors of modern African-Americans were, and they were people from all up and down the west coast of Africa," Ely said.

"What no one has really looked at is how often can someone trace their roots back to a single ethnic group back in Africa and there the answer came out to be not very often - maybe 10 percent or less."

"We need to establish larger databases of DNA as we continue this project," Ely said.

"However, we do know that, for most African Americans, it is impossible to use only mitochondrial DNA to determine a single ethnic group as the source of the maternal ancestor."

The researchers are now using the genetic database to find useful medical information, like genes linked to a more aggressive form of breast cancer in U.S. blacks.