As the story gets pieced together, John Jackson was a black man, a freed slave perhaps, who came to New Haven, Connecticut, to join the American Revolution. He fought under Benedict Arnold and, after the war ended, sailed to Madagascar as a merchant seaman on a whaling ship. There he met a woman who became his wife, and though she would die within a year of their move to the United States, she bore John a son.

John's boy would have 11 sons of his own, all carrying the mark of their parentage -- a distinctive signature on their Y chromosomes revealing their grandfather's African ancestry.

Today, Bruce Jackson, a descendant of John Jackson, is a molecular biologist at Boston University School of Medicine and co-director of the African-American Roots Project. Through Jackson's efforts, and those of Burt Ely, a molecular geneticist at the University of South Carolina, the project is helping other African Americans understand their ancestry.

'Every African American has major interest in his or her lineage,' says Jackson. 'Among Americans, it's the most sought after information -- where we emanate from -- and especially for African Americans because of the institution of slavery.'

It turns out that Bruce Jackson's grandfather was the son of a black man and a white indentured servant in a Virginia fishing community. Bruce Jackson's mother had the marks of her Irish origin in her mitochondria. And Bruce Jackson himself shows Irish lineage on his mitochondrial DNA and ancestral African DNA on his Y chromosome.

All this genetic information connecting the present to the past can be gleaned from cells inside the cheek. Every cheek cell, like all cells in the body, contains genetic information about a person's heritage. Mitochondrial DNA, found in the cells of both males and females, is passed down through the mother's lineage. Y chromosomes, which are found only in the cells of males, can be used to trace the roots of the father's side of the family. By comparing cheek-cell DNA to a worldwide database of ethnic groups, Jackson's team can reveal the donors' links to Africa.