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The Science and the Investigators
Intro Race and Science The Tests Learning from DNA
Learning from DNA: Reading genetic history with Henry Louis Gates, Jr.
Dr. Rick Kittles, discussing test results with series host Henry Louis Gates, Jr.
Henry Louis Gates and Dr. Rick Kittles
The Gates family had long believed that Jane Gates -- Henry Louis Gates' great-great-grandmother -- had a relationship with a former slave owner, Samuel Brady, and a good deal of circumstantial evidence and family legend, even among Brady's white descendants, seemed to support this belief. To test the theory, Dr. Gates visited Dr. Rick Kittles, a geneticist at Ohio State University. Genealogist Jane Ailes, herself a descendant of Samuel Brady's, contacted two distant cousins who are both direct descendents of Brady, and Dr. Kittles tested their Y-chromosomal DNA alongside Dr. Gates' sample. The results lay the family legend to rest -- Dr. Kittles finds that there is no match; Dr. Gates' Y-chromosome is not the one shared by the two confirmed male Brady descendants -- and neither Henry Louis Gates nor anyone in his direct line of male descent is a descendent of Samuel Brady.
Dr. Mark Shriver, at work in his lab.
Dr. Mark Shriver
Gates had always known that his family line had included one or more white ancestors -- there were light-complected people on both sides of his family, going back several generations. To learn more about his admixture, Dr. Gates turned to Dr. Mark Shriver, at Penn State University, and contributed a DNA sample. What he learned surprised him, even given his awareness of his white heritage. According to Gates' admixture test results, his genetic heritage was 50% European and 50% African -- half of his ancestors, according to Dr. Shriver, were white. That meant that perhaps four of his eight great-grandparents were white, or all eight had a white parent of European descent.

But who were these white ancestors? Working with Dr. Kittles, Dr. Peter Forster, and Dr. Fatimah Jackson, Dr. Gates did further lineage testing, and found even more surprising results. His mtDNA tests revealed the most matches in northern Europe and only a single match on the African continent, in northern Africa, near Egypt -- very unusual for an African American; who would expect to find European matches in his Y-chromosome lineage, among his male ancestors. But Dr. Thornton and Dr. Heywood, working with documentation of Dr. Gates' family, parts of which had lived as free people since the time of the American Revolution, developed a well-supported hypothesis: that Dr. Gates is probably descended, on his mother's side, from two indentured servants, living during the colonial period, a black man and a white woman.

Map of Africa, with DNA matches highlighted as red points; from Dr. Peter Forster's database project.
In Dr. Peter Forster's database exact matches appear as red dots -- the larger the dot, the more matches he has found. Dr. Gates' matrilineal heritage lies between Paris and Dublin, with only a single matching dot in North Africa, around Egypt
Expressing mock alarm at these results, Dr. Gates (who is chair of African and African American studies at Harvard University), set out to find more answers. He turned again to Dr. Shriver at Penn State, who's been working to achieve finer resolution in his admixture testing using a technology known as a microarray.

The microarray itself is a silicon chip spotted with thousands of single strands of DNA, the densely-packed surface lets researchers analyze a huge number of markers -- 11,155 SNPs in all, as opposed to the 176 markers analyzed during traditional admixture testing -- simultaneously. A sample of the DNA from the person being analyzed is allowed to interact with the DNA on the chip; during the process DNA spots on the chip that match the test subject's DNA sample "hybridize" with it -- they form complementary sequences to the DNA sample, sequences that can be read with a scanner.

Scatter chart displaying the results of Dr. Gates' microarray tests
The results of Dr. Gates' final round of DNA microarray analysis
Using microarray analysis technology, Dr. Shriver has developed a test that can analyze admixture proportions with a high degree of detail, by focusing in on a pre-selected set of genetic markers with a custom-designed microarray chip developed by Affymetrix. Basically, the test lets researchers analyze a much larger set of markers from a smaller group of populations, providing clearer results in cases like Dr. Gates, where previous testing has not been able to reveal matches among present-day African population groups.

Dr. Shriver first identified groups of genetic markers commonly found in the European genetic components of African Americans' admixture, and excluded these markers from the chip, and therefore from the analysis. This made it possible to zero in exclusively on Dr. Gates' West African genetic origins.

The test results were then compared against a database of individuals from several West African ethnic groups. The results of the analysis show where the subject's markers are clustered relative to know African ethnic group. In this way, he found a close match for Dr. Gates among the Mende people -- introducing him to his distant relatives and revealing, at last, his roots in Africa.





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