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The Science and the Investigators
Intro Race and Science The Tests Learning from DNA

Image of an admixture test result, presented as a bar graph


Sample admixture test results for an individual, displayed as a bar graph of relative amounts of geographically identifiable ancestry.
To get an overall picture of your genetic heritage, admixture tests focus on autosomal DNA -- those 22 pairs of non-sex chromosomes that exist in every cell. In each pair, one has been inherited from your mother and one from your father and since this process has taken place over and over again for generations, these autosomes contain recombined segments of DNA from all your ancestors.

DNA sequences in autosomes are 99.9 percent identical in every human. The slight variations in the remaining one-tenth of one percent are what make each of us unique and different, as well as what create the differences that (through a lens of belief and received social ideas) we perceive as race and ethnicity

The variations examined in admixture testing are "highly informative" single nucleotide polymorphisms, grouped together into haplotypes and haplogroups whose frequencies have been found to correlate strongly with membership in one of four general anthropological groups: Sub-Saharan African; European; East Asian; and Native American.

Admixture test results displaying the genetic heritage of a group of Americans with some African heritage, mapped against a group of present-day Africans.

Image of an admixture test scatter chart.
The test itself takes your DNA sample and compares it (for the purposes of AFRICAN AMERICAN LIVES, DNA samples were compared at 176 SNPs) against a reference database of DNA samples from people belonging to those four ancestral groups. Based on how the genetic markers in your DNA match up to genetic markers in the database, the test estimates your admixture, defined as what proportion of your gene pool comes from one population, what proportion from another population, what proportion came from a third or a fourth. The results are plotted in either bar graphs or scatter charts (as seen in AFRICAN AMERICAN LIVES), which display, respectively, either a breakdown of the components of your personal genetic makeup, or your genetic heritage in the context of other people who have contributed DNA to one of the databases used in the test.

It is important to keep in mind that the markers this test looks for do not necessarily define the physical characteristics of these population groups, nor are they markers found exclusively in these groups. Rather, these markers are simply common among people with similar recent -- recent in the evolutionary sense, over the past 10,000 or so years -- ancestry. Some markers are shared among several groups (there is considerable overlap between Native American and East Asian populations, for example). This is why the test looks for large numbers of markers, haplotypes, and haplogroups -- the larger the sample, the more confident the tester can be in making a deduction about your ancestry.

Imagine a town which has 4 different clothing stores. Store 1 specializes in yellow clothes, but also sells red, green and blue clothes. Store 2 specializes in red, but also sells yellow, green and blue clothes. Store 3 specializes in green clothes but also sells red, yellow, and blue clothes. Store 4 specializes in blue clothes but also sells clothes of the other three colored. Let's assume every person in town shops at all these stores. Now if we go out on the street and look at you, an individual who lives in this town, based on what you are wearing, we can form a probability statement on how much time you have spent in each of the 4 stores. The clothes you are wearing are analagous to your unique gene sequences; and the clothing stores are the population groups to which you may trace your ancestry.
-- Tony Frudakis, DNAPrint Genomics


So if we are all one race, we've all shopped at the same genetic stores, to take Dr. Frudakis' example. Though the anthropological groups are a useful shorthand for understanding the results of these tests, (and a helpful reminder that we are more alike than different), the tests can both reveal important parts of our genetic inheritance and show us, very clearly, that racial identity is not as concrete and compartmentalized as some might think.

And things are not always as they seem. Interestingly, as seen in the series AFRICAN AMERICAN LIVES, Dr. Mark Shriver's own admixture test showed that he had some 11% West African, 3 % Native American and 86% European ancestry. Though Shriver has always identified as "white," he means that 11 out of one hundred -- approximately one out of ten -- of his recent ancestors were black. Or one great-grandparent came from the West African population group.


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