Genetic Ancestry Testing
Genetic ancestry testing, also called genetic ancestry inference, looks at your DNA to estimate where your ancestors may have originated from. There are different types of tests, lineage-based (mitochondrial DNA & Y-chromosome) and autosomal (genomic DNA), which will tell you different things about your genetic background.
Lineage-Based Genetic Ancestry
Every person has two types of DNA – one is your nuclear DNA (or genome), which includes your 22 pairs of autosomes and your sex chromosomes (X and/or Y), and the other is your mitochondrial DNA (mtDNA).
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Your mitochondrial DNA is much smaller than your nuclear DNA (with only 16,569 base pairs in comparison to the approximately 3 billion in your genome) and is located in the cellular organelle of the mitochondria. MtDNA is inherited separately from your genome, and has a particular form of inheritance. What makes mtDNA unique is that it
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Nuclear DNA, found in the cell's nucleus, is pictured on the left; mitochondrial DNA, found in the mitochondria of the cells, is pictured on the right.
is passed down directly from mother to child, allowing inferences into your direct maternal ancestry. But, mtDNA testing only looks at one ancestral lineage out of your total ancestry.
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Patterns of inheritance for Mitochondrial DNA vs. Y-chromosomal DNA. Lineages highlighted in pink represent mitochondrial DNA inheritance along the direct maternal line; lineages highlighted in blue represent the Y-chromosomal inheritance along the direct paternal line. The pink circle represents the mitochondrial DNA, and the smaller blue chromosome represents the Y-chromosome.
Y-chromosome tests, on the other hand, only reveal direct male ancestry. The Y-chromosome is part of your nuclear DNA, but is unique because it is a sex chromosome. Only biological males carry a Y-chromosome, and thus it can only be inherited from a father to biologically male offspring. This allows us to look at direct paternal ancestry. But like mtDNA testing, Y-chromosome testing only looks at one ancestral lineage out of your total ancestry.
It is important to understand that mtDNA and Y-chromosomal ancestry tests do not provide a complete picture of your total genetic background or ancestry. They provide information only on your direct maternal or paternal histories, which do not represent all of your ancestral contributions. With this in mind, lineage-based testing can reveal important information about your maternal or paternal histories, and can be important for family genealogical and pedigree research.
Autosomal Genetic Ancestry
In comparison, autosomal ancestry tests look at your total genetic background by analyzing your nuclear DNA (or genome), which is produced by combining your parents chromosomes. At the time of conception, your parents chromosomes undergo a process called recombination (or crossing over), where segments of their chromosomes are swapped. This process produces new combinations of chromosomes every generation, and is how ancestry is traced in the genome.
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Autosomal ancestry tests look at these swapped or ancestral proportions of your chromosomes. They analyze specific markers, also called single nucleotide polymorphisms (SNPs),
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Image to the left depicts process of recombination (noted as crossing over), where paternal and maternal chromosomal segments are exchanged to produce new combinations of chromosomes every generation. The image to the right shows the difference between nuclear DNA and mitochondrial DNA inheritance.
in your genome associated with world populations. These markers are a product of random evolutionary change in your genome, and can be used to reconstruct population affinity. This is where commonly discussed 'ancestry percentages' are derived from. Genetic ancestry testing companies, such as Family Tree DNA, 23andMe, or AncestryDNA, have panels of markers associated with reference populations that they compare your genome to. The ancestry percentage, such as “17% South Asian” in the illustration below, is based on the percentage of markers that this person's DNA has matched with in comparison to their reference panels and populations.
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Example of an AncestryDNA autosomal testing result, with ancestry percentages displayed on the left.
What is important to know about autosomal genetic ancestry tests is that markers may differ between companies. While these tests may be informative in grasping a general understanding of your genetic ancestral background, the specific percentages generated by these tests are not absolute estimations of ancestry. Rather, these percentages are the product of the specific genomic test panels developed by genetic ancestry testing companies themselves. This is why you may get slightly different results if you submit your DNA to multiple genetic ancestry testing companies.
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In addition, autosomal ancestry results are highly dependent on comparative reference populations, which are groups that are analyzed for population-specific markers to determine genetic affiliations. Reference populations are often portrayed as being ‘ancestral’, when in reality they represent surveys of present-day population diversity, which can only provide an estimate of past genetic diversity for geographic regions.
Personal history, Identity, & Genetic Ancestry
Acknowledging that there are caveats of lineage-based and autosomal ancestry testing, there is much that genetic ancestry testing can do to inform your genetic background. It can tell you the general regions of the world that you share genetic affiliations with, and it can help you understand the specific histories of your fathers and mothers genetic background. This information can be important for how you view yourself or your identity.
Many scholars have written about the effects of genetic ancestry testing on personal identity formation, encompassing the experiences and information which inform your personhood, such as genealogical research. The information that genetic ancestry testing generates could potentially add to your identity by providing confirmatory or even new information, or it could provide information that conflicts with your previously established identity. Because of this, it is important to understand how testing results are produced to fully comprehend how this information may relate to you and your personal identity.
Genetic ancestry inferences may provide useful information in cases where family genealogical research is limited. This may be the case for some in the African American community, as the historical realities of slavery and segregation in the United States have often limited the amount of genealogical information that can be gleaned from the historical record. If you are interested in reading more about this topic, Columbia Professor Dr. Alondra Nelson’s book The Social Life of DNA, pictured above is a great resource.
Ultimately, how you define your identity and incorporate genetic ancestry testing results are up to you. But, by understanding how these tests are created and how the results are generated, you can establish a better understanding about how your genetic ancestry results can inform your own personal history and identity.
Further Reading
Bandelt, H., Yao, Y.-G., Richards, M. B., & Salas, A. (2008). The brave new era of human genetic testing. BioEssays, 30(4), 1246–51.
Bolnick, D., Fullwiley, D., Duster, T., Cooper, R., Fujimura, J. H., Kahn, J., … TallBear, K. (2007). Genetics. The science and business of genetic ancestry testing. Science, 318(5849), 399–400.
Duster, T. (2014). Ancestry Testing and DNA: Uses, Limits - and Caveat Emptor. In B. Prainsack, S. Schicktanz, & G. Werner-Felmayer (Eds.), Genetics as Social Practice (pp. 59–71). Surry: Ashgate.
Greenly, H. (2008). Genetic Genealogy. In B. Koenig, S. S.-J. Lee, & S. Richardson (Eds.), Revisiting Race in a Genomic Age (pp. 215–234). New Brunswick: Rutgers University Press.
Nash, C. (2012). Irish DNA: Making Connection and Making Distinctions in Y-Chromosome Surname Studies. In: K. Schramm, D. Skinner, & R. Rottenburg (Eds.), Identity Politics and the New Genetics, (pp. 141-166). New York: Berghahn Books.
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Nelson, A. (2016). The Social Life of DNA. Boston: Beacon Press.
Royal, C. D., Novembre, J., Fullerton, S. M., Goldstein, D. B., Long, J. C., Bamshad, M. J., & Clark, A. G. (2010). Inferring Genetic Ancestry: Opportunities, Challenges, and Implications. American Journal of Human Genetics, 86(5), 661–673.
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Shriver, M. D., & Kittles, R. A. (2004). Genetic ancestry and the search for personalized genetic histories. Nature Reviews. Genetics, 5(8), 611–618.