A human being’s DNA contains the genetic instructions that created him and continue to maintain his body. But these valuable instructions are surrounded by other sequences that don’t appear to help the body do or make anything. It is this “junk DNA” that contains the clues to a person’s ancestry.
Autosomal DNA Testing: Genetic Portraits
The DNA on a human’s 22 pairs of non-sex chromosomes is tested through autosomal DNA testing, similar to a paternity test. Each pair contains one chromosome inherited from that person’s father, and one inherited from that person’s mother. “Since this process has taken place over and over again for generations,” writes the African American Lives website, “these autosomes contain recombined segments of DNA from all your ancestors.”
DNA testing company DNA Tribes uses this information to compare someone’s genetic signatures with similar signatures found across the world. “Your top matches are the places in our database where your DNA profile is most common,” they write in their FAQ. They also offer special Native and African panels for people whose Native or African ancestry doesn’t show up in their top matches.
Admixture: Another Autosomal DNA Test
An admixture test suggests what percentage of a human’s ancestry came from different regions. Ancestry is broken down into four anthropological groups: Indo-European, East Asian, Native American and Sub-Saharan African.
Admixture is different from the blood quantum method that people sometimes use to describe mixed ancestry. Someone with 25% European ancestry has inherited 25% of their ancestry from the Indo-European group. This does not necessarily mean that one of their grandparents was European. Also, “junk DNA” does not determine physical characteristics, so a person may carry the ancestry of a certain group but display none of that group’s typical physical features.
Mitochondrial DNA: Testing Maternal Ancestry
Mitochondrial DNA is passed from a woman to all of her children. A person who tests their mtDNA can trace back his or her mother’s mother’s mother and so on. While scientists are still unsure of how far back in time mtDNA testing can trace, the general consensus is that it measures maternal history over thousands of years. Native American groups have six mtDNA genetic signatures unique to them, while Europeans have seven, popularized in Bryan Sykes’ book The Seven Daughters of Eve.
Y Chromosome Testing: Testing Paternal Ancestry
There is a section on a man’s Y chromosome that remains relatively stable over centuries as it is passed from father to son. Tracing back this non-recombinant Y chromosome reveals direct paternal ancestry, that is, a father’s father’s father and so on. Some groups of people with a shared surname conduct Y chromosome projects to determine their links to each other. DNA testing companies caution that Y chromosome results aren't accepted as paternity test results.
Limits of Genealogical DNA Tests
With three billion genes inside one human and billions of humans who have been migrating for thousands of years, DNA testing is not an exact science. To create genetic portraits and admixture analyses, DNA testing companies build databases of different DNA sequences and make statistical comparisons to determine probable ancestral origins.
Y chromosome and mtDNA testing only reveal two of a person’s many lineages, but the information they reveal may upset someone who doesn’t keep in mind that they carry many, many lineages in his or her DNA. While revolutionary, genealogical DNA testing is still a young science that cannot reveal a person’s entire heritage yet.
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