How Does DNA Ancestry Work?

posted 5/15/22 by Anna Gdalevich

These days, it’s pretty popular for people to buy DNA kits in order to learn more about their ancestry and family history, but how do these services actually use people’s DNA to determine their origins? The answer is through looking at their genome.

Users take a sample, usually saliva, and send the DNA kits back to where they bought them from. The companies then analyze the DNA in the sample. They look at multiple parts. Paternal lineage can be determined by looking at the Y chromosome, and maternal lineage can be determined by mitochondrial DNA. The most important part, however, are the 22 non-sex chromosomes, as these contain the largest amount of and most detailed information.

Generally, DNA ancestry services each have very large libraries of genetic information for different groups of people all around the world that the user’s information is compared to. An algorithm breaks up the user’s genome and compares it to reference datasets such as “British” and “Chinese”. The algorithm then gives a probability for where the piece of DNA comes from. For example, if the analysis says that someone is 40% French, 40% of their DNA has traits similar to those found in the “French” dataset. Different DNA ancestry services can have differences in their analysis, such as saying someone is 38% French instead of 40%, but this does not mean one of the tests was inaccurate, different companies just have slight differences in their reference datasets.

To get into the nitty-gritty of the process, the DNA is analyzed to identify genetic markers, and these are used to create haplogroups, who are a group of people that share a common ancestor. These are then used to determine origins using the reference datasets. They are also used to match people with possible relatives. Their DNA is used to see how related they are, as closer relatives share more identical DNA segments. This is how these services identify possible long-lost relatives.

The data for the different ancestry references comes from a mix of self-reporting and independent research. However, there are discrepancies in how thoroughly different groups are sampled. Most people can describe their immediate background fairly well, but doing further research on some groups can be more difficult than others. For example, people of European descent often get more detailed ancestry information than people of Native American descent because of the larger sample size and disparity in historical records. This is why “French” can be separated from “German”, but multiple Native American tribes are simply grouped into “Native American”. This makes it difficult for some to get broad data about their ancestry. Unfortunately, it can be difficult to make up the difference due to the historic underreporting of certain groups.

Sources

Letzer, Rafi. "How Do DNA Ancestry Tests Really Work?" LiveScience, 4 June 2018,

www.livescience.com/62690-how-dna-ancestry-23andme-tests-work.html

"How Do Ancestry DNA Kits Work?" CRI Genetics, www.crigenetics.com/blog/howdoesancestrydnatestingwork#:~:text=An%20ancestry%20DNA%20test%20is%20based%20on%20the,broader%20examination%20of%20the%20maternal%20and%20paternal%20lineages