How Well Do Ancestry DNA Tests Actually Work?

Companies that offer saliva-swab DNA tests to assess your ancestry and health are rising in popularity. But do they really work?

Sabrina Stierwalt, PhD
5-minute read
Episode #262

image of double helix representing a dna test

It’s Thanksgiving week in the US which for many means gathering together with family. If you’re sitting around the holiday dinner table and wondering, "Can I really be related to these people?," you might be tempted to take one of the increasingly popular mail-in genetics tests. Companies like AncestryDNA and 23andme offer you a look at how your genetic ancestry breaks down in terms of percentages of your lineage coming from different regions around the globe. They also offer the possibility of connecting with potential relatives based on matches in your DNA to other users in their database.

23andme further offers a report on your genetic health risks, including whether you have genetic mutations for diseases like cystic fibrosis that may affect any future children and how likely you are to develop Parkinson’s or Alzheimer’s. It is this disease risk assessment that got the company into some trouble with the FDA back in 2013. At the time, the Food and Drug Administration required that 23andme discontinue the disease prediction portion of their service until they could offer more proof of the accuracy of their tests as well as evidence that their customers understood their results. However, as of April 2017, the FDA has restored authorization to 23andme to provide the information on genetic disease markers based on evidence from peer-reviewed scientific studies directly linking those diseases with the genetic mutations the company tests for.

What are the benefits of exploring your ancestry via your DNA?

One of the main benefits of genetic DNA testing is that it’s easy. All of our cells contain complete copies of our DNA, so, as we know from watching crime scene investigation shows, our DNA can be tested from almost anywhere including our hair or skin without the need for a blood test. In the case of most genetic testing companies, you mail in a vial of your saliva.

Once it arrives at the lab, your spit is typically subjected to what is known as admixture testing. The specifics of your DNA are compared to a library of other DNA samples from around the world to determine from where the best matches arise. You are then provided a breakdown of your lineage into percentages associated with each of 20-35 geographic regions.

This level of information—possible ancestral connections quantified by probabilities—can be very helpful when tied to related, non-DNA-based investigations into family history. Perhaps family lore suggests you once had distant relatives in Scandinavia, but you have not been able to find any photographic or otherwise more concrete evidence. A DNA-based link to Scandinavia could help bolster those family stories.

Alternatively, perhaps you have little to no information on your lineage and so any clues, even if not 100% certain (or even 50% certain), is valuable.

What are the limits of DNA tests of your ancestry and disease markers?

If you choose to submit your DNA in search of a peek into your possible lineage, here are a few caveats to keep in mind:

1. There is no such thing as a complete database of human DNA samples, or even a sufficiently globally-representative one. When a database of existing samples with known lineage is searched for matches to your DNA, the quality of that match depends on the size of the comparative samples. Each company has its own database so you may get different results from different companies. In April 2017, AncestryDNA reported that a database of more than 4 million people is used in its searches and 23andme claims possible comparisons to over 2 million people. These numbers are significantly larger than even a few years ago and quickly increasing.

2. If you’re not a white European over the age of 30, you will not get the most thorough match possible. The size of the database of possible DNA matches—known as reference populations—is not all the matters. Variety plays a role too. So far, the matches produced for non-white customers, and anyone with lineage traced outside of Europe, are typically far less accurate because they draw from a much smaller subset of DNA samples available for comparison. For those customers, doing some investigating into the make up of the databases used by different companies is key.


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About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.