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.
3. While it can be obvious when DNA samples are not a match, definitively linking your DNA to that of your ancestors through similarities is not always so black and white. Assuming with each descendent, half of someone’s DNA is passed on to the next generation, you’re unique contribution is already down to below 1% after only ~7 generations. If I want to know if I’m related to Galileo, it will thus be challenging to link specific aspects of our respective DNA samples that aren’t also shared with just about everyone else. We do have mitochondrial DNA which is passed on only by our mothers and DNA linked to Y-chromosones which is passed on only by our fathers. These types of DNA can serve as more direct links to distant ancestors, but those ancestors only make up a small percentage of our lineage.
4. Interpreting your DNA ancestry results requires understanding confidence intervals. Most companies lay out your results in terms of how confidently you can trust them in terms of probability. Some even allow you to change those confidence interval settings to see the range in your results. For example, my results may tell me that I am 2% Japanese at a 75% confidence interval but that probability of Japanese lineage may increase if I’m willing to look at less confident results. These confidence intervals also mean that any results presented as accurate to the decimal point are probably less certain than they appear to be.
5. Understanding the limitations of percentages and probabilities is especially important in the interpretation of any findings-related disease mutations linked to genetic diseases. For example, having a particular gene variant known as N370S means you are three times more likely to develop Parkinson’s, which can sound intimidating. But the normal risk is only around 0.3% so a tripling of that risk brings you to still less than 1%. Not to mention much of the risk in developing Alzheimer’s or Parkinson’s is thought to be nongenetic or from genes that aren’t tested by companies like 23andme. Also, consider how it will affect you to know that you have a 5% chance of colorectal cancer, for example. 5% is still fairly low but worrying about such a result could be a source of stress.
Results require generalizations about genetics of large populations of people throughout time and across the globe.
6. Testing your DNA for disease markers should never replace seeking or continuing any form of treatment. The most thorough analysis of your genetics is best done through a face-to-face appointment with a genetic counselor who can personalize (and thus improve the accuracy of) your results by, for example, incorporating your family history of disease.
Do ancestry DNA tests work?
The answer is both yes and no. Direct-to-consumer DNA tests typically work as promised in the fine print—they offer a look at which regions of the world are home to people with DNA signatures most similar to yours. For many, that is enough.
However, those results require generalizations about genetics of large populations of people throughout time and across the globe. Will they be able to tell you that you are for sure related to Cleopatra? No. For those answers you’ll have more success digging through your family history. Our ethnicity is also not entirely dictated by our genetics. We each have a collection of lived experiences and cultural ties that also help play an important role in who we are.
Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.
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