Is there a genetic marker for ethnicity?

More and more commercial DNA companies are popping up, But what do they actually tell you?
More and more commercial DNA companies are popping up with the likes of 23andMe, Ancestry.com and Family Tree DNA. But what do they actually tell you? (Getty Images)
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This week's question comes from Alphonse Kolodziejczak in Buffalo, New York: "More and more frequently, my friends and acquaintances are telling me about surprising information concerning their ethnicity which they discovered through a commercial company's DNA analysis. Is there such a thing as a genetic marker for ethnicity? How accurate are these commercial DNA reports?" 

Here's Dr. Caitlin Mullarkey, Assistant Professor in the Bachelor of Health Science Program at McMaster University's answer:

Humans are 99.9% identical in our DNA. The difference between each human genome is very small-a mere 0.1% on average. Yet, in analyzing these small differences we can begin to understand what makes us unique. The variation (or difference) between human genomes is not randomly distributed across the globe. This makes sense at a basic level, because we are more likely to have offspring with people that live nearby.  As a result, the closer geographically two individuals (or populations) are, the more genetically similar to they tend to be. If we were to gather DNA from across the globe, we can start to connect certain genetic signatures to geographic spaces.

Ancestry tests provide estimates of the proportion of each person's genome that belong to certain genetic signatures. But how reliably can you use your DNA as a marker of identity? Here, we need to proceed with caution in the application and interpretations of these tests at the individual level.

Geography is an imperfect proxy for race and ethnicity. It may be tempting to use these words in this scenario, but there is no such thing as an ethnicity or race gene. Furthermore, the information in these assembled databases is acquired at a population level and not based on individual ancestry. Finally, we need to keep in mind the markers or signatures companies are looking for when generating these ancestry reports. Ideally, you would want your individual DNA results to be compared to the comprehensive "world DNA database", but, no such database exists. Companies have their own proprietary DNA databases. The customer often does not know the quality, comprehensiveness, or size of these reference DNA databases. This brings into question the confidence you can have in each result. Individual ancestry testing for most of these companies comes with a low statistical confidence interval. In fact, you can obtain different results from different companies based on which databases they employ. In short, don't put too much stock in your ancestry test results.