What DNA ancestry tests can — and can’t — tell you


So I recently took one of those at-home DNA
ancestry tests. All I had to do was fill up a vial with a
disgusting amount of spit and mail it off for analysis. We’re gonna be here for a very long time. I just spit it back up in my nose. A couple weeks later, this is what I got:
It’s a neat little pie chart with these specific percentages that were color-matched to different
regions on a world map. The report told me I was mostly Southwest
Asian — no surprises there, considering both my parents are from Iran. That percentage — 86.7% — I understood
that to be the portion of my DNA that’s West Asian. But it turns out, that’s not exactly what
ancestry tests are telling us at all. This is an ad for one DNA ancestry test, 23
and Me. An ethnically ambiguous woman travels the
world, and a circle animates around her, sort of like the pie chart in my test results, as
if to say, this woman’s DNA is 29% East Asian. And here’s an ad for
a different ancestry test. “52% of my DNA comes from Scotland and Ireland.” And somehow this information compels him to… wear
a kilt? Alright, so what are ancestry tests really telling
us? Can you help me understand what my results are telling me? Because I’m getting mixed messages from
ads and how other people talk about their results. This is Wendy Roth. I’m an associate
professor of Sociology at the University of British Columbia. OK. First of all, these test results are not
about your entire DNA. They’re about a tiny, tiny fraction of your
DNA. To understand how genetic ancestry tests work,
let’s start with the DNA itself. There are about 3 billion base pairs in our
genetic code. Those are the As, Cs, Ts, and Gs that form
the instructions that make us… us. Of these 3 billion base pairs, 99.9% are exactly
the same in all humans. But for the remaining .1%, one person might
have an adenine where another person has a guanine. These single-letter differences are called
Single-Nucleotide Polymorphisms or SNPs. Groups of SNPs can help explain why some people
are taller than others or why some people have green eyes while others have brown eyes. But most SNPs have no known effect at all. What many DNA tests are looking at are a relatively
small number of SNPs, specific positions in this .1% in our DNA, in order to give you
your results. When a testing company receives your sample,
they compare your pattern of SNPs to
different reference populations in their database. These reference populations contain SNPs known
to exist more frequently in different modern populations in the world. Then the testing company will give you a percentage that represents how strongly your pattern of SNPs resembles that group. But this process has a bunch of important
limitations and this is where things get complicated. Lots of markers are found in multiple
populations around the world. First, even trying to classify humans into groups in the first place is tricky. Human genetic diversity isn’t organized
neatly into groups like countries or continents. Take a look at the distribution of this SNP
that affects how a person absorbs folic acid. It’s commonly found in Mexico, but also
in Chile, or even China, just as often. So let’s say that a particular marker is found
in the South Asian population 30 percent of the time. There’s still a possibility
that when you inherited this marker you got it not from somebody who was South Asian, but
from somebody who was in some completely different group that also happened to have that marker. Second, testing companies put together their
reference populations based on academic research and other people that have taken genetic ancestry
tests. And most testing companies aren’t clear
about how many people are represented in their reference populations. So each company might have different reference
databases, which helps explain why you might get different results from different companies. So what does this all mean for my results? This is a probability with a margin
of error. So it’s not that you overall are
eighty-five percent West Asian, but that the particular spot that they happened to look at,
eighty-five percent of those locations are associated with Western Asia in their reference
population. So what about these other results? Am I really 2 percent African? You’ve got a lot of, you know, sort
of small trace percentages here. Percentages that small are really not meaningful,
again because that could be affected by having one person in the database. And if that one person gets reclassified later
on because they get a larger sample, that percentage will disappear. Ultimately, DNA ancestry tests are really
just giving us a probability, the testing company’s best guess. And that uncertainty isn’t made very clear
in the results. Buried in my results I found this “confidence
slider.” It turns out, my results were presented at
about 50% confidence by default. When I increased it to 90%, my results got
much more vague. All of a sudden I was “broadly” West
Asian and a lot of my genetic markers were unassigned. So, DNA ancestry tests don’t actually tell
us where our ancestors lived – they’re really just giving us probabilities of where we’re
likely to have relatives today. But so what if people misinterpret their results? Well that has consequences. They can make us believe that our ethnicities
have these bright-line distinctions between them, like in a pie chart. When people are presented with
test results and these percentage breakdowns and they are led to think that these tests
can tell you your race or they can tell you who you are, that that leads to a way of thinking — makes us feel that there are very stark and clear biological differences between races. One study found that DNA ancestry tests reinvigorate
age-old beliefs in essential racial differences, that our socially constructed racial categories
like “white” or “black” are essentially different from each other. Some groups have even turned to genetic ancestry
tests to try and prove their “racial purity.” DNA ancestry tests can be useful. Search YouTube and you’ll find hundreds of
stories of people using them to find lost relatives and fill in their family histories. And, to people who don’t know a lot about their ancestry, the tests offer the best available
estimate. But it’s important to remember that, despite
their marketing, these tests are just a company’s best guess at matching your genetic markers
to different parts of the world. What they’re not going to tell you is whether
you should wear a kilt or not. DNA ancestry tests might not be as informative
as you want them to be, but more and more people are still taking
them. And this giant database of genetic information is
becoming super valuable to an unexpected group: Law enforcement. We’ve teamed up with Verge
Science, to look into how your privacy is at risk because of genetic ancestry tests, even if you’ve never taken one.

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