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  • 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 Asianno 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

  • 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 usus.

  • 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 thisconfidence 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 thinkingmakes 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 likewhiteorblackare essentially

  • different from each other.

  • Some groups have even turned to genetic ancestry tests to try and prove theirracial 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.

So I recently took one of those at-home DNA ancestry tests.

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DNA祖先測試能--和不能--告訴你什麼? (What DNA ancestry tests can — and can’t — tell you)

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    ayami 發佈於 2021 年 01 月 14 日
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