Your best bet at telling whether or not she is your aunt is to use an ancestry DNA test. (National Cancer Institute)
Let’s say someone contacted you out of the blue to say she is your long-lost aunt. You are interested but cautious and so want a bit more proof. You decide to get a genetic test done since aunts and nephews share a lot of DNA (25 percent on average).
So you Google “aunt nephew genetic test” to see what’s out there. You quickly find there’s a lot. Unfortunately, none of the tests on the first page of results are your best bet. And the second page isn’t much better.
This is because the best tests for confirming relationships more distant than parent/child are actually those marketed as tests that reveal ancestry or find long-lost relatives. These are offered by companies like 23andMe and AncestryDNA. They aren't perfect, but they are heads and shoulders above the ones that you’ll find in your Google search.
When Less Than 50 Percent Is Not Enough
Most relationship genetic tests look at tens of markers to determine if two people are related. They compare these few spots on both samples of DNA to see how many the two people share. In general, the more markers they share, the more likely the two of them are related. (It also helps if they happen to share some of the rarer markers.)
While a test with tens of markers is fine for a paternity test, it is not nearly as useful for more distant relationships like half siblings or first cousins. Or for that matter, aunt and niece or nephew.
With a typical uncle/aunt genetic test, you get a number that shows how likely it is for someone to be your aunt or uncle (or niece or nephew). In other words, you rarely get a definitive result and instead are often stuck with a frustrating "maybe."
What’s even worse is that with so few markers, these tests can sometimes miss a relationship completely. In this case, your aunt may not look like your aunt simply because the markers the test happens to look at aren’t the ones that show your relatedness. You will come away thinking the two of you aren’t related, even though you really are.
While this is just the nature of biology and how these tests work, it is not a particularly satisfying result. Most people would prefer a definitive answer when they submit to a genetic test. And for that they may want to turn to certain ancestry tests.
Sometimes you Need Hundreds of Thousands of Markers
Your best shot at a definitive result on a genetic test with distant relationships is to use an ancestry test. Instead of tens of markers, these tests look at hundreds of thousands of markers (or more). Except for rare circumstances like being a chimera, what was hard to see with other genetic tests becomes obvious.
If a person is really your aunt, the test would come back that the two of you share around 25 percent of your DNA. 23andMe even offers results in a picture like this:
The blue bits are the shared DNA. These two people share around 25 percent of their DNA.
Here is what the results would look like if the two of them weren’t related:
Nothing wishy washy here. It is pretty obvious that the first two are related (lots of blue boxes) and the second two aren’t (no blue boxes). A picture is indeed worth a thousand words!
Now, of course, these tests aren’t perfect. The top result tells us the two people are closely related but it doesn’t distinguish between half siblings, aunt/uncle and nephew/niece, or grandparent/grandchild.
You may also not get a perfect 25 percent shared DNA. Because of how biology works, an aunt and a nephew could easily share less or more DNA. And if the numbers are too off, you may not be able to tell a first cousin from an aunt for example.
Still, it will definitely tell you that the two of you share a pretty recent common relative. Which is why a paper trail establishing relationships is still so useful and will continue to be so no matter how powerful genetic tests become.
For people who want to dig a bit deeper, the key to these ancestry tests has to do with the fact that they use so many markers that you can see the chunks of DNA you share with a relative. Instead of being isolated markers, you see a whole set of them strung together along the DNA like Christmas lights. Click here for why this is significant.
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