Ancestry
Back to Ancestry
My twins each did a 23andMe DNA test and the results verified that they are identical; however, their ancestry composition is not!
How is that possible if their genomes are identical?
-A curious adult from New Jersey
I can see why you’re confused. The same DNA should give the same ancestry results. And yet they haven’t.
This is pretty common with DNA ancestry tests and it isn’t just a 23andMe thing. Companies like Ancestry.com or MyHeritage will give these sorts of results too.
This does not mean these companies are doing shoddy work. They aren’t—they are doing outstanding, cutting edge science that brings DNA testing to many, many people.
It is just that the analysis is complicated enough that it is incredibly difficult to get an exact result. There is some wiggle room.
So don’t take percentages as exact numbers. As we talk about in a previous answer, they can vary by 10 or even 20% pretty easily and sometimes even a bit more. So two people might have the same amount of East European even if a DNA test says one has 20% and the other person has 40%.
But even when the percentages are small, this kind of thing is a bit more unsettling when you get different results from the exact same DNA. These differences mostly come from how the computer algorithm splits up the DNA into thousands of windows, analyzing one window at a time. And how blank spots (or “no calls”) in the data affect how the DNA is interpreted and/or split up into those pieces.
These no calls are an inevitable consequence of any test like the ones these companies run.
23andMe reports that on average they get good reads on more than 98% of the markers they look at which is really very good. But it still means that each test has thousands and thousands of spots that could not be read.
And importantly, the same markers do not come up as no calls in different tests. Each time someone’s DNA is read, you can end up with a different 2% being uninterpretable.
These spots on the DNA that can’t be read in a particular assay can tip the ancestry scales one way or another. What might look English with a smattering of these missed reads, looks German with a different set of no calls.
So in the first case a piece of one identical twins’ DNA might look English while the same or an overlapping piece of DNA will look German with the second identical twin. It wouldn’t take too many differences like this to shift enough DNA to make the two not look identical from an ancestry point of view.
All of this is true even though when looked at with another test, the relatedness one, it is obvious these two people are identical twins. Even with the no calls the companies can see that the two have the same DNA.
DNA Landmarks
These companies use what is called reference DNA to figure out where your DNA came from. This reference DNA is the DNA of people whose families have stayed in a part of the world for a long time. Or who have very detailed family trees.
The companies then compare your DNA to the DNA landmarks they found in these families. The parts that match that group of Germans is German, the parts that match that group of people from the British Isles is English and so on.
Sounds easy enough but most people are not like the reference group. They have lots of different ancestral DNA scattered in chunks throughout their DNA. This is where things can get tricky.
To get around this mixed DNA, the computer program analyzes small sections of the DNA at a time. These “windows” have around 100 or so DNA markers in them which translates to thousands of such windows.
The no calls we talked about earlier can affect it so a window can be interpreted in different ways. If by chance the program interprets it as German for twin 1 and English for twin 2, then the two will appear to be a little bit different.
Connect With Us