Did you know that AncestryDNA is using fully-identical regions to distinguish full siblings from half siblings? And apparently has been doing so since 2016? I didn’t.
If you’re asking yourself, ‘How did we miss that?‘, I am to! And I don’t have a good answer. I didn’t read their scientific White Paper closely enough. Mea culpa.
Then again, if you’re asking ‘Isn’t a FIR a tree? The other kind of tree. What does it have to do with DNA?‘, read on.
A fir, not to be confused with an FIR (any excuse for a good botany pic!):
Autosomal DNA and Relationships
Autosomal DNA, the kind most commonly used in genealogy, indicates how closely two people are related. The more DNA they share, the closer the connection is likely to be.
It’s not so simple, though, as you can see from the graph below (modified from Figure 5.2 in the AncestryDNA Matching White Paper). Each relationship has a range of possible shared DNA—some 2nd cousins share more than average and some share less—and the ranges of different relationships overlap.
The lower the amount of shared DNA, the more overlap there is among relationships. Barring endogamy or other factors, a match of 500 cM could only fit one of three relationship categories (the groups that include 1C, 1C1R, or 2C, represented by the red, navy, and yellow lines, respectively), whereas at 200 cM, there are at least four possible categories: 1C1R/navy, 2C/yellow, 2C1R/light green, 3C/black, and possibly even 3C1R/light blue.
Even the closest relationships can fall into overlapping zones. A half sibling (dark green line) who shares an above-average amount could match on more DNA than a full sibling (medium blue) who shares less than average. There is a way to tell full and half siblings apart, though.
Fully Identical Regions
We have two copies of each autosomal chromosome (the ones that don’t determine sex). For each chromosome, half siblings can only match one another on one of their two copies, the one they inherited from their shared parent. They won’t match on the other copy, which they each inherited from a different parent.
Full siblings share two parents, though, so they can match on both copies of each chromosome. They will have some sections of DNA where they inherited the same segment from their mother and, in the same spot, where they inherited the same segment from their father. This is called a “fully-identical region” (FIR), and half siblings won’t have them. Full siblings will also have regions where they only match through one parent, and some where they don’t match at all.
Genetic genealogists have long known that we can use FIRs to distinguish full versus half siblings, but we are only able to visualize these regions at 23andMe and GEDmatch. Here’s a visual comparison of three chromosome in my two kids at GEDmatch:
The solid green regions are where they inherited matching DNA from both me and their dad, the mostly yellow regions are where they match through just one of us, and the mostly red regions are where they inherited different DNA from each parent.
Half siblings will have red and yellow sections when viewed at GEDmatch but no solid green (FIRs), like this:
FIRs at AncestryDNA
At AncestryDNA, matches are categorized into groups based on how much DNA is shared. For example, a match who shares between 650 and 1300 cM will be in the “First Cousin” category (although other relationships are possible), a match between 200 and 650 cM is in the “Second Cousin” category, and so on.
Full siblings normally fall into the “Immediate Family” group and half siblings into the “Close Family” one. If those categories were delineated solely by a centimorgan threshold, full siblings who share less than 2200 cM would be classified as “Close Family” and half siblings who share more than that would fall into the “Immediate Family” category.
In the past, that’s exactly what happened. But it seems that AncestryDNA updated their algorithm in 2016 to consider FIRs. The weird thing is that no one noticed! Or if they did, they didn’t make a fuss over it. I certainly missed it.
This figure, also from the AncestryDNA Matching White Paper, shows how the consideration of FIRs (labeled IBD2 on the horizontal x axis) helps to distinguish full (orange) and half (red) siblings, and even to tell identical twins (purple) from a parent or child (blue).
Considering only total shared centimorgans (the vertical y axis), a match of 2200 cM could be either a full sibling or a half sibling. Both red and orange points fall there. However, when you plot FIRs (IBD2) on the x axis, the two relationships are distinct.
Parent–child and identical twin relationships can now be similarly differentiated. Of course, most people with those matches never doubted the relationship once age was considered.
This change is a big improvement for close relationships. That it flew under the radar for so long is pretty amazing!
Take home lesson: AncestryDNA is doing a better job of telling full sibs from half sibs than they once did.