To find DNA matches, we search for pieces of DNA that are identical between two people. However, there are a number of reasons why DNA sections might match, making DNA matching a complicated scientific problem we’ve worked hard to solve at AncestryDNA. (Read more about how we find AncestryDNA matches.)
DNA can be identical because of a recent common ancestor
In family history, we look for pieces of DNA that are identical because two people share a recent common ancestor. This DNA is called identical by descent (IBD).
Figure 1. Shows DNA that is identical by descent. The blue and red bars at the top represent two people’s DNA. Their children (one row down) inherit a mix of this DNA. After several generations, the cousins at the bottom share only a small section of DNA (highlighted in yellow) from their common ancestors at the top.
DNA can be identical without two people being related
However, DNA can be identical even if two people aren't related. For example, two people may have identical pieces of DNA because:
- they're human (human genomes are 99.9% identical)
- they come from the same region
- they share other ancient history
- of some other reason
DNA that’s identical for reasons other than a recent common ancestor is called identical by state (IBS).
How do we determine if DNA is identical by state or descent?
The challenge of DNA matching is ensuring shared DNA comes from a recent common ancestor—rather than other shared history. Since DNA sequencing alone doesn’t tell us the difference, we’ve developed a scientific method to distinguish between DNA from recent common ancestors and DNA that’s shared for other reasons. This lets us better calculate the relationships with your matches and helps you focus on the ones most important for your family history. But don’t worry; we’ll still show you all your DNA matches and the amount of DNA you share with them.
As science and our methods advance, we’ll continue improving our delivery of DNA matches that help you explore and uncover your family history. (Get tips on making the most of your DNA matches.)