To find DNA matches, we search for pieces of DNA that are identical between two individuals. But there are a number of reasons why two people’s DNA could be identical. This is what makes DNA matching a complicated scientific problem that we’ve worked hard to solve at AncestryDNA. (Read more about how we find DNA matches.)
DNA that is identical by descent
In family history, we’re interested in DNA that’s identical because two people both inherited the same piece of DNA from a recent common ancestor. This is called DNA that is identical by descent, or IBD (see figure 1).
DNA that is identical by state
But there are other reasons why two people’s DNA could be identical. After all, the genomes of any two humans are 99.9 percent identical. (And the genome of a human is 50 percent identical to the genome of a banana.) Pieces of DNA could be identical between two people because they are both human, because they are of the same ethnicity or come from the same region, because they share some other more ancient shared history, or other reasons. We call these identical pieces of DNA identical by state (IBS), because the DNA is identical for a reason other than having a recent shared common ancestor.
How do we determine whether DNA is identical by state or descent?
The real challenge of DNA matching is to make sure that any identical pieces of DNA we find are really identical because of a recent shared ancestor—rather than ancient shared history. Looking at the DNA sequence alone doesn’t tell us the difference, so we’ve developed a ground-breaking scientific method that helps to distinguish DNA that is IBD from DNA that is not. Our method looks at not only the amount of identical DNA between two people, but also its location in the genome and other features. (Get more details on the process in our white paper.) If we find that you have identical DNA with a potential DNA match that appears to be IBS and due more to an ancient shared history, we won’t show them in your list of matches.
This means that we are doing our best to only show you DNA matches that are identical because you are related through a recent common ancestor. The confidence score associated with each match also allows you to see the strength of evidence that you and your match are actually related. DNA matches with high confidence scores are meaningful for genealogy because it’s likely that you have a common ancestor within the last ten generations.
Keep in mind that calculating DNA matches accurately is a tough scientific problem. As we update our methods behind DNA matching, we’ll continue to do a better and better job at delivering DNA matches that you can explore to unlock your family history. (Get tips on working with your DNA matches.)