By Julie Granka, Ph.D.

When someone takes an AncestryDNA test, we compare their DNA to the DNA of the millions of other test-takers in the AncestryDNA database. We’re looking for “DNA matches”–people who share identical DNA with one another and might be related through a recent common ancestor.

But, identical DNA can be shared between people for reasons other than a recent common ancestor. Identical DNA may also be shared when both people are from the same ancestral origins.

So, Ancestry developed a special tool to keep just the shared identical DNA that is from a common ancestor. This tool is called Timber, and it helps Ancestry give you a more accurate estimate of the relationship between you and your matches.

Shared DNA doesn’t always mean a recent common ancestor

The main idea behind identifying a DNA match is to look for pieces of DNA that two people both have because they each inherited it from a recent common ancestor. In our DNA matching white paper, we detail the steps we take to turn your genetic data, and that of the other people in the AncestryDNA database, into these suggested DNA matches.

One of those steps is to identify pieces, or segments, of DNA that are likely to be identical between pairs of people. But if two people have identical DNA, it doesn’t necessarily mean that they inherited it from a recent shared ancestor. Pieces of DNA could be identical between two people because they are of the same origins. As part of the genetic processes that occur over time, a population of people can end up sharing a specific segment or segments of identical DNA among all its members. This happens because of endogamy within a population and isolation (genetically) from other populations.

Timber gives you more accurate relationship estimates

So in order to find DNA matches that are due to recent ancestors, we need a filtering step. At AncestryDNA, we use an algorithm developed by the science team called Timber. The basic idea is that if two people appear to have identical DNA at a particular place in the genome, but they also appear to have identical DNA with thousands of other people at that same spot, then the shared DNA between the two people was probably inherited from a shared region rather than a recent common ancestor. In other words, that shared DNA is probably not from a common ancestor within the last 6 or 10 generations and probably isn’t meaningful for genealogical research. In cases like this, Timber can filter out those identical pieces of DNA entirely–giving these people a better estimate of how they are related.

Timber mostly filters out shorter segments of shared DNA

The DNA you share with your matches is distributed across segments of DNA. If you think of you and your match’s DNA like two books, shared DNA segments are like identical sentences or paragraphs in both books. The length of the longest segment you and a DNA match have in common can help determine your relationship. Sharing fewer, longer segments usually indicates a closer relationship. Shorter segments of shared DNA can indicate a distant relationship or identical DNA due to a shared region.

When we looked at the shared DNA segments removed by Timber for a study of 300,000 Ancestry customers, we saw that most of the removed segments are shorter (<10 cM). See the chart below.

After finding what appear to be identical segments of DNA between pairs of people, Timber will filter out those that do not appear to have been inherited from a recent common ancestor. For identical segments of the indicated sizes, bars show the proportion filtered out by Timber in a set of over 300,000 individuals. (Note: CentiMorgans (cM) are a unit of distance in the human genome.)

However, we also found that in some cases, longer identical segments (>15 cM) were also removed by Timber. What this shows is that there is much more than just the sizes of the identical DNA segments to consider when trying to identify shared DNA from a recent common ancestor.

Timber makes genealogical research easier

By using Timber, we can find shared DNA that is more likely to be due to recent common ancestors. Timber is exceptionally powerful at removing DNA matches that are due to a shared region, rather than a recent common ancestor. This narrows down your list of DNA matches to a more manageable and informative set.

Timber is a personalized filter that is uniquely possible with AncestryDNA’s enormous database. As a result, AncestryDNA test-takers can receive DNA matches that are more reliable for genealogy research.