AncestryDNA now reports longest shared segment with DNA matches—yay! There’s some confusion about longest shared segment v. total shared DNA. Here’s how their Timber algorithm affects these, and why it’s good for your family tree.
Most of the time, I leave the difficult task of naming things to others. (I didn’t even name my own children or pets.) Naming is just not one of my talents. So I appreciate it in others. And let me tell you, AncestryDNA nailed the name of the tool they use to filter their matching DNA segments. It’s called Timber. It is so appropriate because essentially they are chopping down all the DNA segments that don’t help you find a common ancestor with your DNA match.
AncestryDNA and the Timber algorithm
Think of your shared DNA with your cousin Jim as a forest, in which each tree represents a segment of DNA. Some segments are giant sequoias and others are just little saplings. But Ancestry doesn’t just look at the segments you are sharing with Jim. It can see these segments across the entire database. So if 892 other people have that exact same segment or “tree in their forest,” it is not likely to have come from a recent common ancestor between you and Jim. It could just mean that you both have Irish ancestry, for example. Ancestry uses Timber to chop down that all-too-common Irish segment/tree so we can get a better view of your familial relationships within recent generations.
Most of the segments they remove are small saplings, but according to this table, the Timber algorithm filters out pieces of DNA up to 50 cM:
This table also helps us better understand why we shouldn’t trust small segments of DNA, as, according to the table, the proportion of segments under 10 cM that are filtered out is very, very high. For example, 95% of the time, segments that are 5-6 cM long are shared by hundreds of other people, and therefore do not represent a connection to a single recent common ancestor. (Ancestry’s recent white paper explores this more, if you’re interested.)
Timber and Longest Shared Segment Update
While I have always appreciated the role of Timber in ancestry’s DNA matching algorithm, it is causing some confusion in the newest AncestryDNA match update. In this recent update, we finally get some long-anticipated information: the size of the biggest piece of shared DNA. This is SO HELPFUL when trying to determine if a match is worth your time or not, especially if you have any endogamous relationships in your family.
However, many people are finding that they have a largest shared segment that is actually bigger than the total amount of shared DNA.
How does this make any sense?
Well, it all comes down to Timber. Here’s the very general process your DNA sample goes through when it arrives at the lab:
- Your DNA is processed (they figure out what values you have)
- Phasing (they separate your DNA jumble into two camps- one from mom and one from dad)
- DNA matching with the entire database
- Identify the longest segment with each DNA match
- Timber to remove segments that are not likely to be from a recent common ancestor
- Estimate your relationship with your DNA match
So, you see, if your total amount of shared DNA is lower than your longest segment, part of that longest piece must have been shared by hundreds of other AncestryDNA members, and thus, was chopped off by Timber.
Don’t let the learning end here!
We know the world of DNA testing and genetic genealogy can be a lot to step into, that’s why we’ve put together this guide on finding birth relatives using DNA. Check it out and see what you can learn about your family!