I have told people in the past that we share a single segment of meaning IBD DNA with the vast majority of our genetic matches (where IBD means Identity-by-Descent, or a valid matching segment of DNA from a recent genealogical relationship). I usually say that we share a single segment of DNA with 99% of our matches, but that’s been an off-the-cuff estimate. I wanted to have better data to cite, so I took a closer look at this issue.
At FTDNA, you can download a list of all of your matches:
I downloaded my list and removed all of my targeted test-takers (anyone that I tested or I asked to test). These close test-takers would skew the data.
After removing them from my match list, I have a total of 2,491 matches at Family Tree DNA.
Family Tree DNA also allows you to download a list of all the segments you share with your matches:
I downloaded my list of total shared segments, and removed all segments smaller than 7 cM (these segments are either too small to be valid or too old to be genealogically relevant). This is perhaps the most controversial aspect of this analysis, but see “Sharing Large Segments With a Match Does Not Validate Small Segments Shared With That Match.”
After I removed segments smaller than 7 cM, I shared a total of 2,554 segments of DNA with my 2,491 matches.
I share more than 1 segment of DNA with 63 matches, or 2.5% of my matches. Accordingly, I share a single segment of DNA with 97.5% of my 2,491 matches at Family Tree DNA.
Of those 63 matches, I share multiple segments with the following breakdown:
- 2 segments = 55 matches
- 3 segments = 6 matches
- 4 segments = 2 matches
Thus, among my matches, it is extremely rare to have a non-targeted test-taker share more than a single segment of DNA (only 2.5% of my matches). It is even more rare to have more than two segments, with just 8 people (<1 %) sharing 3 or more segments.
Of course these numbers will vary from person to person, and will vary significantly in endogamous populations. Interestingly, this might be a way to identify people that come from endogamous populations, but may not know it. It will be interesting to see this analysis from people that have endogamous backgrounds.
Using a 5 cM Threshold
Notably, when I only removed segments smaller than 5 cM, I had 212 matches (8.5%) that shared more than 1 segment of DNA, and they broke down like this:
- 2 segments = 192 matches
- 3 segments = 15 matches
- 4 segments = 4 matches
- 5 segments = 1 match
Thus, most of these additional matches had a second segment between 5 cM and 7 cM, which is a danger zone for DNA evidence.
After this analysis, I maintain that most people will share a single segment of IBD DNA with their genetic matches (endogamous populations will vary).
Among my Family Tree DNA matches, I share a single segment of DNA with 97.5% of my matches.
It will be interesting to see whether I can find common ancestry with the 63 matches I’ve identified as sharing more than 1 segment of DNA, and whether it is easier than other matches. I have a ton of questions I don’t have time to answer right now, unfortunately. For example, what is the total amount of DNA shared by these matches compared to the average of all of my matches? Do any of these matches match both my parents? What are the segment size distributions for the segments shared with 63 matches? Is it usually one large segment and one small segment, or does it vary?
I would also like to repeat this process using the Tier 1 Matching Segment Search tool at GEDmatch. Using the tool, I get a total of 4,732 segments of 7 cM or greater shared with non-targeted test-takers (it reports that it analyzes my 9,787 matches). However, it’s a lot trickier because I have to be wary of repeated uploads and re-used pseudonyms, so identifying the total number of matches is an issue that requires more time.
Of course this analysis is strongly influenced by FTDNA’s matching algorithm (see “Family Tree DNA Updates Matching Thresholds“), so please keep that in mind.