How Many Segments Do You Share?

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: ... Click to read more!

Using Shared Matches – A Quick Example

I logged into my results at AncestryDNA today, and I had new fourth cousin match: Vivian Reese Wescott (ALL names in these screenshots are changed to protect privacy unless noted otherwise). This is a significant match to me, my 22nd closest match (not counting family members that I’ve tested). The relationship is estimated to be fourth cousin.

When I open up Vivian’s profile, I can see that she’s a new member, and likely hasn’t seen her results yet (I check frequently, but she hasn’t logged in since September 23rd). I also see that she doesn’t have a tree associated with her profile:

The first thing I would normally do is review her tree for clues as to our relationship.

Since I can’t do that, I’ll skip that step and now I’ll look to see how much DNA we share in common: ... Click to read more!

Inheritance of DNA Segments

DNA is randomly inherited. As a result, a match that shares 100 cM DNA with a parent will likely NOT share exactly 50 cM with the parent’s child; rather, there are a range of possibilities (100 cM, 50 cM, 0 cM, and everything in between, for example). On average it will be about 50%, but there is lots of room for variation.

Prompted by a great question in the Genetic Genealogy Tips & Techniques Facebook group, I used the “People who match one or both of 2 kits” tool at GEDmatch to look at the random inheritance pattern of DNA between my father and myself with regard to matches sharing about 35 cM (the examples here worked out great, but you can pick any size).

We can see the randomness of inheritance in this table. And we see a surprise (that I just discovered today with this exercise!) that reminds of the fact that matching DNA can come from BOTH parents! ... Click to read more!