Friday, July 20, 2012

Known Relative Studies at FTDNA: Third Cousin Comparison and More Random atDNA Inheritance

I don't write about Family Finder very often for my known relative series since most of my close relatives have tested at 23andMe. Fortunately, my Travis third cousin recently decided to take the Family Finder test at Family Tree DNA. As a result, I have a new third cousin comparison to report.

Our only (known) common ancestors are our great great grandparents Abraham and Ruth (Stolebarger) Travis, so any matching DNA that we share is inherited from the Travises. Abraham's father Asa and Ruth's parents John and Sarah are two of my major genealogical brick walls, so it is really interesting to be able to isolate DNA that came from those lines.

My Travis third cousin and I share 45.96 total cM of DNA with a longest block of 14.84 cM. This is on the low end for third cousins and the most likely relationship suggested by FTDNA is actually fourth cousins. Only about 25 cM comes from segments longer than 5 cM, so just including those in my calculations (to keep it consistent with my 23andMe comparisons), that means we only share about .37% of our total DNA. Since third cousins are expected to share about .781% of DNA, this is a bit low, but it is in line with my other third cousin comparisons so far (averaging .39%). That's random atDNA inheritance for you!

Family Tree DNA offers a unique perspective on these comparisons, so I will share how this match looks on the different settings that are possible on their Chromosome Browser tool. In the chart below, the blue bars represent my twenty-two autosomal chromosome pairs. The orange bars are the sections where my Travis cousin and I have stretches of matching DNA. This chart is displaying the lowest setting in order to show all matching segments over 1 cM. Many of these are probably false positives, but it is still interesting to be able to see them.

My third cousin comparison showing all matching segments over 1 cM

The next image is set to show only matching segments over 3 cM. As you should be able to see, our main matches are on Chromosomes 2 and 14. The only other match that didn't drop off is the one on Chromosome 6. This match falls under the threshold of what you would see at 23andMe, so over there we wouldn't have known about it at all. Remember, this could be a "false positive" since a pretty large percentage of segments this size prove to be, but I will reserve judgment until I am able to compare it to my chromosome map (when it is more fully developed) to confirm if this segment falls in an area that I can positively attribute to my Travis ancestral line. That should help determine whether this is an authentic matching segment or not.

My third cousin comparison showing all matching segments over 3 cM

The final chart shows only the two largest matching segments. You can see them signified by the orange on Chromosomes 2 and 14. When you scroll over these spots, a window will open (as above) describing the exact starting and ending points of the matching segment(s).

Third cousin comparison showing only matching segments over 5 cM

To summarize, I compared a known third cousin to myself to identify the portions of our DNA that match each other. Due to the sizes, we can be confident that two of the matching segments are authentically inherited from our common ancestors. What this means is that I can now attribute those larger segments as originating with Abe and Ruth Travis.

Abe and Ruth Travis

If my Travis cousin were to upload his data to Gedmatch, I could compare him to my mother, sisters and some of my other cousins who have tested at 23andMe. It would be very interesting to see the variety of inheritance patterns. Hopefully, I will be able to do so in the future.

On another note, I actually have four matches on Family Finder who are predicted to be more closely related to me than my Travis third cousin. I have not found a common ancestor with any of these people mainly because, with the exception of one, they do not respond to my emails. It does make you wonder what would happen if everyone responded with great family trees, ready for comparison.

This should give some hope to those of you who are struggling to confirm common ancestors with your matches. We tend to focus on the larger matches, of course, but some of these seemingly lesser marches could still be quite significant.  As I have often emphasized, autosomal DNA inheritance starts getting pretty unpredictable at about the third cousin level. This comparison is a good example of that because we wouldn't usually expect a .37% match to have such a (relatively) recent connection. So, take a closer look at your match lists and give it another go. You just might be surprised by what you find!

*Another third cousin comparison: I found my third cousin today at 23andMe!*


  1. Very interesting account, and well written. I enjoy your mix of science and ancestry, made very understandable.

  2. Your personal experience with DNA has been clarifying and very educational for those of us still wrestling with the implications of DNA test results.

  3. Very nice write up and find CeCe! Thank you for sharing. :)