This is certainly not a new question. But it was raised again in the context of genetic genealogy. How much of your family tree have you identified? If you and/or your DNA match haven’t built out your family trees, how can confident can you be in determining your most recent common ancestor?
Since the majority of my most recent research has been with the goal of connecting my family tree with those of my AncestryDNA shared matches, I thought I’d try to answer this question. I started with a table based on one shared by The Genetic Genealogist.
As you can see, I’ve identified everyone in my pedigree from myself through my 3rd great grandparents. I’m missing two of my 4th great grandparents and 36—holy cow!—of my 5th great grandparents. Not surprisingly, the number rises even more the farther back I go. All told, I know only 477 out of 2,046 direct ancestors going back 10 generations. Only 23%.
Why does this matter?
The majority of my AncestryDNA matches fall into the 4th through 6th cousin category. This means we are expected to share ancestors in the 3rd great grandparent to 5th great grandparent range. I’ve sometimes found a closer relationship and sometimes a more distant relationship, but generally that range is correct, give or take a generation or two for either myself or my match.
If I—or my match—haven’t identified those ancestors, then I’ve little chance of determining how our DNA matches. It also makes it much more of a challenge to break through brickwall ancestors if I can’t find commonalities—even if just by surname—in my shared matches’ family trees. I have been building out trees for some of those matches. But with more than 500 matches in the 4th cousin or closer category, I can’t do that for everyone.
Knowing these ancestors not only permits us to identify a common ancestor, but also allows us to say with confidence that we don’t share ancestors on another line.