What’s our tree completeness?
It happens all the time.
We identify a batch of autosomal DNA matches all in the roughly 15-20 centimorgan (cM) range … and they all have the same couple in their tree.
Our most recent common ancestors (MRCA)!!!
Um… not so fast. There’s something else we need to consider:
Just how complete are those trees that we’re comparing?
Because, of course, a match in the range of 15-20cM could be a fifth cousin, meaning we’d share a set of 4th great grandparents. Or a sixth cousin, sharing 5th great grandparents. Or a seventh cousin, sharing 6th great grandparents.1
And — sigh — The Legal Genealogist is pained to admit it but…
I for one am darned hard-pressed to identify all that many of my ancestors out to those levels.
I’m doing a bit better than I was two years ago, the last time I took a careful look at this.2 I can now put names on 37 of my fourth great grandparents (up from 36), and on 34 of my fifth great grandparents (up from 28), and on 24 of my sixth great grandparents (up from 22).
But — sigh — the problem is that I have 64 fourth great grandparents, 128 fifth great grandparents and 256 sixth great grandparents. So I’ve only identified 58% of my fourth great grandparents, 27% of my fifth great grandparents, and 9.38% of my sixth great grandparents.
Which means, of course, that there are a boatload of people I could have received that 15-20 cM segment from. How can I know that it’s from this one potential set of fourth or fifth or sixth great grandparents and not from that one out there among all those unidentified sets of fourth or fifth or sixth great grandparents?
Sure, there are things we can do to narrow this down. If my match is on my German-born father’s side, I’m not going to worry a whole lot about the holes in my tree completeness among my mother’s colonial American ancestors. But I need to know there are holes and think about their impact on the match before I ever start to come up with a theory about where that match and I might find our real MRCA.
And so I remain exceedingly grateful to Jonny Perl and his fabulous DNA Painter website and its many invaluable tools. This is where you’ll find that Shared cM Project tool to see the range of relationships supported by a particular amount of shared DNA.
And it’s where you’ll find that tree completeness tool — the tool that produced that chart you see above, telling me I have done some good work since the last time I really focused on this — but also that I still have a lot of work to do.
Those with a free DNA Painter account can have one tree for free to go along with their one free DNA Painter profile, and free accounts can import their tree data out to the fourth great grandparent level. Those of us who think DNA Painter is about the next best thing to sliced bread and subscribed the minute it became a subscription service can have as many as 50 trees to go along with our 50 profiles and import our tree data out to whatever level we have (except a lot of pedigree collapse may punk out around the 8th great grandparent level).
So shelling out the $55 subscription price for a year is a small price to pay for the utility of being able to see all of the issues with our tree completeness for ourselves and for the profiles we manage — not to mention supporting a terrific set of tools and future development of more.
The bottom line in DNA analysis is thinking through the question of just how we and any of our matches are related: just which set of most recent common ancestors can we reliably say we both descend from?
Reconsidering that match means reconsidering our tree completeness.
Our match’s tree… and our own.
And here’s a tool that helps us stay on track doing just that.
Cite/link to this post: Judy G. Russell, “Reconsidering that match,” The Legal Genealogist (https://www.legalgenealogist.com/blog : posted 29 Aug 2021).
- See the batch of relationships supported by a 15-20cM match as shown by “The Shared cM Project 4.0 tool v4,” DNAPainter.com (https://dnapainter.com/tools/sharedcmv4 : accessed 29 Aug 2021). ↩
- See Judy G. Russell, “Filling in the blanks,” The Legal Genealogist, posted 28 July 2019 (https://www.legalgenealogist.com/blog : accessed 29 Aug 2021). ↩