A Segment-ology TIDBIT

Up Front – it’s the one with the highest average cM among Match cousins.

Setup: You’ve pretty much determined a particular couple are bio-Ancestors to youself (or someone else) – often by a consensus of Match Trees in a group (usually a Cluster) – see here. However, this bio-couple had a number of children. Which one of them was the bio-Ancestor? It gets harder and harder the more generations back you are researching.

Process: I’ve had good outcomes by determining as many DNA Match cousins as possible for the bio-Ancestor couple. Line up the DNA Matches and the shared DNA cMs under each of the children, and then determine the average cM for each child. In general, one of the averages will be somewhat more than the others – even when you don’t know the link. That’s because you are a closer cousin with Matches who descend from the same child as you do.  For instance, you may be a 5C through most of the children – sharing an average of 25cM with those Matches; and you would be 4C with the Matches who descend from then one child who is your Ancestor – sharing an average of 35cM with them. Of course, our results may vary somewhat from the Shared cM Project, but it’s the concept we are focused on here.

When I do this analysis, I drop down into the smaller segments, in order to get a fair comparison among all the cousins I can find. The more Matches we use, the more it averages out to the Shared cM Project and the correct bio-Ancestor child.

[22CI] Segment-ology: Which Sibling Is the Bio-Ancestor? TIDBIT by Jim Bartlett 20240403