Testing Siblings - Redux

I have written about the importance of testing siblings on any number of occasions, each time as background to a story. Here is another one from the last few days.

I received an email from a fellow named David, saying that he had seen indications on GEDmatch that we may be second cousins. He gave me a list of eight family surnames, none of which match any of mine.

I expected to tell him that the matching segments are likely from two or more distinct common ancestors, but first I looked at our kits on the GEDmatch One-to-One tool.

Three segments, all less than 10 cM. Nothing near second cousin level.  Obviously his match must be one of my cousins, listed under my email.

So I looked at David's top matches and one of my paternal second cousins stood out.

Well, 248 cM certainly looks like a second cousin - with segments of 42.6 cM, 34 cM and 33.4 cM and five more over 19.5 cM. But still, these could have come from several different common ancestors. On the other hand, a segment over 40 cM and two more over 30 cM looks encouraging.

I also recognized one of David's surnames as someone on that cousin's other side - I think a maternal grandmother.

This second cousin has a full brother who has tested as well as a first cousin, who is also my second cousin. Here is the brother.

Where David matches the sister with 248 cM, he matches the brother with only 116.8 cM. That's less than half. And of those 116.8 cM, only the segments on chromosomes 10 and 16 are among David's matches with the sister.

David's match with their first cousin is not worth mentioning.

So I wrote all this to David, bcc'ing the female cousin, so they can examine surnames. (This particular cousin is good about that.)

But my point, of course, is that had the sister not tested, all David would have had is the brother's second-cousin-once-removed level match. Would he have followed it up? Who knows!

Test first cousins. Especially test second cousins. But don't neglect the siblings. It matters.