The genetic genealogy community has been known to disagree about the usefulness of small segments. Although less contentious, large segments is also "a thing."
Blaine Bettinger asked on Facebook a couple of weeks ago
How many matches do you have using a threshold of 25 cM for a GEDmatch One-to-Many autosomal DNA comparison?Shared cM Project, where he asked people to tell him the sizes of matches they had with known relatives. The chart on the right shows the results.
It is anecdotal and self-selecting, to be sure, but for many people it feels better than the theoretical tables that we have been working with until now. ISOGG has even added it to their Autosomal DNA Statistics page.
So Blaine's most recent crowdsourcing challenge has been large segments, something which I admit I have not paid alot of attention to. It is one thing to evaluate our matches by total matching segments and quite another to look at individual large segments.
The default threshold for GEDmatch is for kits which have a match of 7 cM or more, but as I mentioned recently in passing, we can change that threshold to suit our own needs.
So I looked at my own matches and there were twenty-six. For eighteen close relatives (up to second cousins) the range was from thirteen to thirty-seven. But of course most of those were just us matching each other. Once I eliminated the matches up to second cousins, I was left with eight. Most of the others were in the 10-18 range. Aunt Betty had twenty-two, Herb had twenty-three and oddly enough, one of my sisters had nineteen.
Meantime, other people were reporting back to Blaine that after removing close reatives, they were getting segments of 25 cM or more with fifty-sixty, even a hundred other people. This surprised me, so I began looking a bit deeper.
Matches with strangers
My interest in this exercise was not my usual how-are-we-related-to-these-other-Pikholz-descendants, but rather the strangers. I suppose that decision was trivial because none of my family members had matches of 25 cM or more with any Pikholz from Rozdol or with any descendants of Nachman or Peretz Pikholz. Or, for that matter, Vladimir or Joyce.
My list of strangers was the shortest - only four people. The first thing I did was to look at the strangers who appeared as matches for several of us Eva, for example, matches six of us at 25 cM or more. So I looked at Eva's matches from 15 cM.
It was no surprise that all six are on the same segment. It was a bit of a surprise that she didn't match any other Pikholz at 15-25 cM. I would have thought there would be a few there. The key here is Rhoda, who makes it clear that this match is on the side of my father's paternal grandparents, but with no additional matches, that's as far as I can go.
Another match named Al showed quite the same sort of results.
Another match on my grandfather's side but with no smaller matches and not much else to say.
A third one told a different story. There are two, actually - a mother and daughter. This is the mother.
The first seven are more or less the usual group, some descendants of my great-grandparents. But they are followed by three more distant Pikholz descendants, two of whom have matches in the 18-19 cM range. Those two would be Judy and Leonora who are related to me through both parents of my great-grandfather.
Anna is a bit more specific. She is a fourth cousin of mine on my great-grandfather's mother's side. I am not sure how important that is because my great-grandfather's parents are some kind of cousins, but nonetheless it gives a bit of direction.
The daughter's matches are about the same. I wrote, hoping to find some names or geography we could work with. But I was disappointed. These matches are from the mother's unknown father. They are hoping for some direction from me. We are corresponding but for now, I don't think anything will come of it.
But while I am mentioning Anna
Our matches with our fourth cousin Anna, are unusual to say the least. Anna and her half-brother (who have both done Family Finder tests) are related to us through their Pikholz-descended father. Both their mothers are non-Jewish, so any Jewish DNA comes from the father that they share.
To confirm that there is no significant Jewish DNA from the mothers, I simply counted their matches. I have 4471 Family Finder matches and other members of my family have more or less (mostly more) than that. Anna has 2366 and her half-brother has 2115. These numbers are consistent with having one non-Jewish parent.
If we look at the chromosomes below, we see that Anna matches everyone in my family except my sister Sarajoy and me. Her brother does not match the two of us nor does he match our second cousins Rhoda and Terry.
On Chromosome 8, both have a nice set of matches with Aunt Betty, Uncle Bob and Herb - who, remember, are their third cousins once removed.
Both have very large matches with Marty on Chromosome 15 - Anna's is 50 cM!
But Chromosome 3 is remarkable. Anna's brother has a nice set of matches with five of us, two of which are a bit more than 20 cM. But Anna has seven matches, all over 30 cM and four of them are 57-69 cM! This is huge for four fourth cousins and three third cousins once removed. And keep in mind that Sarajoy and I are not there at all.
If Anna were not known to be a cousin, these numbers would jump off the page - but only by looking at the largest segments or the individual chromosomes.
In fact, if we only looked at the Family Finder match list (on the right), we would see nothing remarkable at all. We would not even see that Anna's matches with us are significantly different from her brother's.
There are lessons here galore. Lessons about looking specifically at the large matches. Lessons about looking at the chromosomes, not just at the total cMs and the overall suggested relationships.
And perhaps most important is the lesson about testing cousins and siblings. Before Anna tested, her brother's results were anything but inspiring. If someone had said "Why do we need her? We have her brother!" look what we would have lost out on.
And even with Anna, if all we had from our side had been Uncle Bob, Terry, Rhoda, Lee, Judith, Sarajoy and me, it would have been a fine test collection of seven people but we would have missed the best results.
I referred to Anna as a known fourth cousin. That is true now. It wasn't true six months ago, before we had seen Anna's results. For it was this set of results that clarified our relationship with Anna's family.