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I can't resolve why the following strange set of match results is occurring.

We have a fairly simple set of known relationships as shown in this tree:

enter image description here

All the DNA Testers are shown in green.

Jenny and Don are 2nd cousins once removed (2C1R) and they share 138 cM as they should.

Jenny is also a 2C1R with Teresa, Dennis and Jeremy.

Jenny shares 135 cM with Teresa, 107 cM with Dennis, and 238 cM with Jeremy. According to the Shared cM Project 4.0, these values are all well within the range of 14 cM to 353 cM for a 2C1R and average a bit more than the expected value of 122 cM.

Now Don in a 3rd cousin to Teresa, Dennis and Jeremy.

Don only shares 12 cM with Teresa, 18 cM with Dennis, and zero with Jeremy. Shared cM indicates the average should be 73 cM and the range could be 0 cM to 234 cM.

My question is what reasons might there be for Jenny to average 160 cM with Teresa, Dennis and Jeremy, but Don only averages 10 cM with the 3 of them.

This seems strange, particularly because Jenny and Don have the strong match between them of 138 cM.

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When I see situations like this, the first thing I would look for is other common ancestral lines that might be affecting the match levels. For example, if Jenny and Jeremy have another common ancestor, then possibly some of their match cM is not due to their common ancestors in this family. That would bring down the average significantly - but not enough to explain the low matches between Don and his 3rd cousins.

If Don also has another common ancestor with Jenny, and an NPE in this line, then it's possible that Don's entire match with Jenny is unrelated to this family, and the low matches with the other presumed 3rd cousins make more sense (I think there's something like a 20% chance of no match with a 3rd cousin, but no match with one and low matches with two others begs another explanation).

That's a lot of supposing, and (IMO) an overly-complex hypothesis (it's certainly worth trying to develope a simpler one). But I would look for other common ancestors than the ones shown.

I have entire ancestral lines like this, where families lived in the same locale for generations, went to the same churches, etc., with much intermarriage between families. So, if one living descendant shares an ancestor with another, chances are they share more than one. There aren't a lot of proven NPEs in my lines, but there are a few. I don't know if that's a possibility here - I think there's a tendency to leap to the NPE conclusion whenever the numbers don't make sense, so I'm somewhat reluctant to suggest a hypothesis based on it (yeah, I know, did it anyway).

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    Only after reading my own answer did I realize that rather than developing a hypothesis to explain the existing matches, what's called for is more samples in each line (and any others), to cross check the numbers, if that's possible.
    – cleaverkin
    Jan 13 '21 at 4:33
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+50

The set of shared DNA amounts measured do not seem peculiar to me, even though they are not hitting the exact average amounts expected for the relationships shown. The highest variance from the average is for the relationships between Don and his three third cousins, but even there the amounts measured fall within the expected range that the Shared cM Project reports.

DNA inheritance is an extremely random process. When we use a summary statistic, like total cM matched, most of the variation present is hidden from us. This is good as it allows us to think about possible relationships easily, but it isn't adequate when we want to understand the details of what may have been inherited.

To understand what sort of thing is going on in the present case we need to explore deeper and think about what particular DNA segments may have been inherited; a total cM number isn't adequate. And in thinking about the process, it's important to remember that DNA isn't like sand or a liquid where a representative, evenly allocated half of a parent's DNA is passed down to the next generation. The DNA comes in chunks (segments), and one half of the somewhat large chunks are not passed down, potentially eliminating entire remaining segments from earlier ancestors (that is, not just passing down half-sized sub-segments of previously inherited segments).

Consider a halfway-plausible segment inheritance suggested by the capital letters shown in this version of the relationship diagram:

segment inheritance

The above is showing Jenny as inheriting seven segments from Harold & Maude, namely A,B,C,W,X,Y, and Z. Don is shown as getting A,D,M, and for the three closer cousins: Teresa getting M,V,X, Dennis M,V,Y, and Jeremy V,W,Z.

So Jenny shares segment A with Don, X with Teresa, Y with Dennis, and segments W,Z with Jeremy. These segments (or collections of segments) are representing the inheritance of distinct pieces of DNA which total the 107 to 238 cM measurements of Jenny's sharing with each of the other tested people.

You'll see I've shown only one segment, M, being shared in common between Don and any of his third cousins, and shows that one of them, Jeremy, shares no segments with Don. This represents one possible way that a particular random arrangement of segments might have been inherited to cause the measured total shared cM numbers actually measured, 0 to 18 cM.

We can't completely know what segments the earlier generations had, but we can identify some segments that they must have had. Jenny's ancestor Betty must have also had all of Jenny's segments, ABCWXYZ, and Don's ancestor Fred must have had all of Don's segments from Harold & Maude, ADM.

Bill, the ancestor of the group of three cousins, must have had all of the segments that his descendants received, which would be MVWXYZ.

The foregoing is just an illustration of the sort of inheritance that must have taken place. Actual details might be available from the data the DNA testing company may provide. At least some companies (e.g. 23andMe) give identification of the segments that match. This takes the form of showing the chromosome, position, and length of each of the segments that matched. It is the total of all such segment lengths (in cM, not bases) that give the summary statistic of the total shared cM value.

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