DNA Matching Puzzle

[ikas]

I am hoping someone can explain a DNA matching puzzle I have on MyHeritage and what it might mean.

I have a DNA match with MH (79.7cM across 5 segments largest 22.7cM). I have seen her tree and have identified our MRCA. We have exchanged e-mails and we are both confident about the MRCA as we have both done a lot of work on this line.

I was examining another of my matches CS (39.4 across 2 largest 31cM), his father GS (48.4cM across 3 largest 30.4cM), and GS’s mother IS (54.3cm across 4 largest 29.8cM) but I have been unable to identify our MRCA as they do not have trees.

What I cannot understand is that my shared match list with MH includes CS where she matches him 33.1cM but does not include GS or IS. That implies to me that CS inherited the DNA segments that match MH and myself from his mother? Is that correct? However in that case why do I match GS and IS.

I have limited experience of DNA matching especially on MH so might just be a misunderstanding on my part.

[Gadget]

Have you tried using the one to many facility with them. It's accessed via the Chromosome Browser (DNA Tools -> Chromosome Browser).You can select up to 7 matches and it will show you how they might match at a segment level. 

[ikas]

Good Idea Gadget. I forgot about One to Many on MH. Sadly there is no triangulation when myself and all four selected. There is no triangulation when Myself, MH, and CS are selected.

Do you think I am justified in thinking my DNA match with CS must be DNA inherited from his mother? Or is that false logic?

[Gadget]

I think it could well be segments for both lines.

Have a look at this post that I made recently:

Segments that seem to be shared by three or more people are not necessarily triangulated. For example, if you compare yourself with two people, call them A and B, you may see that you share a segment with A and you share a segment at the same genomic position with B, but A and B may not match each other on that segment. That’s because you may be sharing your maternal segment with A and your paternal segment with B and A and B aren’t related to each other at all.
If a match is shown as triangulated, it means that you, A, and B, all match each other precisely on that segment, and therefore all of you are probably related, and you probably got that triangulated segment from the same common ancestor.