Can this be explained by DNA

Question

I just want to ask the advice of some of our DNA experts here.

Recently I received a new top match on Gedmatch. Unfortunately the was no tree attached. I then went on to look at the other persons matches. I went to the first match with a tree and took a look. Low and behold on this tree one of the persons Great Great Grandmothers was a Colwell which happens to be my surname. This being a fairly unusual surname I thought I may be onto something here so I did a one to one check with this person but to my surprise the was nothing matching.

What I would like to know is can this be explained genetically or is the fact that the surnames match a pure fluke.

My Gedmatch No H760680 matches A? as follows

chr 3 - 10.9

chr 9 - 11.2

chr21- 22.5

A? matches A? 2

chr 7 - 23.1

Thanks for any help you can give

Chris

Comments

Unless you have permission from these matches to post their gedmatch numbers on an open forum, please anonymise the information in your question.

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Thanks for the heads up Lynda have done as requested

Thought it would be ok as I left the names out.

Chris

Answer 1

Gedmatch is estimating a bit over 4 generations to the common ancestor with A1. That would make you 3rd cousins. By pure chance you may not share any DNA with people who are genuinely your third cousin. See https://isogg.org/wiki/Cousin_statistics

It could be the case that A2 is such a person. I'd suggest a polite email to A1.

Tim

Answer 2

If I understand your process correctly, it looks like you did a one-to-many comparison for yourself, identified A1 as the strongest link to you, and then did a one-to-many using A1's code.  That would not be a good way to work.

Let's say you and A1 are 3rd cousins, then you each share one pair of g-g-g-parents out of 8.   Anybody who matches A1 to the same degree will only have a 1/8 chance of being related to you at all.

The better way to follow up is to do a "people who match one or both of two kits". That will immediately list anybody who shares DNA with both you and A1, and what amazed me was that it looks like there is only a single person at the default settings.