Matching segments, but different ancestors

[david123]

I'm trying to understand the scenario where two people have a matching segment with you, but they've inherited it from different ancestors.

https://www.familytreedna.com/learn/...ents-location/

I can understand small segments happening at random, but can it happen for longer ones, say the 7cM+ size we'd be looking at for matches?

I suppose what I'm really wondering is what the implication is for a group of people who have the same "big" match at the same position. Is it likely to be an issue, and if so how do you account for it?

Thanks

-David

[MoberlyDrake]

Usually one of the two is matching you on the strand of DNA you got from your father, and the other is matching you on the strand you got from your mother.

Occasionally I find a group of 3 or more matches on the same segment, none of whom match each other. I have no explanation for that, as it seems to me that, if the segment is sizable, any group of 3 or more matches, should either all match each other, or split into 2 (and no more than 2) groups.

[david123]

Thanks. I think my real question is where does the matching strand come from?

From what I read, small matches are easily IBS / coincidence. For larger matches (again say 7+ or 10+) would that tend imply the maternal and paternal grandparents had a common ancestor in their recent past?

If it does then ideally the paper trail would identify that. And if it doesn't go back far enough then fingers crossed you’re part of the pair that can document everything to the same ancestor!

-David