Do these persons share with me through one parent?

[leonardo]

I have a match on chromosome 12 with one individual, at the location of 69,000,000- 89,000,000. I have a match with another person on chromosome 12 at the location of 74,000,000 to 79,000,000. Both are males. When I use triangulation tool on 23andMe, find that these two individuals share genomes with each other, not on chromosome 12 (sharing the same location with me; a common match for all), but on chromosome 3 at the location of 66,000,000 - 70,000,000 (5.2cM). It appears to me that we all share a common ancestor, and that these two persons would match me through the same parent (my mother), but I would like some assurances. Any thoughts on this from the forum community?

[AJL]

It's not certain. After weeding through multiple kits of relatives of mine, it has become clear that some matches in common are through a common ancestor, and some aren't. If one of their parents' backgrounds is very different to the other's, though, you might have a good clue.

[leonardo]

It's not certain. After weeding through multiple kits of relatives of mine, it has become clear that some matches in common are through a common ancestor, and some aren't. If one of their parents' backgrounds is very different to the other's, though, you might have a good clue.

Thanks AJL. That makes a good deal of sense. How about this scenario: on a specified location (segment), on a particular chromosome, I share ancestry with two persons of apparently different ethnic ancestry (say Italian and Polish), yet I believe I share with both through the same parent (who, by the way, appears to have ancestry from these two nationalities). By segment, I mean perhaps 34,000,000 to 46,000,000. Is this possible on a segment this size to have matches from the same parent from two divergent nationalities?

[AJL]

Because of recombination one match could come from a mix of someone's parents, but then there should not be overlap between the Polish person and the Italian one unless they also share ancestry. Does that make sense?

[leonardo]

Because of recombination one match could come from a mix of someone's parents, but then there should not be overlap between the Polish person and the Italian one unless they also share ancestry. Does that make sense?

Yes. So, it is unlikely a person can match two unrelated persons through the same parent on a segment that is only about 10cM in length? If so, then one match must come from one parent and the other match from another?

[AJL]

Yes, essentially if you (1), a second person (2), and a third person (3) overlap, you must also see if 2 and 3 overlap at the same spot. If they all do then you share a common ancestor, but it is quite frequent that you will match 2 and 3 on the same spot without their matching each other.

As a complication, they may also match each other on another spot and it may not be connected. Let's say 1 is half Chinese, half Swedish; 2 is half Chinese, half Brazilian; and 3 is half Swedish, half Brazilian. I guess you can see how you could match two people who also match each other without being related to the others through a common path?

[leonardo]

Yes, essentially if you (1), a second person (2), and a third person (3) overlap, you must also see if 2 and 3 overlap at the same spot. If they all do then you share a common ancestor, but it is quite frequent that you will match 2 and 3 on the same spot without their matching each other.

As a complication, they may also match each other on another spot and it may not be connected. Let's say 1 is half Chinese, half Swedish; 2 is half Chinese, half Brazilian; and 3 is half Swedish, half Brazilian. I guess you can see how you could match two people who also match each other without being related to the others through a common path?

Thanks AJL. Probably like most,I have both scenarios. I was curious about the latter scenario, whereby I share with two persons, who also share with each other, but not at the location where I share with them. It does make sense that, if you match on different segments, it could be just coincidental, in the sense that not all three of you share the same common ancestor.