The tools for working with matches at 23andMe are outstanding. The only drawback is the lack of response from matches. There are no trees at 23andMe, and few people have posted ancestral surnames. I'm finding, however, if I can find a common ancestor with a 3rd cousin match I can identify segments which lead to resolving how many of my matches relate to my mother and I.
I've been able to identify 3rd cousin matches by searching for trees at Ancestry.com. I search using the member directory. Most 23andMe matches don't have Ancestry trees, however. They generally don't have trees at MyHeritage either. Most testers at 23andMe have little ancestral information, if any at all.
The best way to ID 3rd cousin, or 2nd cousins for that matter, is compare segments using Genome Mate Pro, or a spreadsheet. If you compare your matches at 23andMe with GEDmatch cousins, for instance, you'll find matches who have tested at all 3 major companies. Since AncestryDNA matches often have some sort of tree you can sometimes find matches with trees who share the same segment, or overlapping segment, with a 23andMe match. You may then be able to identify these segments as coming from common ancestors.
You can compare segments with all your Family Tree DNA matches, if you've tested with them, or transferred your AncestryDNA raw data over to them. You will find more trees and surnames at Family Tree DNA than you'll find at 23andMe. You may find segment matches and common ancestors comparing here.
Once you have a map with identified segments 23andMe matches will be easier to identify, even if they aren't sharing with you, or open sharing. The relatives in common feature will allow you to identify matches. I didn't copy the names for privacy reasons in the screenshots below.
The first shot below is from the common relatives feature, at 23andMe, displaying matches shared with a match I selected. This match is a 3rd cousin. The shared matches are 3rd cousins with me; as stated below in the first column. The percentage of DNA shared with me is also given. Column two gives the relationship of these shared matches with the match I selected. The first common match is actually the mother of the match I selected. The second common match is the uncle of the selected match, as stated below. The uncle isn't sharing with me, and has a different surname. I wouldn't have guessed their relationship with out the same surname. The fact the relationship is provided is so helpful. I have had contact with the person described as mother here. We have found common ancestors. So even without any surname information posted by these three I have determined how they are related. I have the shared segments for the mother and daughter and can marked them with the shared ancestral couples' surnames. These segments have helped me identify many more matches. I've been able to confirm some of these at Ancestry.com, with trees. I don't have the uncle's segments, but know how he is related.
My mother and I don't have 2nd cousin matches at 23andMe so I'm focusing on 3rd cousin matches with substantial shared amounts of DNA. I'm using the large segments to infer how those sharing smaller segments might be related. Below you see a 3rd cousin match who is a 1rst cousin to the match I selected to compare with. This would definitely lead me to believe we three share common ancestors. Both are sharing with me and share DNA in some of the same places on the chromosomes. I've marked their shared segments and again have identified many shared matches who match in the same place on the chromosomes.
|Relatives in common feature at 23andMe|