In order to determine if 3 people have a common ancestor, you need to have the following two requirements:
1. All 3 of them must match each other on the same part of the same chromosome
2. All 3 of them must be in common with each other
I usually get the most responses from my ftDNA Family Finder matches shortly after I've assigned all of my matches a known relationship. You know, that little orange button that says "assign" beside all of your match names?
Clicking on the "Assign" button brings up a list of relationships you can choose from. If you know for sure what the relationship is between you and your match, then select it from the list. For example, I have had my mom and grandparents tested, so I have set theirs accordingly. But for everyone else, I set to "Distant Cousin." I set this for everyone right off the bat so that I can use the In Common With feature on ftDNA's website.
Using the In Common With Feature:
To use this feature, go back up to the top and filter by "In Common With."
Then you can select the person you want to run an In Common With Report for. All of the people that you have assigned a known relationship for will appear in the drop down list. It is alphabetized by last name.
When you select a match to run an In Common With report on, a list of all of their matches will be displayed. This feature is very useful, for example, when you have an actual known relative who has taken the test, like a parent or a brother, and you want to see who all matches to them (or just the opposite, you can use the next option down, which is "Not in common with."
This feature is also important because being in common with someone is one of the two requirements for finding a common ancestor. The second requirement is sharing matching DNA on the same segment of the same chromosome.
A last word of caution though. I often have trouble getting my In Common With feature to work properly. Oftentimes I run it for someone and the results come up empty. I have started using a 3rd party tool called DNAGedcom. This tool allows me to download ALL of my In Common With data at one time. The only downsides: I have to set a relationship to every single match which can be time consuming and I have to re-download each time I get new matches.
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