I have been mainly working with my Dad’s Norwegian DNA at 23andme and at Family Tree DNA. Often he will have a match at one company and there will be a match to someone else on the same segment location at the other company. So how to tell if they match each other? Since one could have the DNA segment that Dad got from his mother and the other could have the segment Dad got from his father, the only way to be sure it is the same segment is if they also match each other on that segment. This is what is known as triangulation.
If they have both uploaded to GEDmatch, I can compare their two kits there and see if they match on that segment. Often however one or the other has not uploaded or the GEDmatch site is down. So I needed another way to figure this out.
It occurred to me that I could check if the new match also matches me there, since my results are at both web sites as well.
Obviously when they both match me on that DNA segment, I know they match each other. If one matches me and the other does not, then I know they are not a match. But what if they both do not match me? Then I must have inherited that segment from one of Dad’s parents and they are matching the DNA piece from his other parent.
In the case above, Dad has a 23cM match at Family Tree DNA with an adoptee at the same spot where he has many smaller matches over at 23andme. So do those folk match DM?
So to figure this out I log into my account over at Family Tree DNA and select in common with (ICW) my father on the Family Finder matches page by clicking just under his profile so that more choices appear including the ICW option:
Next I put the surname of the match I am interested into the name box by clicking on Name: in the gray bar so that the blue box appears for me to fill in. It was a very common surname, so there would have been too many pages of names to look through if I just did the surname box without the ICW selection.
And the result is that DM is not in my family tree DNA match list.
Over at 23andme there are many people who match Dad at places on this spot in chromosome 3. Only two of them match me, so those two are not a match to DM, but the others are. The ones that do not match me match my brother and first cousin so we know they are real matches. Here is what that section of my spreadsheet of my matches to Dad looks like. As you can see, I use colors to show the triangulations when people match each other (the bottom group are all one family):
Thus when using my match results to triangulate between the companies, if the groups or individuals I am trying to compare do not match me at either company then they are a match to each other as long as they are a true match to Dad. I consider the match good if the segment is 10cM or greater. Smaller than that it is not for sure, but often matches to other cousins or relatives will confirm that the match is a real one, so IBD rather than IBS (see the article at http://dna-footprints.com/203/the-abcs-of-dna-ibd-vs-ibs/ for good explanations of IBS and IBD)
This same technique can be used to triangulate between matches at Family Tree DNA where you cannot compare your matches directly to each other. I also have written an automated tool for working with match data from family tree DNA called Kworks for the DNAadoption site; it will make a spreadsheet of matches with an ICW grid filled in, but that is a blog post for another day.