(This article was revised 11 Aug 2016 to fix some incorrect statements)
Be careful if you’re triangulating at FamilyTreeDNA. I just found out they can match twice on a segment.
If you look in your Chromosome Browser Results file which is downloadable from the Chromosome Browser page, you may find matches with a second person that overlap. For instance, look at this match my uncle has with David:
On chromosome 5, 7 and 12, there are three matches that overlap. The matches on chromosome 7 are identical. This would seem to indicate that one half of my uncle’s chromosome matches with one half of David’s chromosome and the other half of my uncle’s chromosome matches with the other half of David’s chromosome.
You don’t notice this when you use the chromosome browser. It will show just one of the matches:
This doesn’t happen often. There are only 198 overlapping matches out of the 178,955 matches in my uncle’s file. But that’s often enough to worry about.
The match of my uncle with David is reported on the Chromosome Browser as having 26 shared segments totalling 87.36 cM. On the Family Finder it is reported as 107.87 cM, and in the chromosome match file I downloaded, after including the 3 overlapping segments shown above and 2 others, there are 59 matching segments totalling 203.56 cM. So what is going on here?
The overlapping matches in the Chromosome match files are not separate matches by FamilyTreeDNA on the two halves of the genome. They don’t do that. Any overlap would look like just one match over the both genomes.
What most (if not all) of those overlapping segments are from are from the incorrect way Family Tree Maker is listing people in the chromosome match file. They are being merged by person name and then by chromosome number and then by location on the chromosome. If two people have identical names, their information is being put together as one in the chromosome match file. This is incorrect and needs to be fixed by FamilyTreeDNA. What they need to do is incorporate the kit number into the matching, so that three John Smith’s are not put together.
See also my recent post: FamilyTreeDNA’s Chromosome Match File for more problems with the file that FamilyTreeDNA needs to fix.
And, if you hadn’t noticed, FamilyTreeDNA made some major changes today and updated their Family Finder interface. They now phase your relatives and show which matches are on your fathers side, mothers side, or both sides. Of course you need more than just one person tested and some known relationships entered before a paternal and maternal side can be assigned:
I also notice they changed the ordering. Your matches are now ordered first by relationship range and then by shared centimorgans. It used to be ordered first by relationship and then by largest segment. As a result, all the matches changed order significantly. But it seems that the relationships and cM values did not change.
For more information about this set of FamilyTreeDNA changes, see Roberta Estes’ post: Family Tree DNA Introduces Phased Family Finder Matches