Mary and I are related through my mother. Our matching DNA on chromosome 2 is from 74188298 to 105591639. But I can't assume that everyone who matches me in those segments are matches on my mother's side. It's so very important to look at chromosome matching in detail after you have identified a known relationship. GEDMatch is just one tool that will help do that.
Another good tool is DNAGedcom. It is a free site that works only with files from FTDNA and 23AndMe. Note that it cannot work with Ancestry files.
DNAGedcom will pull my DNA information and my matches right from my FTDNA account and will upload the files to the website. I can then run a report that will graphically show my matches for each chromosome. These graphics are far better than GEDMatch and FTDNA for seeing the shared segments.
Here's a part of my DNAGedcom report for chromosome 2. I can see the name and email address for each person who matches me at FTDNA. If they have put surnames on file, I can hover over their information to see those surnames. I can also hover to see the names of the people who match me in common with that person.
Mary is not on this report, as she has not uploaded her results to FTDNA. The large fuchsia line at the top is for my Dad. The yellow boxes show the areas where I have matching DNA with Mary.
Notice the top yellow box. I have matches that are up to 85044774 that match my Dad (fuchsia on the left), yet are in the range where I match Mary. Then there are several matches starting at 84408203 that do not match my Dad. Is the matching DNA random, or IBS (identical by state)? Or is there a real match?
Is part of my match with Mary IBS? I need more confirmed matches to know if the DNA matches are real or random. The point is to not make assumptions based on one match. Keep working with your matches to find more confirmed cousins and to build your knowledge of what your DNA reveals.
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