A couple of weeks ago, I posted about a great connection I made through 23andMe with a new cousin. In that post, I talked about the match in 23andMe and why I was so excited to see her show up. This time, I’ll share what I learned about our DNA match.
For some reason, when I viewed Cousin P in my Relative Finder, the M/P designation did not show for her. This is the designation 23andMe uses to tell you on which side of your family your match and you are related. It only works if you have had at least one parent also tested with 23andMe. Well, both of my parents have tested and I have many other matches where the designation is shown, so I am at a loss why it didn’t show that day. Must have been a bug. It shows now though. But, on that day, my next step after realizing she is a person of interest was to find out which of my parents to which she is related.
That’s done easily enough using the Family Inheritance: Advanced feature. Doing that revealed that she is related to me (purple) and my mother (blue) on Chromosome 16. Our segment match length is 38cM.
I then wanted to compare Cousin P against my mom, and my uncle (mommy’s full brother). What was interesting here is that Cousin P hardly matches my uncle – their segment match is 6cM (green)
Since 6 cM is on the cusp of matches that might be excluded as not being significant enough to determine a match that is identical by descent vs. DNA just be identical by state. Though my mother and uncle have the same parents, if only one of them had tested, and it was the one with the small amount of matching DNA, we could have missed this match. Lesson to be learned: try and get as many family members tested as possible. 
The next step is to then enter Cousin P into my spreadsheet I keep for sorting my matches. This is important to do for triangulation purposes as it allows me to see if those that match me and my family members, also match others who match us. While you can do some of this comparison on the 23andMe site, I keep the spreadsheet for better record-keeping and HIGHLY encourage everyone to do something similar if you aren’t already.
My Analysis Spreadsheet has a tab for my maternal matches, and a tab for my paternal matches. I then sort matches by Chromosome Number. My columns are Chromosome, Name, % Shared DNA, Start of Segment Match (Base Pair Position #), End of Segment Match (Base Pair Position #), Distance of Match (cM), # SNPs in common and Notes. Below is a snapshot of my spreadsheet, the tab for my maternal matches, with the Name column removed for privacy. As you can see, I’m not always diligent in filling it out uniformly, but I capture the essence of the info needed.
Where you see clusters of color represent cases of triangulation – cases where my matches not only match me/my family members, but other matches of mine/my family members. The purple cluster is where Cousin P lies. The first row of the cluster is Cousin P. She matches us from base pair number 11,000 – 51,000. The second two purple lines are another match of mine who I’ll call Cousin W. Cousin W matches my mother and I from base pair number 17,000-24,000 and base pair 47,000-51,000. Upon seeing Cousin W matching my mom and I in the same place, I then went back to 23andMe and used the Advanced Family Inheritance feature to compare all three of us — I wanted to see is Cousin P also matches Cousin W for it is possible that they wouldn’t.
I was pleased to see that all three of us match! In the figure below, the
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The blue shows where Cousin W matches my mom; the green shows where Cousin W matches Cousin P., and the light blue shows where Cousin W matches my uncle. Sweet. Thus, I was able to color the three rows in my spreadsheet as one color and I now know that Cousin W, Cousin P, and my mother all share a common ancestor.
Now knowing this, I then took a closer look at Cousin P’s family tree – information she’d filled out in her 23andMe Profile. In the next post of the series, I’ll share I learned then!
