Shared cM Project | Now Add TWO Testers

Connie Davis

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The Shared cM Project has a new feature which lets you enter the shared cM of two testers, rather than just one. This can narrow down a list of possible relationships to DNA matches while using siblings or cousins from the same generation. Learn more about how to use the updated feature.

Have you wondered what to do when you have two DNA testers, such as yourself and a sibling, and you share different amounts of DNA with the same mystery match? Jonny Perl and Blaine Bettinger have added a new option to the Shared cM Project (SCP) to help you. With this option, you can enter the amounts of shared DNA of two testers with the same relationship to the match to investigate the relationships between them. 

Shared cM Project

First a little bit of background: The Shared cM Project (SCP) shows you the probabilities of how you might be related to your DNA matches. For an introduction to the SCP and strategies to learn how you are related to your matches, check out this article.

The new option at the SCP provides an efficient way to work with the match data from two testers. Here’s an example from my own research. I am searching for my mother’s 2x great-grandparents. My brother and I have both tested, but because we have our mom’s DNA, we are both irrelevant in this situation (hers is much better to use!). In the diagram below, the missing couple is at the top and my Mom, Oletia, is pink. I’m fortunate that my mom’s brother (blue) and sister (yellow), also tested for me. Siblings can provide valuable information. My fourth tester is my mom’s first cousin (in green).

The new option for the Shared cM Project allows me to enter the data of two testers who would have the exact same relationship to the match. Looking at the diagram, I could use the new option with any match who shares a common ancestor at the generation of the grandparents or further back. This means Oletia, her brother or sister, and their first cousin could all be used to help me figure out how they are related to the mystery match. 

The mystery match is in a genetic network related to the unknown 2x great-grandparents. This table shows how much DNA each of the testers shares with the match:

It’s worth noting the considerable variability in shared DNA with this mystery match–even among siblings. Test as many siblings as you can! (Here are more reasons to test multiple siblings.) If I only had the sister’s DNA (79 cM) and entered it into the shared cM project, I would see this table of all the possible relationships and the probability of those possible relationships.

That’s a lot of possible relationships! Quite a few are less likely, with probabilities of 2% or less. I have estimated this match is one generation younger than my testers, so I am going to focus on the once removed (1R) relationships for now, but I might need to go back and revise this hypothesis later. Below the table in the SCP, I can see the results displayed on a chart. I’ve outlined the position of the testers in yellow and the potential locations for the mystery match in red:

I know the first cousin shares 0 cM with the match. The new option at the SCP instructs me to use the two results with the biggest difference. I enter both numbers, the highest value and lowest value, into the new option in the Shared cM Project. It’s important to put a 0 cM amount in the first box like I’ve entered here:

When I look at the chart below, I see there are now fewer possible relationships:

I don’t have to stop there, though. I can look at the Shared cM Project histograms if I make a slight adjustment. The zero value doesn’t show on the histograms, so as a workaround, I use the value of 1 instead and then click on the square for Half 2C1R. (This would mean that the mystery match actually descends from the great-grandmother). I can now see red arrows marking the median for both values I’ve entered. Here’s what I see for Half 2C1R:

I next check 5C1R:

That doesn’t seem very likely, since the 79 is so far to the right. Next I check 3C1R:

The 79 is still on the high end but definitely possible. Next I check half 3C1R:

This relationship is also possible. I’m going to start by looking for a half 2C1R, 3C1R, or half 3C1R relationship. Any of the five once-removed relationships are possible and I need to consider the twice removed and “even” (same generation) relationships, too. I now have information that will help me explore the mystery match’s tree to look for their shared ancestor.

It’s always fun to have a new tool–or rather, a trusted tool with a new option. This option saves us from needing to manually remove options based on the shared DNA of two testers.

All of the steps I used here (and more!) are covered in our DNA Skills Workshop, a hands-on, six-week long masterclass experience. The Skills Workshop teaches the methods that professional genetic genealogists use with every DNA case. The course participants I’ve worked with as a coach have SOLVED mysteries and built their confidence in “doing the DNA!”

Check out Our DNA Skills Workshop

If you’d like to try learning these strategies on your own, pick up a copy of Your DNA Guide–the Book, the textbook that comes with the Workshop. (It is sold separately.)

Purchase Your DNA Guide--the Book

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<a href="" target="_self">Connie Davis</a>

Connie Davis

Connie Davis is a Genetic Genealogy Coach for Your DNA Guide. She is a professional genetic genealogist focused on using documentary evidence and genetic genealogy to solve family mysteries and confirm traced ancestors. Her personal research includes early colonial United States, southern United States, the Midwest, and Canada. Many of her ancestors heard the call of the west and became California pioneers. She has experience in African American research and an interest in reparative genealogy.


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