One of these days I need to take some time to write about calculating relationships between two people based on the amount of DNA they share. It seems like such a simple thing, but nothing is simple if no one has ever explained it to you. I run into a lot of people who just don’t get it, so they’re chasing down theories that make no sense.
In the meantime, here’s a quick intro from Your DNA Guide. Chart. Video. What more could you ask?
The way they explain it, “Blaine Bettinger at thegeneticgenealogist.com spearheaded the effort that became the Shared cM Project. He collected the shared cM data for known relationships from genetic genealogists just like you. This free tool gives you a good estimate of how much DNA should be shared for the different relationships.”
Maybe I can add that “cM” stands for centimorgan. A centimorgan is a unit of measurement for the distance between chromosome positions. Don’t worry about understanding it. It’s just a measurement word like feet and inches.
Blaine’s current chart is at August 2017 Update to the Shared cM Project. I like to go to his site directly so I can be sure I’m getting updated information. He has a summary of his posts at The Shared cM Project.
I also like Jonny Perl’s Shared cM Project 3.0 tool. Put in the number of cMs you share with someone and this tool with calculate the likely relationships.