All of us genetic genealogists are extremely grateful to Blaine Bettinger for collecting statistics on the actual amounts of DNA shared for known family relationships. He just updated his numbers for that project this past March. Click here for the details on his blog
This update is also included in the wonderful online calculator at DNApainter where you can input either the cM or the percentage shared and dynamically see the probabilities of various relationships. Click here to read what the programmer, Jonny Perl, has explained about its new features on his blog.
I refer people to this calculator all the time so that they can see the full range of possible relationships for a specific amount of DNA. In the above screen shot of that tool I have used red arrows to show where you would put the number of cM or the % shared.
An exciting new feature is that if you click on any of the colored boxes it shows you a histogram of the frequencies within the range for that relationship.
Let me demonstrate using this calculator by comparing a few of my family members.
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The question I often see is are we really 2nd or 3rd or 4th cousins? The answer is usually “maybe.” A 2nd-4th cousin designation by your testing company is purposefully vague. Best to look at the amount of shared cMs in segments greater than 7cM, number of segments, and the sizes of those segments; plus, of course, who else this new DNA relative matches!
DNA inheritance gets more and more random the further away the relationship is. The amount of DNA you share with someone more distant than a 3rd cousin is impossible to predict and even those 2nd and 3rd cousins seem highly variable. So the statistical study conducted by genetic genealogist Blaine Bettinger is deeply appreciated by all of us hunting down the relationships with our DNA connections.
Shared DNA statistics from Blaine Bettinger, used by permission
Blaine has created this beautiful chart. His blog has several posts explaining the study which is the source of these new statistics. See http://www.thegeneticgenealogist.com/2015/05/29/the-shared-cm-project/ for all his posts on this study. I had previously discussed his project when recruiting people to add their statistics; apparently he is still taking in statistics so click here to add yours.