We are learning how to use probabilities to figure out genealogical relationships. The first post in this series explained how to combine independent probabilities (just multiply them), and the second described hypotheses and how we can test them numerically. But where do we get the probabilities in the first place?
The probabilities used in this series (and in the tools I’ll be describing) come originally from simulations done by AncestryDNA scientists. Specifically, they come from Figure 5.2 in AncestryDNA’s Matching White Paper. I’ve modified the labels in this version to list some of the relationships included in each color-coded group.
I used an online plot digitizer to extract numeric values for the lines on the graph then converted the data into a table. (The whole gory process is described here).
I’ve used this information—with great success—almost daily over the past year in my own genealogical work and in the research I do for clients. I will forgive you, however, if your eyes glaze over at the very sight. It’s boring. It’s too much information. It’s easy to cross rows. And it doesn’t help below 40 cM at all.
Enter Jonny Perl, who is rapidly becoming a knight in shining armor for the genealogical community by creating fabulous online tools to rescue us from our distress.
He’s converted the table above into a more visual interface that not only tells you relationship probabilities but also highlights where your DNA match could theoretically fit into your family tree. Dr. Andrew Millard made some refinements to get more precise values between the points in the table. And I extrapolated the numbers below 40 cM.
Go ahead: try it!
Let’s do a simple example together. In the “Filter” field at the top of the window, enter 140.
This table will appear, ranking the possible relationship groups from most to least likely:
And below that, you’ll see a stylized family pedigree with the possible relationships highlighted. (The dimmed relationships are ruled out for a match who shares 140 cM with you.)
The percentages from the tool are what we need for the probability calculations described in Part 1 and Part 2 of this series. Remember to convert the percentages to probabilities first by dividing by 100. For example, if your first hypothesis places this 140-cM match as a 2nd cousin, the probability you would use in the calculation is 18.88% ÷ 100 = 0.1888. And if your second hypothesis places the match as a 2nd cousin once removed, the probability would be 6.42% ÷ 100 = 0.0642.
In the next post of this series, we’ll consider the mystery of the American GIs in Germany. Which brother was the grandfather?