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… for the Shared cM tool …

For roughly the forty-‘leventh time in just the past few weeks, The Legal Genealogist answered a question the same way just a couple of days ago.

A Facebook poster couldn’t understand why a paper-trail third cousin once removed showed up as a DNA match with whom he shared 111 centiMorgans (cM) when a published chart the poster had been using said he should expect to share much less.

And my answer was the same as I’d given so many other times: put that chart away and use the Shared cM Project tool on DNA Painter instead.

shared cM tool

Why? Because all those published charts show the purely mathematical shared DNA amounts: we share 50% of our DNA with a parent or a child (or, on average, 3,400 cM); 25% with a grandchild or a grandparent or an aunt, uncle, niece or nephew (or, on average, 1,700 cM); 12.5% with a great-grandparent or a great-grandchild, a first cousin, a great-uncle, great-aunt, great-nephew or great-niece (or, on average, 850 cM); and so on with the amounts dropping by half for each generation as the relationships get more distant.1

Nice, neat, mathematical progression.

And, of course, real life isn’t nice or neat.

Taking a first cousin relationship, for example, where the chart tells us we should expect to share 850 cM of DNA, actual reported documented paper-trail cases tell us the amount we might share is going to be in a range from a low of 330 cM to a high of 1,486 cM, with the average being 865 cM.2

And how do we know that? Because one genetic genealogist — Blaine T. Bettinger, author of (among other things) The Genetic Genealogist blog — set out to collect real-world data, and has published and updated the data collected over a period of years. Called the Shared cM Project, it’s “a collaborative data collection and analysis project created to understand the ranges of shared centimorgans associated with various known relationships.” As of the last updated, in March 2020, citizen scientists had contributed data for nearly 60,000 known relationships.3

So… overdue thank-you number 1, to the thousands of citizen scientists who contributed that real-world data and to Blaine as the visionary who understood the value of collecting and analyzing that data.

Using that data, however, wasn’t quite so easy at first. Some of us who are … um … math-challenged, shall we say … struggled to apply it in our day-to-day research.

And then in 2017, Jonny Perl, developer of the DNA Painter website, recognizing that this was “an incredibly useful dataset that helps genealogists start to figure out just how they might be related to an unknown match,” went ahead to develop and release a web-based version in which we could enter a number — how much DNA we shared with a match in cM — and then filter the results visually.4

So… overdue thank-you number 2, to Jonny Perl who recognized not just the value of this data but also the difficulty faced by those math-challenged folks like me in using it, and produced a tool to give us easy access to it.

There still was a missing piece, however. The first version of the tool would highlight the possible relationships but it said nothing about how likely any possibility was. So entering 500 cM, for example, we could see that the match could be anything from a great-grandparent to a half first cousin once removed (with a lot of other options), but nothing that told us which possible relationship to look at first.

Enter Leah Larkin, who was already working with Jonny on DNA Painter‘s What Are the Odds tool, and a whole bunch of statistical analyses.5 Which, as of late in 2017, added probabilities to the Shared cM tool so that, now, entering 500 cM, we see that one set of possible relationships has a 90% chance of being right, while two other sets of possibilities have only a five percent chance.

So… overdue thank-you number 3, to Leah Larkin who is not just The DNA Geek but also a math geek — whose combination of those geeky skills added critically-needed qualifiers to this tool.

No, this tool alone won’t tell us that someone with whom we share 500 cM of DNA is a half first cousin rather than, say, a half great-niece or -nephew. But it does tell us to look at both of those first before we consider, say, a second cousin or first cousin twice removed.

And for that we all owe them all an overdue thank you!

Cite/link to this post: Judy G. Russell, “An overdue “thank you”,” The Legal Genealogist ( : posted 5 Dec 2021).


  1. ISOGG Wiki (, “Autosomal DNA statistics,” rev. 11 Nov 2021.
  2. Blaine T. Bettinger, “Meiosis Groupings,” The Shared cM Project Version 4.0 (March 2020), The Genetic Genealogist ( : accessed 5 Dec 2021), PDF at 7.
  3. Ibid., The Shared cM Project Version 4.0 (March 2020), PDF at 1.
  4. See generally Jonny Perl, “Introducing the updated shared cM tool,DNA Painter blog, posted 27 Mar 2020 ( : accessed 5 Dec 2021).
  5. See Leah Larkin, “The Limits of Predicting Relationships Using DNA,” The DNA Geek, posted 19 Dec 2016 ( : accessed 5 Dec 2021).
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