Month: May 2019

Project Tool Profile: Shared cM Tool

One of the tools we’re using in this genetic genealogy project is The Shared cM Project 3.0 tool v4 (https://dnapainter.com/tools/sharedcmv4). This calculator provides an estimate of how a DNA match may be related to the DNA test-taker.

It does this by converting the amount of DNA shared between two people (expressed in a number of centiMorgans, or cM for short) into a list of percentage probabilities to determine if a match is likely to be, for example, a 3rd cousin, once removed or an even more distant cousin.

In the case of our Sainsbury DNA genealogy project, we’re looking for DNA matches in the range covered by the “Other Relationships” column in the relationship chart below.

Looking that this chart, and considering how many generations have passed since our “mystery ancestor” migrated to North Somerset, the average cM we’d expect to find in our distant cousins (these would be descendants of our ancestor’s siblings, or cousins) is around 12 cM. However, it could range as high as 50 according to this chart.

So the DNA matches we’re finding with Sainsbury or variant spellings in their family tree and with shared DNA values up to 50 cM could suggest something like an 8th cousin relationship–which is about the relationship we’d expect for anyone who shares a common Sainsbury ancestor with us who was born around the year 1700.


The Shared cM Project 3.0 tool v4 . Red outline shows the level of cousins we’re trying to identify as a step towards finding the origin of our shared ancestor, Richard Sainsbury aka Saintsbury

So far, we’ve got the full range of cM counts in our database of about 120 “unknown” matches with Sainsbury or variant spellings in their tree!

This chart show the breakdown by amount of cM:

Number of DNA Matches (by cM amount)

So what’s next? Well, it’s a multi-pronged approach. We’re working to trace back the Sainsbury/Sansbury trees of the matches we’ve identified to see which couples they lead to who could have been the parents of our ancestor.

We’re also examining these matches to see if they might have occurred for some reason other than the fact that they have a Sainsbury in their tree. If they do, we’d need to be very careful about using information from their Sainsbury lineage to help us answer our genealogy question.

Copyright notice: Image adapted (red outline and placement of “How to read this chart” box) and used under a CC 4.0 Attribution License.