Month: August 2022

The Shared cM Project is a collaborative research project that uses data from 100s of genealogists to generate shared DNA ranges and averages for nearly 50 different genealogical relationships. The most recent version of the Shared cM Project, Version 4.0, was released on March 27, 2020 (I had so much free time in the early days of the pandemic!). See “Version 4.0! March 2020 Update to the Shared cM Project!”

Several years ago, Jonny Perl – creator of the invaluable genealogical website DNAPainter.com – graciously converted the results of the Shared cM Project into a dynamic online tool. With this FREE tool, a genealogist can enter an amount of DNA shared (in cM) with a match at a testing company, and the possible relationships for that shared amount will highlight in the graphic. In addition, clicking on any of the relationship boxes produces a pop-up that reveals the histogram for that relationship (arguably the most valuable aspect of the Shared cM Project). ... Click to read more!

TL:DR Executive Summary:

1. The size of a “small segment” is not strictly defined but is generally less than 10 cM and preferably less than 7 cM;
2. A very large percentage of small segments are not valid matching segments (i.e., are false segments), and our ability to distinguish valid segments from false segments is either impossible or very limited;
3. Although we cannot reliability utilize a small shared segment as genealogical evidence in most cases, that does not negate the potential value of a genealogical relationship;
4. There is no evidence that: (1) triangulating a small segment; or (2) sharing a large segment in addition to the small segment; or (3) finding a shared ancestor, increases the probability that a small segment is valid;
5. Even if a small segment is somehow identified as valid, perhaps an even bigger challenge is determining whether the small segment (which can be 10, 20 or more generations old) came from the identified ancestral line or another known or unknown shared ancestral line.

Small segments have long been a controversial subject in the genealogy community. Some love them, some hate them. Here we will look at all the available evidence surrounding small segments, some of the misconceptions associated with them, and some of the ways we might be able to utilize small segments in our research.

What are “Small Segments”?

One of the biggest problems surrounding the discussion of small segments it that there is no strict definition of the term. A “small segment” to one person might be anything 5 cM or less, while a “small segment” to another person might be anything less than 10 cM. And there are many other variations. For purposes of this analysis, we will define a small segment as any single segment of DNA less than 8 cM (although segments as high as 10 cM or more can be problematic as well). ... Click to read more!