Making the most of DNA Painter

One of the best features of DNA Painter is the interactive Shared cM Tool. Simply enter the total number of centiMorgans shared between you and your match, and the site will generate a list of the possible relationships and the probabilities for each relationship cluster. You can use the tool to check that the relationship you’ve assigned to your match falls within the expected range, or you can use the list of relationships to focus your search on your family tree for the most likely connection. The list includes not just the standard cousin relationships, but also possible half- cousins.

The underlying data for the tool is taken from the Shared cM Project, a collaborative citizen- science project that genealogist Blaine Bettinger launched in 2015. Genetic genealogists are encouraged to submit their data for known relationships, which is then collected and analysed. The latest version of the project, as of August 2017, incorporated data for more than 25,000 known relationships.

Such a tool is only as good as the user-submitted data upon which it is based, but the sheer number of submissions makes outliers easy to identify. However, you should be aware that any relationship beyond about the fourth- or fifth- cousin level is difficult to assign with confidence. This is because of the random nature of autosomal inheritance, the limitations of genealogical research, and pedigree collapse (procreation between a couple who share an ancestor, such as cousins, which reduces the size of a tree).