How Low-cM DNA Matches Can Help You Solve Genealogical Brick Walls

Many researchers using DNA to try to find their own biological parentage, or a friend or client’s biological parentage, only pay attention to the DNA matches with whom they share the most genetic similarity. In cases where the top of your list of AncestryDNA or 23andMe matches contains first- or second-degree relatives, this can indeed suffice to solve the mystery. However, many researchers find, upon receiving DNA match results, that a series of second, third, and fourth cousins populate the top of their match list. The researcher then sets about trying to discover the common ancestors of these mid-level relatives. Once the researcher has discovered the ancestor candidates, they then tediously map out all of the potential ancestor’s descendants and strive to figure out which branch of the family the friend, client, or they themselves might descend from.

Some genetic genealogy methods, like the now-well-known and incredibly helpful Leeds Method, encourage you to ignore matches with less than 90 cMs (centimorgans). However, after using the Leeds Method to identify candidates for possible common ancestors, you should not stop there. You should pay close attention to matches with 74 cMs or less, and also to matches with 29 cMs or less.


Because, let’s say that you have figured out that you likely descend from ancestors John and Jane Smith, who lived a century or two ago. John and Jane Smith may have had several children (we’ll call them Alfred, Betty, Carl, Denise, and Earl), and your next task involves figuring out which of these children may have been your great grandparent. You may not have any super close matches in your DNA match list from any of these children–not because you (or your client) don’t descend from any of these siblings, but because your (or your client’s) parent, sibling, uncle, aunt, or grandparent simply has not taken a DNA test.

However, if you have a very low cM match who descends from one of the siblings (from Alfred, for instance), the small size of the match may indicate which branch you (or your client) do not descend from.

Matches with 75 to 360 cMs can represent a second cousin, a first cousin twice removed, or a half first cousin once removed. But below 75 cMs, those relationships are no longer possible. Matches with 30 to 215 cMs can represent a second cousin once removed, a half second cousin, a first cousin thrice removed, or a half first cousin twice removed. Again, below 30 cMs, those relationships are no longer possible. Matches of 1 to 109 cMs can represent third cousins, second cousins twice removed, and other, more distant cousins. Therefore, if you find a match who is a great grandchild of Alfred and shares only 10 cMs with you (or your client), then, at best, this match is a third cousin of you (or your client), meaning that you might share a set of great great grandparents with this match, but you definitely do not share a set of great grandparents with this match. Ruling out Alfred’s branch of the Smith family tree allows you to focus on Betty, Carl, Denise, and Earl’s branches.

AncestryDNA and 23andMe’s DNA match lists, however, can drone on with hundreds or thousands of distant matches, spanning dozens of pages. How can you sift through all these results efficiently to locate low-cM matches from relevant families? Both Ancestry and 23andMe offer ways to do this, although Ancestry has better searching flexibility than 23andMe. Ancestry’s DNA match system allows you to filter results by only showing you: matches with whom you have a common ancestor (as indicated by a green leaf symbol), matches who have a certain surname in their family tree, or matches who have a certain surname in their username or display name. Ancestry further allows you to sort matches by “Shared DNA” amounts, and provides custom fields in which you can enter minimum and maximum cM numbers.

23andMe also allows you to filter your match results by surnames and other details that may appear in your matches’ profiles or family trees, although 23andMe users do not seem to build family trees in their accounts with the same frequency or thoroughness as Ancestry users.

Unfortunately, at this time, neither Ancestry nor 23andMe allow any way to sort matches from most distant to least distant, which would prove especially helpful for using this method. Thus, finding the more distant matches, especially in 23andMe, involves simply working your finger muscles and scrolling down. Interestingly, FamilyTreeDNA does allow sorting match results by most distant to least distant, although companies like FamilyTreeDNA and MyHeritage have far less people in their database than the better-known Ancestry and 23andMe.

So remember not to feel discouraged when staring at a list of DNA matches with no “smoking gun” close matches. Take a look around in the depths of your more distant DNA matches. Knowing which branches of a family you or your client do not descend from can be just as valuable as knowing which branch of a family you or your client do descend from.

Are you struggling to figure out your parentage or ancestry and feel like you may need to hire professional help? Contact Josiah Schmidt today to start with a free consultation!

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