Shared Centimorgans: See How You’re Related
Want to know how you’re related to your DNA matches? Here’s how to answer this kind of genetic genealogy question using your total shared DNA and the free Shared cM Project.
It’s the most basic of DNA testing questions: how I am related to all those matches? The testing companies can’t tell you for sure, unless it’s a super-close relative like a parent or sibling. So it’s up to you to figure it out. Here’s how to use information about your genetic match and the Shared cM Project to figure out the possibilities for how you may be related.
But first, a definition:
A centiMorgan (cM) is a unit of measure for DNA. Your total shared cM tells you how much DNA you share with another match. In general, the more DNA you share with a match, the higher the cM number will be and the more closely related you are.
Now, here’s how to use the amount of your shared DNA to determine your possible genealogical relationship to your DNA match:
1. Find the total amount of DNA you share with your match.
Every company tells you how much total DNA you are sharing with your match. Some of them also estimate your possible genealogical relationships. Here’s what you’ll see at each testing site:
23andMe: On 23andMe, you can see the percentage of shared DNA from the main DNA Relatives home page:
You can use that percentage in the Shared cM Project or you can click on that match to see a detailed view, to find the total shared DNA in cM.
AncestryDNA: With AncestryDNA, next to each person in your match list, you will see how much DNA you share (below: “332 cM across 16 segments”).
Clicking on the little gray icon with an “i” in it will bring up a chart much like what we’ll describe below at the Shared cM Project to help you see various genealogical relationships that might fit your genetic relationship.
Family Tree DNA: On the main match page for your Family Tree DNA Family Finder results, you will see a genetic relationship range reported in the third column on your match list, followed by the total amount of shared cMs and the size of the biggest piece of shared DNA in the fourth and fifth columns, as shown below.
Living DNA: Known as the test that gives you the most British Isles-specific ethnicity reports, Living DNA is still working on its DNA matches experience. At this point, it reports just total shared DNA and a simple predicted relationship, as shown below:
MyHeritage DNA: On the MyHeritage website, the total amount of shared DNA, total number of shared segments AND even the size of the largest segment is shown on the main match page under the title Match Quality:
2. Take shared DNA data to the Shared cM Project
The total shared cM and any estimated genealogical relationships are a great start, but they’re not usually THE one and only, super-confident answer you’re looking for when you ask, “How are we related?”
Your next step is to take the total cM of shared DNA to the Shared cM Project. Blaine Bettinger at thegeneticgenealogist.com spearheaded this collaborative effort. He collected the shared cM data for known relationships from genetic genealogists just like you. The resulting free chart gives you a good estimate of how much DNA should be shared for different relationships.
How to Use the Shared cM project
To use the chart, take the total amount of shared DNA you have with a match, and look up that number in the chart to get an idea of what kind of genealogical relationship might best fit the genetics that you see. For example, if I share 69 cM with my match, we might be third cousins. But we might also be second cousins once or twice removed.
Now, how you determine which is the correct relationship? Simply put: do genealogy research! It’s time to use traditional records and research skills to better understand the genetic clues in your family history mysteries.
Jonny Perl and Leah Larkin have taken the Shared cM Project even farther. He has created an interactive Shared cM relationship chart on the DNA Painter website. You can input the shared cM value and it will highlight the possible relationships for you:
Notice in the above case that it is about 34% likely that you have one of those four listed relationships. A half 3C would share one of your 16 2X great grandparents. A 3C1R is a third cousin once removed, and indicates that this person is on a different generation than you are; they are either older or younger than you.
It is important to realize that these numbers are just estimates and honestly, as long as the probability of your relationship isn’t zero, you are probably OK. These numbers are just for you to use as a guide to help you know when you should start looking for your common ancestor with your match.