What’s a CentiMorgan, Anyway? How DNA Tests for Family History Measure Genetic Relationships
If you’re doing DNA tests for family history, you may see lots of predicted cousin matches: 2nd, 3rd, 4th, etc…..But what does that predicted genetic relationship actually mean? Learn about centimorgans, the powerful genetic genealogy unit of measure, and how it helps your research.
How DNA Tests Measure Genetic Relationships
When we are looking at genetic relationships, there are also many ways we can measure them. But ultimately, we want the testing company to tell us how likely it is that a particular individual shares a single, recent common ancestor with us. One factor in this calculation is to take into account the total amount of DNA we share with that match.
Currently, all the testing companies are reporting this sum in centimorgans (cMs). Every company reports to you the total number of shared cMs, as outlined below.
AncestryDNA: Click on the match to access the personal profile page for that match. In the second section, under Predicted Relationship, you will see the confidence level. To the right of the confidence level, you will see a grey circle with a little “i” in it. Clicking there will show you the total amount of shared cMs as well as how many pieces of DNA you share.- Family Tree DNA: On the main match page for your Family Finder results, you will see the total amount of shared cMs in the third column.
- 23andMe: You can see the percentage of shared DNA from the main DNA Relatives home page. To convert the percentage into centimorgans, just multiply your percentage by 68 (that will at least get you close). You can also see total shared cMs in the chromosome browser tool (go to Tools > DNA Relatives > DNA).
- MyHeritageDNA: The total amount of shared DNA is shown on the main match page under the title Match Quality. MyHeritage also has a new DNA Match Review page. Click here to read more about that.
Centimorgan: A Genetic “Crystal Ball”
It is very tempting to think of a cM just like you would think of an inch or a centimeter, and for all practical purposes, that is okay. But it is actually much more complicated than that.
A cM is actually more like a crystal ball: it helps us predict how likely a piece of DNA looks exactly as it did a generation ago. This, in turn, helps us calculate how far back we should be looking for the common ancestor between two people.
But for our practical purposes, you can use the total amount of shared DNA, in combination with this chart compiled by Blaine Bettinger and the Shared cM Project, to better assess your genealogical relationship with your match based on your genetics.
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.
How do you figure out which one? 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.
My series of DNA quick reference guides can help you get the most out of your DNA tests for family history. I definitely recommend the value-priced bundle of all 10 guides. But I especially recommend the guides listed below if you’re to the point where you’re trying to understand what genetic relationships mean:
- A Guide to AncestryDNA
- A Guide to Family Finder at Family Tree DNA
- A Guide to 23andMe DNA
- Breaking Down Brick Walls with DNA
- Organizing Your DNA Matches
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DNA Matches: What You Can Do with All Your Genetic 4th Cousins
(Update 2020) When genealogists take an ancestry DNA test, they are looking for more than just their ethnicity results. They are also very interested in receiving information on other people who have tested who closely genetically match them. They want to know who the closest matches are, and if those matches have family tree information that they can share.
However, with all the people testing these days, (some being genealogists and some not) the volume of matches can become overwhelming very quickly.
Are you one of those people who have way too many genetic “4th cousins or closer” among your DNA matches? Have you ever wondered “What do I do with all these matches?!” If so, keep reading. We’re going to explore some of your options, and most importantly, how to determine how genetically close your cousins really are.
Doing the DNA Math on Your Cousins
Math can provide us with a degree of certainty in genetic genealogy. Each of us has two biological parents. We have four biological grandparents, and eight great-grandparents.
However, the farther back we go the less we can rely on math.
For example, on paper you should have sixty-four 3rd great grandparents. However, many of us find that the same person occupies more than one slot on our pedigree chart. While this significantly decreases the workload for traditional genealogy, it adversely impacts your genetic genealogy. Especially when it comes to that long long list of 4th cousins you have in your match list at any of the three major DNA testing companies.
Depending on how intermarried your lines are, you may be seeing individuals on your match list that genetically look like your fourth cousins, but they are genealogically your sixth cousins – EIGHT TIMES! So how can you tell the difference?
There are two parts to that answer: one you can control, and the other you can’t.
Distinguishing DNA Matches with Genetic Tools
While your fourth cousins and your eight-time-sixth cousins may look similar genetically, there are often small clues in the genetics that can help you tell the difference. This distinction can sometimes be detected by a testing company who, through research and validation, has been able to fine-tune their algorithms to detect these subtle differences.
Your Genetic 4th Cousins
You can participate in this double checking process by using some of the genetic tools that are available to you at Family Tree DNA, or at Gedmatch.com. But since you may not be an aspiring geneticist, let’s focus on the genealogical work you can do to determine if a match is truly a fourth cousin.
Use Google Earth to Plot Your DNA Matches
A fourth cousin designation just means that you and your match are separated by between six and twelve degrees (people). So that might be five back on your chart to your common ancestor, and five down to your match, which would make you true fourth cousins. It could also be some other permutation of that.
For our example, let’s assume true fourth cousins. That means that the two of you share one of thirty-two 3rd great grandparents (16 couples). In order to find out which set, you have two genealogical identifiers: surname and location.
Therefore, the first thing you should do is make a list of the surnames and locations of those thirty-two 3X great grandparents.
Now, most of us do not know all 16 of those couples, so you are going to have some holes. Feel free to fill in those holes with surnames on subsequent generations that will carry through to this fifth generation.
A great tool to plot your own list of ancestors geographically is the free downloadable Google Earth software.
Check to see if you have the latest version of Google Earth downloaded to your desktop or laptop computer. On your desktop, look for a grey and white globe. If you see a blue and white globe, you have the older original free version of Google Earth. However, a few years ago, Google made their Google Earth Pro version free to everyone, and it is now the standard.
If you do have Google Earth Pro (the grey globe software) then you’re ready to go.
The grey Google Earth globe on the desktop.
If you don’t have it, then you will need to download it.
How to Download the Free Google Earth Software:
- Go to http://www.google.com/earth/download/gep/agree.html
- Click the blue download button
- Read the Terms and Conditions
- If you agree to them, click the Agree and Download button
- Follow the installation guide
- When complete click Run Google Earth
Now that you have Google Earth, you can begin by creating a folder in the Places panel in Google Earth devoted to your 16 couples. Here’s how:
1. In the Places panel, right-click on MyPlaces and select Add > Folder:
Right-click on MyPlaces > Add > Folder
2. Name the folder and then click OK:
Creating a folder in Google Earth
3. Now you will see your new DNA folder for your 16 couples in the Places panel. If you don’t see it, look toward the bottom of the list. You can move the folder to any location within the list by dragging and dropping it.
Create a folder in Google Earth for DNA 16 couples
Once you have your DNA folder created fro your 16 couples, you can then easily plot your surnames and locations.
How to Plot Your Surnames and Locations in Google Earth:
1. Click your new DNA folder to select it. This will ensure that the placemark you are about to create will be stored in that folder.
2. In the search box (upper left corner of the Google Earth software) type in the first location and click Search. Google Earth will fly to that location on the map.
Type the locaton in the Search box and click Search.
3. In the toolbar along the top of the screen, click the placemark button to place a pushpin in that location:
Click the Placemark button in the Google Earth toolbar.
4. In the Placemark dialogue box, enter a title for hte pushpin placemark. Click the OK button to close the box and set your placemark.
5. Repeat the process for all the locations.
Then evaluate the fifth generation of your fourth cousin matches for genealogical information that lines up with any of the items on your list.
You can also plot the surnames and locations of your matches in Google Earth. This is where Google Earth really comes in handy. The free software makes it very easy to see when your ancestral home may be bordering the locations of your matches. Those with whom you find a similarity become your best matches, and your best chance of determining your connection. Those without an obvious connection cycle to the bottom of your pile for a genetic evaluation. You can perform these same kinds of searches for your second and third cousins as well.
As you begin to become more familiar with the fifth generations of your matches, you may also start to see patterns of surnames or locations emerge among your matches. These then become the surnames and locations that might be able to fill the missing spaces in your pedigree chart.
More Genetic Genealogy and Google Earth Gems
If you are new to using Google Earth, I have several suggested resources for you by Lisa Louise Cooke:
Learn more in Premium episode 131.
- How to Use Google Earth for Genealogy (FREE VIDEO)
- Google Earth for Genealogy Video Training digital download
- If you are a Genealogy Gems Premium member, Tune in to Genealogy Gems Premium podcast, episode 131, for tips on how to run a more thorough genetic evaluation on those 4th cousin matches.
Here’s a video of the authors discussing three common DNA misconceptions:
Authors: Diahan Southard and Lisa Louise Cooke