Using shared cM counts to help find the common ancestor: part 1 of 3

Last month I suggested that soon I’d be sharing some results of a citizen science DNA project I’m working on. However, I realize now that the proposed blog draft is too long, with too many diversions to explain some basics that might help newer readers.

So I’m taking a step back and planning several short posts first. The theme is ‘Using shared cM counts to help find the common ancestor.’


A cM, short for centimorgan, is the unit of measurement we use in genetic genealogy to evaluate how substantial a DNA segment is. The higher the cM value shared between two people, the more matching DNA they share. We can use this to help estimate how closely we are related to another tester.

But before we start looking at the DNA results, we have to remember that genetic genealogy is just a tool to support genealogy. Genealogy is the big picture – it comes before, during, and after the DNA.

So let’s make sure our genealogy background is ready before we apply genetics to it.

The first step is to pull together our family trees, back to 1800-1820 or so if we can. (It’s sometimes possible to identify a common ancestor from before 1800 with autosomal DNA, but let’s start with 1800 as an initial goal.) For me, that’s back six generations, to my 32 great-great-great grandparents. 

Image 1.


Are there any brick walls in your tree for those generations? Here’s my analysis of my roots:

12 of my 32 3x-great-grandparents were born in the U.S., all between 1800-1820.

  • 2 of them are solid brick walls—I have no idea who the parents of James Hammond and Nancy Jameson were
  • 2 of them have some evidence of parents but not “proof” (e.g. I’ve seen suggestions on other people’s trees but no documentation to indicate how reliable that information might be)
  • 8 other U.S. lines do go back further than six generations, ranging from NY, PA and New England to as far south as Maryland and Virginia.

4 of my 32 3x-great-grandparents were born in Germany. I’m not actively working on those lines right now, and I find almost no DNA matches on those lines. (For example, Germany has not permitted AncestryDNA to market DNA kits there.)

16 of my 32 3x-great-grandparents were born in Ireland. For most, I have names and the county in Ireland, but not all. My biggest gap across all 32 ancestors in that generation is in County Galway, where my great-grandfather John F. Harrigan was born in 1868. I haven’t identified all my 3x-great-grandparents on this line yet.

So—there are definitely some places where DNA might help me. Maybe I’ll find new cousins related to me through those Hammond or Jameson brick walls. Or distant relatives may emerge in other lines where I need more evidence to prove I’m descended from the people some published trees suggest. Or maybe I’ll match someone in Ireland!

Look at your own trees. Identify your brick walls and places where your evidence may be weaker than you’d like. Consider how you will share your tree with people whose DNA matches you. Will you have a public tree online? Will you print a pedigree chart from your genealogy software, save it as a PDF, and email it to your matches? Or something else?

Before we dive into the ‘genetic’ part of ‘genetic genealogy’, there is one more task to do. Make sure we all understand the various kinds of relationships we may encounter. Let’s look at my brick wall couple James Hammond and Nancy Agnes Ann Jameson. Image 2 shows some descendants.

Image 2.



When two people share a common ancestral couple, if they are the same number of generations away, they are some number of cousins. If the Most Recent Common Ancestors (MRCA) are their grandparents, they are 1st cousins. If the MRCA are their great-grandparents, the two descendants are 2nd cousins, etc. In the chart above, Alta and Lois are 3rd cousins. Jim and Ann are 4th cousins. If two people are a different number of generations distant from the common ancestor, choose the closer one to determine the degree of cousin. If the two matches are Lois and Jim, Lois is the closest to the MRCA, at the 3rd cousin level (because the ancestors are her great-great-grandparents). Jim is one degree farther away, so he is Lois’s 3rd cousin once removed. Kevin and Lois are 3rd cousins twice removed. Kevin and Ann are 4th cousins once removed.

Now with that out of the way, we are ready to talk about ‘Using shared cM counts to help identify the common ancestor’ in the next post.


Ann Raymont © 2017



2 thoughts on “Using shared cM counts to help find the common ancestor: part 1 of 3

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s