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

What if your analysis of your match doesn’t match the shared cM chart?

Most people are able to find their Most Recent Common Ancestor (MRCA) when they share over 90 cM of matching DNA. Below that, it begins to get a little more challenging. In this February 2017 series of blog posts, I’ve been focusing on identifying the MRCA with people who share more than 60 cM with us, projected to be 4th cousins or closer. You may not have much in that range and have chosen to work with matches who share 45-60 cM with you. That’s okay. There are a few key reasons why we want to focus on this group.


Why focus on people who share 45 cM or more of matching DNA with us?

  1. It’s easier to find a MRCA with someone no more distant than that. Farther back, the holes in our trees may be a challenge. Our common ancestor may be a blank in our pedigree charts.
  2. Some of us have brick walls at that distance. If my brick wall is not my match’s brick wall, I can get a great lead. For example, I don’t know the parents of Ellen Davis, who was married in Holmes Co OH in the 1830s. If I have a DNA match whose tree has a Lorenzo Davis who was married in Holmes Co OH in the 1830s and his tree does name his parents, I have a great new lead.
  3. Suppose in the case above, that my match doesn’t have the name of Lorenzo Davis’s parents either, but we feel confident that our match is on our Davis line. Many testing companies have tools that will give us a list of people who match both of us. That may point us to another distant relative that will give us a new lead on where and whom our Davis family came from.
  4. The Shared cM Project results are admittedly less reliable as predictors of relationship the farther back we go. This data is self-reported from volunteers whose conclusions re: degree of kinship may be flawed or who may have even made typos in transcribing data. The closer the relationship, the more likely the number of participants was high and that their kinship conclusion was accurate. The more distant the relationship, the fewer the participants reported information and there is greater reliability risk.


What if you and your match share 60-90 cM (or even 45-60 cM), you’ve compared trees that go back to 1800 with no gaps, and you still have no idea who your common ancestors are. A few possible explanations may help.

Pedigree collapse is always a possibility to consider.  Imagine that your 32 individual 3x-great-grandparents each contributed an equal amount of DNA to you. A little over 3% of your genome would come from each of them—that is, 6.25% from each of the 16 ancestral couples in that generation.  If your grandparents were second cousins to each other, instead of having 16 unique ancestral couples in that generation, you’d have 14 couples appearing once in your pedigree chart, and one couple would appear twice. Instead of having 6.25% of your DNA from that couple, you’d have (on average) 12.5% of your DNA from that pair.

You may share more DNA than expected with someone else descended from that couple.  You might appear closer in time to the MRCA than you are.

The same thing is true if your common ancestor is further back. With pedigree collapse, you may share more DNA that normal from that distant ancestor, so the numbers might be closer to the figures you’d expect with a cousin whose common ancestor was closer.

If you have ancestors in a community where the culture or geography made intermarrying common (island populations, Ashkenazi Jews, early Colonial America, etc.), you may have multiple examples where the names of ancestors occupy more than one slot on your pedigree chart. This is called Endogamy. You’ll have more DNA from them than you would from forebears who did not intermarry. Your DNA shared total with a distant cousin from this branch might look like a number in the 3rd or 4th cousin range, but the MRCA could really be much farther back.

Double cousins? Another common situation is siblings marrying siblings.  For example, in the 1860s, Lew Griffith married Betsy Carpenter, and Frank Griffith married Julia Carpenter. The Griffiths were not related to the Carpenters. Their DNA test-taking descendants have 64 different names in their pedigree charts for their 4x-great-grandparents: no pedigree collapse or endogamy. But those distant cousins don’t have just one set of common 5x-great-grandparents; they have two sets in common. They have Griffith DNA and Carpenter DNA. They are not just 5th cousins, but double 5th cousins. Their total DNA may look like 4th cousins, but the common ancestors are really further back.

Matching on more than one line? This situation is a little like endogamy and a little like double cousins. Simply stated, you and your match could have more than one family in your pedigree charts in common, even if no one married close relatives. If it’s pretty far back, it may or may not affect your total shared cM much. You might have no matching DNA from one ancestor, and you and your match may have a matching chromosome segment from the second ancestral line. But I mention this to point out that just because you have matching DNA and have found a matching person in your tree doesn’t mean that DNA came from that person. You may need to flesh out the complete tree for you and your match back that far, to be confident the DNA didn’t come from a blank name in your pedigree chart. Testing closer cousins to see if they match the distant relative too can also help confirm that your matching DNA came from the line you suspect it did.

Half-cousins? Alternately, instead of having twice as many ancestors in common, two cousins could have half as many ancestors in common as expected. I’m descended from Tempest Tucker McCurdy and his second wife. Linda is descended from TT McC and his fourth wife. We are 3rd cousins, but actually only half-3rd cousins. We only share 17-18 cM. This situation doesn’t apply to the problem where you share 60-90 cM with someone but can’t find the common ancestor, but it does fall into the category of “What if your analysis of your match doesn’t match the shared cM chart?” If one of your matches is significantly less than the average for the relationship expected (in this case, the average for 3rd cousins is 79 cM, but we share less than 18 cM), consider if you might be half-cousins. (You might be full cousins after all; the range for full 3rd cousins is 0-198. But it’s worth investigating.)

NPE? This is a genealogy acronym for Not the Parent Expected. If you share 60-90 cM of matching DNA with someone, and both your pedigree charts go back six or seven generations, and there is no sign of a common ancestor…. Perhaps one of those ancestors was raised in a family that wasn’t the birth family, whether they ever knew it or not.


So—with those caveats in mind, take a look at your match list, focus on those matches greater than 60 cM or so, and see if you can identify the common ancestor. Work your way down to matches over 45 cM. Offer to share your family tree and evidence, and with luck, you’ll get valuable information in return. As well as the pleasure of new cousins!

P.S. You can definitely work with smaller matches. But finding the MRCA will be harder: try to identify any stronger matches first.


Ann Raymont © 2017


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