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Cluster Overlap

Do you ever feel like you’ve figured out a new way to cluster matches and then realize that those clusters you thought you had found included matches from distinctly separate areas of your tree? Well, that was my experience when I expanded my mountain/valley analysis of my Crawford line.

As explained in my DNA Clusters: Mountains and Valleys post, I had a ‘missing mountain’ for the parents of Nelson G. Crawford in my original diagram.

Nelson Crawford’s parents were James Crawford and Sally Smith Duggins Crawford. Since Sally had two sons from her marriage to Alexander Duggins prior to her marriage to James Crawford, Nelson had two step-brothers. Knowing that I had matches whose lines go back to Nelson’s sister and a step-brother, I decided to expand my Mountain and Valley diagram to include those lines.

My thought was that I could learn more about this SMITH branch of my tree from these Duggins matches and the matches we share. I only have two matches on ThruLines thru the Duggins side of Sarah’s family. Both of those matches have ONE shared match. Thus, I’m only working with FOUR matches to potentially learn more about this SMITH line. When I followed the link to the profile page for one of my matches to view the shared matches, I was excited to see the shared match marked as having a ‘Common Ancestor’!

When I clicked to view the tree so I could see who the common ancestor was, I was disappointed. The common ancestor wasn’t on my grandfather’s CRAWFORD line, but was Osmond Bland on the line of his wife’s CURREY line.

I discovered a similar situation when I looked at the shared matches for descendants of my ancestor’s sister: Polly Crawford. Two of those descendants had shared matches that were labeled ‘MOTHER’S SIDE’. 

Since my CRAWFORD line is my dad’s side of the tree, these matches labeled ‘Mother’s side’ clearly indicate that I have some sort of cluster overlap going on. I am NOT a DNA expert and can only explain this overlap based on what I know about my tree.

  • I don’t have first cousins marrying each other in the first six generations of my tree.
  • James Crawford and Sally Smith are in the 7th generation of my tree, making them my 4th great grandparents.
  • I’m looking at matches for FIFTH cousins and thus share a smaller amount of DNA.
  • My tree is colonial. The vast majority of my ancestors were in the colonies at the time of the Revolutionary War.
  • My ancestors tend to come from New England, Pennsylvania, Maryland or Virginia.

So, I have learned that I can’t make any assumptions from shared matches of identified cousins. Since I’m wanting to learn about distant generations, those shared matches could descend from a different common ancestor.
Thank you Donna Leeds for prompting me to look at my data this way! Following your blog post, Who Is in a Cluster, helped me discover the potential for cluster overlap in my tree.