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Common Ancestor Puzzle

Do you ever participate in one of Blaine Bettinger’s DNA quizzes? Or, do you ever see someone else’s post about their DNA stats and decide to investigate your own? I know I often take the time to look at the data and see how my data compares.

That happened two days ago when someone (and unfortunately I don’t know who and can’t find the original post) posted about his Ancestry DNA Common ancestors. Basically, the poster indicated how many common ancestors he had at each cousin level and wondered how that compared to other testers.

Thus, I decided to look at mine:

  • One 1st cousin
  • Three 2nd cousins
  • Five 3rd cousins

With over 3,000 4th cousin or closer DNA matches, I expected a lot of common ancestors at the 4th cousin level. Thus, I was shocked to only find EIGHT.
Out of curiosity, I looked at my incomplete ThruLines spreadsheets to see how many cousins I have identified so far.

Not only do the numbers differ for 4th cousins, but they also differ for how many 2nd and 3rd cousins I have descending from a common ancestor. 
This sent me back to my matches list. When I looked at the number of shaky leaf icons in my matches list, the number of the shaky leaf icons for 1st thru 3rd cousins matched the number of cousins using the common ancestor filter.
The next level is a little deceiving. On the matches list this level is 4th thru 6th cousin — not just 4th cousins. So I started counting the shaky leaf icons. I had counted 155 shaky leaf icons and was still in the 4th-6th cousin section. Since I had reached a match length of just 22 cM, I stopped counting.
Once I realized that the shaky leaf is not always synonymous with common ancestor, I went back and started looking for the term ‘common ancestor’ by the shaky leaf. I quickly counted over 8 4th-6th cousins with the words ‘common ancestor’. Studying the first 15 of those ‘common ancestor’ matches, I found it odd that three of the 15 matches had two trees with a tree size of 7 and one tree in the list with a tree size of 2. When I looked at those very small trees, a common ancestor was not shown on the tree.

Since I descend from Isom Gallimore, it is possible that Isom Gallimore is our common ancestor. However, nothing in my match’s tree connects my match to Isom Gallimore. Yet, this match comes up using the ‘Common Ancestor’ filter.

Going back to my match list, I wondered why some of my known cousins don’t appear as common ancestors. For example, several descendants of my 2nd Great Grandfather, George Mentzer, have tested but aren’t listed on the ‘common ancestor’ list. 

This match descends from George Mentzer thru his son Ernest. My tree not only contains information on George Mentzer, but on his children and grandchildren.

Thus, our two trees should contain enough information to connect our two trees. Thus, I expected to find this match on the ThruLines screen for George Mentzer. 

Unfortunately, the ThruLines screen does not show George’s son Ernest and the link to my DNA match — even though our trees both contain Ernest as a son of George Mentzer.

Since this Mentzer match has an unlinked tree, I’m wondering if that is keeping the ‘common ancestor’ and ThruLnes from working for this match.
Unfortunately, I don’t have an explanation for only EIGHT 4th cousin matches showing up  with the ‘Common Ancestor’ filter. 

Can anyone else explain this?