Sunday, April 18, 2021

When Your Pedigree Collapses


Most people, when researching the story of their ancestors, might chart their relationships by sketching out a pedigree chart. For each generation the researcher steps back in time, the number of ancestors will usually double. 

While we each can count two parents, then four grandparents, then eight greats, that is not always the mathematical progression for every researcher's family tree. In some instances—and my mother-in-law's tree is a prime example—some branches of that ancestral tree duplicate the names entered elsewhere in the family. Cousins married cousins of various degrees of relationship.

Genealogists call that issue pedigree collapse. It's not all that rare, either. People from isolated communities—South Pacific islanders come to mind here, or residents of valley communities nestled between indomitable mountain ranges—end up marrying people whose families have been remotely related for generations. In previous centuries—such as during the early colonial period in America—limited populations of marriageable age sometimes meant a decreased count of great-grandparents.

For the unsuspecting avocational genealogist—that would be me—building a family tree for my mother-in-law meant pushing each individual family line back through generations, oblivious to the fact that that ancestor's name had also showed up on another family line, way on the other side of the ever-expanding pedigree chart. Eventually, the "oh, duh" moment hit me, and it would be time to "merge" individuals in that tree. 

Though that "rinse, repeat" mantra has been my tune over the decades, I'm still surprised to discover yet another duplicate person entered in my mother-in-law's tree. This was one of those weeks. In tracking some lines, I've discovered not only duplicates, but triplicate entries, thanks to the many lines which have intertwined themselves through the branches of Perry County, Ohio, relatives since the founding families settled in that region in the early 1800s. I'm sure even more will surface.

So what does that mean for my biweekly count? Since my research goal for April involves one of my mother-in-law's ancestors, I have certainly been adding several individuals to her tree. At the same time, having to comb through the generations to remove duplicate entries does make me feel like I'm going backwards.

However, the numbers don't say so—which is a good point for advocating the practice of keeping track of progress. After shedding so many repeated entries on her tree, I feel so much better to realize that though I may be collapsing, say, three individuals down to one, I've managed to add 197 new names to my mother-in-law's tree over the past two weeks. And that count includes all the individuals whose duplicated names needed to go "poof!" That tree has now reached 20,562 people.

Of course realizing just where the lines converge becomes important, once we introduce the element of DNA testing. DNA matches descending from intermarried lines can sometimes show up with a greater centiMorgan count—making the relationship look closer—but a telltale sign is sometimes the larger count of small matching segments. Such signs are immediately visible when using a chromosome browser, but even knowing to look for the segment count will help provide a clue that there may have been intermarried lines in a family's history. 

During these same past two weeks, I was busy adding 115 names to my own tree, as well. Of course, I'm not encountering as many duplicate entries there, and the total count on my family's tree has made it up to 25,581. But as I stretch back to those colonial times, I will begin to encounter some pedigree collapse on my mother's side, as well. It's just not as pronounced as that isolated region in my mother-in-law's home in Perry County where, for generations, a relatively small group of settlers intermarried.


  1. Intermarried lines: that helps explain all those DNA matches with many segments - even matches who are not all that close to me. Some of my low cM matches have a set of colored dots that looks like a string of Christmas tree lights.

    1. Now, that's a descriptive way of putting it, Lisa: a string of Christmas tree lights! That may indeed be what you are seeing. Of course, it certainly doesn't help with research progress. Untangling intermarried lines can be challenging!

  2. I first noticed the intermarried families on my dad’s side when I noticed the same family names showing up in Canada, then again in the Kankakee Illinois area, and finally in Cloud County Kansas. That led me to research that showed how French Canadians migrated south and then west, in tight knit groups of families. So interesting to find these stories.

    1. Yes, that is indeed a situation to watch for when researching French Canadians, Sara. You are certainly fortunate to have found information on the specific migration pattern and locations settled for that family group.

      I recently learned of another ethnic group with the same type of immigration and intermarriage story: Belgian immigrants who are of Flemish descent.

      Perhaps part of the instigation for these tight-knit communities might have been their common language which united them. In other cases, such as my mother-in-law's family, it was a unity which formed around religion, as they were early Catholic settlers in what was essentially a mostly Protestant culture of the time.


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