The Family Tree that Ate Itself:
Understanding Pedigree Collapse
A typical family tree looks like this, with you at the bottom and spreading out vertically as your ancestors multiply, from 2 parents, to 4 grandparents, 8 great grandparents, 16 great grandparents, and so on.
But probably at some point for all of us a portion of our tree resembles more of a diamond shape. This occurs when related people marry each other, such as cousins marrying cousins. Their children, instead of having 8 unique great grandparents, will instead only have six.
This phenomenon is called Pedigree Collapse. It is especially common in communities that were historically small, geographically isolated, or socially insular. One clear example is rural communities and religious groups who would not marry outside their faith, such as Latter-day Saints. It is also common within certain royal lineages. In my family, you can see that my 2nd great grandparents were cousins. They were early settlers of Utah, living in small communities. There were not many options for marriage, so cousins often married each other. This means their children only had three unique great grandparents, instead of the typical four.
When communities experienced widespread intermarriage within the same group over many generations, this creates something different called Endogamy, which is a population pattern, and not a one-off event like Pedigree Collapse. These groups can be found in parts of the British Isles, the Caribbean, or the Azores. For generations, these populations were limited by geography, weather, and economics. People married within walking distance of home because travel was difficult and newcomers were rare.
A second example is religious minority communities, particularly Ashkenazi Jewish populations in Eastern Europe. For centuries, Jews were legally restricted in areas where they could live and whom they could marry, leading to tightly knit communities with limited marriage pools. Marriage within the group was not a choice so much as a condition of survival.
How Can DNA Help?
Pedigree collapse becomes especially relevant in the context of autosomal DNA testing. DNA tests estimate relationships based on how much genetic material two people share. Under typical circumstances, the more DNA two people share, the closer their relationship is expected to be. A parent and child share about 50% of their DNA. First cousins share much less. Second cousins share even less. These averages form the basis of the relationship labels assigned by testing companies.
When pedigree collapse is present, these assumptions begin to break down. Instead of sharing DNA through a single ancestral pathway, two people may share DNA through several different pathways. Each shared ancestor contributes a small amount of DNA, and when those contributions add together, the total amount of shared DNA can look unexpectedly high. As a result, distant cousins may appear genetically closer than they truly are in genealogical terms.
For example, my DNA matches me to someone we’ll call Person 1 at 283cM. With that much shared DNA, we have about a 59% chance of having the following relationship:
However, in reality, we are 2nd cousins once removed, which had a much smaller likelihood at only 16%:
The average 2nd cousin once removed shares 122cM with a tester, while I share over double that amount with mine. The reason the algorithms suggest a higher percentage is because we have shared inherited DNA from two different branches in the family.
When we see DNA matches, we have to be careful assuming that the shares amount of DNA is only coming from one line. It may be that instead of our matches being unusually close relatives, instead they are all connected to our shared ancestral couple in more than one way. Each connection passes down a small amount of DNA, and when those small pieces add up, the total shared DNA looks closer than expected for a single relationship.
When DNA results look confusing like this, it helps to stop focusing on one match at a time and instead look at the bigger picture. Rather than asking “Who exactly is this person to me?”, it is often more useful to ask, “What do many of my matches have in common?” For example, if several of your matches all trace back to the same couple, like Charles Delaitre Wilbur and Calista Wilton Scammon in our example, that shared connection matters more than the exact amount of DNA you share with any one person. Even if the numbers look different, the repetition of the same ancestors across many matches is a strong clue that pedigree collapse is involved.
It’s also helpful to notice groups of matches who all match each other. When many people are connected in this way, it usually means they all inherited DNA from the same ancestors through overlapping family lines. This kind of interconnected group points to long-standing family overlap rather than a recent or unexpected relationship.
If you can sort your matches by your mother’s side and your father’s side, that can add even more clarity. In families affected by pedigree collapse, the same ancestors can appear on both sides of the family tree. Seeing the same group of matches connected to both sides may feel surprising, but it is a common and telling sign of pedigree collapse.
What Can You Do Now?
The emotional side of this process should not be underestimated. Many people approach DNA testing hoping for clarity and simplicity. When the results instead reveal complexity, it can feel overwhelming or disappointing. Some worry that they are missing something obvious or failing to understand their own family. Seeing pedigree collapse does not mean people are closely related in a troubling way. It simply means the community shares a long, continuous history. DNA testing reveals this history in a way that paper records alone often cannot. Pedigree collapse often tells a story of stability and community. It reflects generations of people who lived near each other, relied on each other, and built lives within a shared world. DNA captures that closeness. What looks confusing at first is often evidence of deep roots rather than hidden fractures.
Understanding pedigree collapse also helps prevent mistakes. Without recognizing it, researchers may assign incorrect relationships, chase ancestors who never existed, or assume events that did not happen. Taking time to understand the broader historical and geographic context protects both the accuracy of the research and the emotional wellbeing of those involved.
DNA is an extraordinary tool, but it does not replace thoughtful interpretation. It reveals patterns, not just people. Pedigree collapse reminds us that family history is rarely neat. It is shaped by culture, geography, faith, and circumstance, all layered across generations. If your DNA results feel crowded, confusing, or contradictory, it does not mean something is wrong. It may mean your family story is richer and more interconnected than you expected. With patience and context, those connections can become a source of understanding rather than frustration. And when the picture still feels overwhelming, having an experienced guide can make all the difference.
If you are feeling overwhelmed by interpreting your DNA results, Price Genealogy is equipped to help you. Reach out today for help with your pedigree collapse; there is a lot to be uncovered.
James/Emily
Sources:
- “Pedigree collapse.” Wikipedia, last modified 2025, accessed December 23, 2025, https://en.wikipedia.org/wiki/Pedigree_collapse.
- DNA Painter, The Shared cM Project 4.0 tool v.4, https://dnapainter.com/tools/sharedcmv4.
Photos:
- OpenAI. Illustration of a person conducting genetic genealogy research. AI-generated image, generated January 5, 2026.
- Genealogy, Public Domain, https://unsplash.com/photos/a-table-topped-with-lots-of-old-maps-tn71061qj5M