Calling all DNA enthusiasts: help me explain this anomaly

because it's driving me nuts

It’s an anomaly. Doctors love anomalies. Dark spot on an x-ray, bright spot on an MRI…. Killing that second inmate is the homicidal equivalent of blood in the urine. It doesn’t fit. I’m interested in things that don’t fit. Tell me why you did it. Your other victims you were almost bragging about. What was different about this guy?

That is a snippet from an episode of House MD I was watching the other day. I found that particular quote so relatable. And currently, I’m working on something that definitely falls into the “anomaly” category. I would love it if others who use DNA in their research, especially if you are familiar with endogamous DNA, could chime in on what I’m about to describe, because it’s throwing me for a loop.

I was recently contacted by a cousin match on Ancestry. She is trying to figure out the identity of her great-grandfather, and I have been trying to help as best I can. I don’t want to talk about her particular mystery ancestor, but rather about something else I noticed analyzing her results that I just can’t get out of my head.

Her match counts. They don’t make any sense!

So anyone who is familiar with working on Acadian or Creole or Isleño DNA knows the endogamy is real strong there. And, in every other case I’ve ever seen, the endogamy causes our match counts to be seriously inflated. The “normal” amount of matches for someone from these communities is anywhere from 100,000 to 175,000 people, in my experience. This is true for me and my mom. My dad, who only has one Creole grandparent, has about 70,000 paternal matches.

However, this cousin of mine has only around 20,000 matches total.

This is made even more puzzling by the fact that all four of her grandparents were from south Louisiana, and all of her lines were in Louisiana prior to the Louisiana Purchase. She has a lot more Spanish ancestry than French, but she also has a significant amount of Acadian admixture showing on her test, an amount similar to what my dad shows. And the rest is essentially Isleño. Her tree and my maternal grandmother’s tree share a lot of overlap. Her tree looks very typical for my cousins on that side.

The next question, of course, is whether her tree is correct. Does she have a recent NPE on one or both sides?

The answer to that is, as definitively as it can be, no. She has already mapped out a huge number of her matches, and her DNA matches her tree. In all my analysis of her tree and matches, I haven’t found any cases where her tree didn’t make sense. In ThruLines, every single ancestor at every generation has numerous cousin matches associated with them. The only gap in her tree is the one we are trying to solve, a great grandfather. This great grandfather may have been German, which may explain the lower match count on her mom’s side, but not on her dad’s side, which is the side she got almost all of her Acadian DNA from. But his side, too, has only about 13,000 matches. They aren’t all hiding in “both” or “unassigned” either, although there’s definitely a good amount of overlap between her maternal and paternal sides. Working with her matches is headache inducing because of the many relationships amongst all of the matches, just like I’d expect.

How could this be? Anybody have any ideas? This is really bothering me, because with my current understanding of How DNA Works, I would have thought that it wouldn’t be possible for someone from all of these endogamous communities to wind up with so few matches. If she only had one connection to the endogamous bunch, perhaps the DNA randomizer just hit in such a way that she barely got anything from those lines. But for ALL of her ancestors to be from the group… I just can’t make that make any sense.

Have you ever seen such a situation? Do you have any hot takes for what may in theory be causing this? If you’re from one of these endogamous groups, how many matches do you have on the relevant side(s)? I need more data. 😂