Endogamy is a word that gets bandied around a lot in genetic genealogy circles, but what it means and how it affects our work is less clear. This post is the first in a series about what endogamy is, why it matters, how to detect it, and how to work with it.
Endogamy is the practice of mating within a specific group. All human populations have practiced endogamy to one extent or another. Some still do. Endogamy can occur because the group is geographically isolated from other people, like Native Hawaiians were; because they prefer to marry within their religion, ethnicity, language, and/or social caste, as most cultures do; or for other reasons, like consolidating power among royalty.
Key to endogamy is that the group is small enough that, over time, marriages occur between cousins. Not necessarily first or even second cousins (although that can occur), but between third, fourth, and more distant cousins. Over and over. And over.
It’s important to remember that endogamy is not incest, which is sexual relations between close relatives, like a father and daughter or uncle and niece. Incest is associated with a substantial risk of early death or genetic disorders in the child, while marriages between even first cousins are much safer.
I’ll discuss the health implications of incest and endogamy in a later post.
Endogamy and Pedigree Collapse
Endogamy causes something called pedigree collapse, but not all pedigree collapse rises to the level of endogamy.
Consider the example below. The home person (at the bottom) is the child of parents who were third cousins to one another. That is, the parents shared a pair of great-great grandparents. As a result, their child (the home person in the diagram) has 30 unique great-great-great grandparents instead of the expected 32. One set of 3-great grandparents shows up twice in the child’s tree. We say the pedigree is “collapsing” rather than doubling in number with each generation back, as we’d expect.
But pedigree collapse is not endogamy. Pedigree collapse is one or a few isolated incidents of cousin marriage, while endogamy occurs repeatedly over many, many generations.
This is endogamy:
This is my mother’s tree. She’s Cajun, a culture that was geographically and culturally isolated in southern Louisiana and, before that, in what is now Nova Scotia. Cajuns have been marrying mainly within their own population since the 1600s.
My mother’s parents were fourth cousins. I don’t think they knew, because my grandmother’s father was born out of wedlock. My grandfather’s parents were third cousins; they definitely knew. There’s no known incest in this tree, but the cousin marriages go on and on, back to the earliest settlers in Port-Royal, Acadia (now Annapolis Royal, Nova Scotia) in the early 1600s, because there simply weren’t a lot of options for marriage partners.
The closest cousin marriages I’ve identified in this tree are between first cousins. Consider Isaure Marie Guidry (1863–1933), my mother’s great grandmother. In the diagram above, she is the left-most female (pink) ancestor just below the horizontal line for 1850. Her parents, Alexis Onésime Guidry and Palmire Dupré, were first cousins through their shared grandparents Louis–David Guidry and Marie Modeste Borda.
To complicate matters even more, Onésime had been widowed before marrying Palmire. His previous wife, Celestine, was Palmire’s older sister, and Celestine had a daughter named Marie. So Marie and Isaure were half sisters their father Onésime, and first cousins through their mothers Celestine and Palmire. (This combination is often termed three-quarter siblings.) But they were also second cousins through Onésime and their mothers. Each unique relationship path is shown in red below.
Technically, Marie and Isaure were second cousins twice over, once through Onésime and Celestine and once through Onésime and Palmire, but you get the picture. It’s enough to make your head spin!
How Does Endogamy Affect Genetic Matching?
Isaure and Marie died more than 75 years before the advent of genetic genealogy using autosomal DNA, but what would their match to one another look like if we could analyze their genomes today? As half sisters, we’d expect them to share about 1750 cM, as first cousins another 850 cM or so, and as double second cousins roughly 200 cM twice over. In many parts of their genomes, they’d match on both copies of their two chromosomes, much like full siblings do. In fact, they might well be indistinguishable from full sisters using the methods we use for genealogy.
While Isaure and Marie are an extreme case, DNA matching is affected to some degree in all endogamous populations. People who are no closer than fourth cousins might share enough DNA to be predicted as third cousins, because they’re picking up “extra” shared DNA through their other relationships.
For example, my mother shares 184 cM with D.M. If you were to plug that number into the DNA Painter SCP tool, you’d see a combined probability of 89.1% that they were either in the second cousin group (38.8% chance) or the second cousin once removed group (50.3%).
In fact, their closest relationship is third cousins, who average only about 50 cM. On the other hand, Mom and D.M. are also third cousins once removed twice over, fourth cousins once removed, and fifth cousins … that we know of. All those distant relationships add to the shared centimorgan tally.
Thus, the overall effect of endogamy is to make many of our DNA matches appear to be more closely related than they really are. This complicates everything, from basic relationship prediction to more advanced and powerful techniques, like the Leeds method and the What Are the Odds? tool.
In subsequent posts, we’ll address how to identify endogamy in a family tree, how to identify it using only DNA match lists, gauging how much endogamy is present, best practices for genetic genealogy, and some health implications.
A Fun Thought Experiment
Is the entire human population endogamous? After all, we only mate (well, mate successfully) with other humans and have been doing so for ten thousand years or more, since the last archaic humans, like Denisovans and Neanderthals, died out. Technically, we’re all (very distant) cousins, and all of our pedigrees collapse eventually.
What do you think?