One of my favourite things about being a genealogist is discovering new cousins. I have a lot of cousins now, some very distant, some a little closer. I have emailed back and forth with some, friended some on Facebook and even talked to some on the phone. It’s been really wonderful making all these connections.

I think one of the things that people don’t realize is what the DNA testing companies mean by “cousin” and what is traditionally meant by “cousin” are not really the same thing. According to the testing companies, my great-aunt counts as a cousin. 2nd-3rd cousin could be a 2nd cousin once or twice removed. What does that even mean?

I think everyone is clear on the idea of a first cousin, the kids of your parent’s siblings. You and your first cousins share grandparents. You and a second cousin will share great-grandparents. But what about when the generations are different? For example, my parents’ first cousins and I don’t share the same grandparents. Their grandparents are my great-grandparents. That’s where the whole removed part comes in. We are one generation apart, and we are first cousins. Therefore we are first cousins once removed. My kids and my parents’ first cousins are two generations apart. They are first cousins twice removed. I really like using this cousin calculator to help me figure out how two people are related. Here is a great chart.

It’s also interesting that not only are the testing companies different on what counts as a cousin, between the different testing companies you’ll likely see variation on how close your cousin is. Here’s a great comparison with one of my distant cousins who happens to be on Ancestry, FTDNA and Gedmatch. Since this cousin and I figured out our actual relationship, I’m going to show you what each company decided our relationship was and talk about some of the things that can influence this.

Ancestry is the most conservative of the testing companies. Here Ancestry tells me we are 3rd-4th cousins, sharing 91cMs.

FTDNA is the most liberal of the testing companies. According to FTDNA, we share 112cM, putting us in the 2nd-4th cousin range. Let me tell you, I’ve got A LOT of 2nd-4th cousins! It was pretty exciting until I realized that none of them were even close to being 2nd cousins, and we’re likely even farther than 4th…

Gedmatch has a lot in common with baby bear’s oatmeal…not too hot or too cold. According to Gedmatch we share 106.4cM and our MRCA is 3.5 generations from us, so again, in that 3rd-4th cousin range.

And the winner is….NOBODY! Everyone guessed too low. When I contacted this cousin I found out she is related to me in two different ways on my father’s maternal side. This cousin is actually my 5th cousin once removed! My dad’s side is from an endogamous population. The Legal Genealogist wrote a really informative post about it, but chances are, if you’re from an endogamous population, you’ve already discovered what that means in practice. Pedigree collapse, when there’s intermarriage in your family line, can also cause this. Most of the time pedigree collapse and endogamy happen together, but not always. My dad’s parents aren’t actually related to each other, despite coming from the same endogamous population. Sometimes segments are just “sticky” (I’m guessing that’s not the technical term, but that’s how I’ve seen it referred to). The testing companies are basing their assessment on how many cMs you share. If a bigger segment happened to get handed down whole to you from a very distant ancestor, it can look like you’re more closely related to someone than you actually are.

Ultimately, it is far more useful to use this tool from DNApainter to establish the relationship rather than what the company (including gedmatch) tells you the relationship or genetic distance is. However, even a tool as good as this one can’t take into account things such as endogamy that deflate the genetic distance between you and a match. The DNA calculator puts the odds of 5C1R at less than one percent (but at least it was an option). Maybe one day we’ll understand enough about how DNA works that the companies will know how to take this into account. But until then, if you come from an endogamous population, keep this in mind.