What is a Double First Cousin?

What you need to know about double first cousins
Marc McDermott

Many people are often confused by the terms “removed” and “second cousin” when entering the genealogy world. If you’re particular like I can be, I am constantly reminding people of the difference between the two when they call their 1st cousins once removed their second cousin, not realizing that the “removed” dignifies generation difference. With that said, often, people’s minds get frozen when trying to wrap their brains around this. So, imagine what happens when you attempt to explain to someone what a double first cousin is?

Well, let us see if I can do a good job of that.

Double first cousins can occur for several different reasons. Most commonly occurs in confined areas where the town might be small, or endogamous populations. However, it occurs when two brothers from one family marry two sisters from another family. Do not confuse yourself and imagine a brother marrying a sister.

If I told you that double first cousins share the same amount of DNA as half-siblings, that might make your head spin. However, if I use another analogy, that might make things easier. Imagine you have two brothers Don and Jon Carlson. Don falls in love with Sue Walton, who just happens to have a sister named Lu Walton. Sue married Don and Lu married Jon; thus, their children would be first cousins, right? Yes. Furthermore, both of their parents are siblings to their Aunt and Uncle. Thus, they are double first cousins.

When we have a first cousin, we usually only are genetically related to an Aunt or an Uncle, and the one we are not related to is just through marriage. In the instance of double first cousins, the double first cousins share DNA with both their Aunt and their Uncle because the Uncle is the sibling to a parent and the Aunt is a sibling to the other parent.

Are our heads still on straight?

Don and Sue have a child named “A,” and Jon and Lu have a child named “B.”

“A” and “B” share the exact same sets of grandparents.

Don and Sue gave an equal 50% of their DNA to “A,” and Jon and Lu gave 50% of their DNA to “B.”

Now, keep in mind that full siblings only share about half of their DNA because the segments they inherit from their parents are not the same. Thus, Jon and Don share 50% of their DNA, and the same goes for Sue and Lu. Due to this, their children, “A” and “B,” will share 25% of their DNA, whereas traditional first cousins share about 12.5% because they don’t share both sets of grandparents.

Keep in mind that DNA is imperfect, and we do not always inherit half from each parent. It is a sliding scale, but this is the best way to explain it to make the math easier.

Traditional first cousins only share one set of grandparents, and because double first cousins share all four grandparents, the DNA amount is generally doubled. This is why half-siblings share the same amount.

Do not be confused. Half-siblings do not share four sets of grandparents, but they share about half the same DNA from their same parent. Thus, they have two sets of grandparents but half of their parent’s DNA, which makes them half.

Tommy and Ben are half brothers who share a father. They have different mothers, but they inherited half of their father’s DNA which means they share the same two sets of grandparents. They share the same amount of DNA as “A” and “B” share with each other.

So, why don’t “A” and “B” share more if they share ALL the same grandparents?

Well, as I said above, their parents did not inherit the same segments. When Sue was born and when Lu was born, they inherited half of their mother and half of their father, but it was completely random what they were given. It is impossible for the same segments to be passed along thus. The same goes for Don and Jon. This randomness is called recombination.

We go through recombination every generation. When Don and Jon’s parents passed along their DNA, it went through recombination. When Don and Jon passed their DNA to “A” and “B,” it went through recombination again. Thus, the imperfectness with recombination is what distances us from our further distant cousins. Segments get smaller as they get passed down, and the smaller the segment generally means, the further the distance, but not always. We won’t get in that for now.

By the time “A” and “B” received their grandparent’s segments that have gone through recombination twice. Therefore half divided by two is 25%, but it went through each parent differently, which means half of their DNA has not been passed along. So we are left with 12.5% identical DNA that was passed along, intact, through recombination from Don, Jon, their parents, Sue, Lu, and their parents and all the way to “A” and “B.”

So, let me recap in different terms.

Half-siblings share the same parent. Recombination occurs from parent to child. DNA comes from the same source but directly from a parent.

Double first cousins’ DNA comes from the same source but not directly from a shared parent. It is still the same amount of DNA because all grandparents are the same. DNA comes from the same source but through all grandparents.

What happens when double first cousins have children? I am kidding; we are not going to unwrap that.

We must remember the randomness of DNA. DNA is imperfect, but, as humans do, we find ways to make math easier. As I said above, it is a sliding scale as to how much DNA people share, and that goes for every kind of relation, regardless if you’re a double first cousin or not. If we were to input two double first cousins into a chromosome browser, we would see identical segments overlapping. If we were to put a half-sibling with another half-sibling into a chromosome browser, we would see similar segments, but they would be larger and less in number. Remember, half-siblings share parts of the same parent. Double first cousin’s parents share half of their sibling’s DNA. Thus, recombination breaks things up smaller. Again, more segments throughout but smaller segment size for double first cousins and half-siblings, larger segments, and fewer segments.

People often pay no attention to websites that display a number of segments. cMs are incredibly vital to determining our relationship, but the number of segments can be just as important for these exact reasons.

I hope your heads are still on straight. If not, take a break and come back. Generally, this is something that I always have to come back to after being out of practice for a while. It is not like other things where you learn it once, and it stays forever.

DNA and genetic genealogy are so meticulous and so new that it is very easy to overload the brain and intimidate the ego. Still, I guarantee the more you remain persistent, the more likely these things will eventually settle in to stay. This goes for all genetic practices, not just the incredibly confusing concept of a double first cousin.

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