Cousins Explained


What is a cousin? Well, Cussin’ is when you say bad words that will cause gosh to send you down to heck. But there won’t be any of that in this episode, no no no. We’re talking about cousins, so don’t worry, Mom. Now, my mom is not my cousin, but she does have cousins. And her cousins are my first cousins once removed. What does that mean, to be removed? And what does it mean to be a first cousin or a second cousin? Well, if you want to find out, you have come to the right episode of Michael Draws on Pieces of White Cardstock. We’ll begin with you. That’s yourself. In my case, it is me. Now, I didn’t just appear out of nowhere. I was born. Through the union of two humans, which I call my parents. I’m gonna put my parents up here, diagonally above me. And we are directly related, meaning I literally dipped into their DNA to make myself. (Well, I mean, I didn’t do it. At least I don’t think I did. I don’t remember doing it.) But, the DNA of my parents, half from my mom, half from my dad, made me, so I share 50% of my DNA with each of my parents. But I, someday, may have children. And those people will be called, well, they’ll be called my children. I’m gonna put them down here. You’ll notice that a row on this chart represents a different generation. If my children have children, those people will be my grandchildren. But then their children will be my great grandchildren. And this continues, well, for as long as reproduction continues to happen. But my parents also had parents. I call those people my grandparents. Their parents are my great grandparents. So, this means that technically, there is no such thing as great parents. No matter how nice yours are, no matter how accepting or inspirational or understanding, or uncondtionally full of love they are, they are not great parents. No one will ever be a great parent, they will only ever be a grandparent. Then the greats get added on. Now parents can have children that are not you. Those are called your siblings. They are on the same row as your row, because you’re in the same generation. Your siblings descend directly from your parents, just as did you. But your siblings can have kids, and those kids are your nieces and nephews. CGP Grey has a great name for these: Rather than calling them nieces and nephews, let’s just call them “niblings”. The children of your siblings: niblings. Their kids follow this exact same pattern. Their kids will be called your grand niblings. Their kids will be your great grand niblings. And so on. Now your grandparents may have had kids that weren’t your parents. Their children, that aren’t your parents, are called your aunts and uncles. Your great grandparents’ children, that are not your grandparents, are called your grand aunts and uncles. And so on. Everything else we put on this graph will be some kind of cousin. And every kind of cousin has a degree and an amount of removal. In order to figure out the degree and the removal amount, we need to locate a most recent common ancestor. An ancestor isn’t just anyone on this tree. It is someone that you directly descended from. So, for example, my parents took DNA from their parents – my grandparents. So, some of my grandparents’ DNA was inside them, and then I grabbed from that. So I got some of this DNA. But my aunts and uncles are not my genetic ancestors, because I did not take any DNA from their pool. They took some from their parents, which are my grandparents, and I also got to touch some of theirs, ’cause it went through my parents. So, me and my aunts and uncles share a most recent common ancestor, that is my grandparents. Of course, they just call them their parents. Me and my sister have a most recent common ancestor in our parents. You and your great grand niece will have a most recent common ancestor that is, well, your parents. Now notice that if we pair you with anyone else currently on this diagram, there will be at least one trip to your most recent common ancestor with that other person that is direct: that passes through no generations. And that is what makes you not a cousin. If the shortest of the two journeys two people must take to reach their most recent common ancestor DOES pass through at least one generation, well then, you are some kind of cousin. For instance, let’s take a look at my aunts and uncles. Now if they have children, the children would go right here. Our most recent common ancestor will be right here. The people that I call my grandparents, and, actually, so do they. But in order to get from me to my grandparents, I have to pass through one generation: my parents. And for these people to get to my grandparents, they too must pass through one generation. The smaller amount of generations passed through is the degree of cousin-ality. In this case, we both pass through one, and the ordinal number for one is “first”. So, the children of my aunts and uncles are my first cousins. Now, because we both have journeys of the same length, passing through one generation, there’s no removal. But the children of my first cousins, which will exist right down here, That’s a whole different story. Me and my first cousins’ children have a most recent common ancestor that is my grandparents. Their great grandparents. So, if we look at these journeys, we have to go through one, two generations for them to get to the most recent common ancestor, but for me, I only have to go through one. The degree is named after the smaller of the two, so these are still my first cousins. But if both of the journeys are of different lengths, then the difference between those lengths is the removal. These people must go through two generations to get to the most recent common, and I only go through one. Two minus one is one. So my first cousins’ children are my first cousins once removed. Their children will still be my first cousins. Their degree is still one because, although their journey takes them through one-two-three generations, my journey to the most recent common ancestor just takes me through one. One is smaller than three, so they’re first cousins. But, the difference between our journey lengths is the removal number. They go through three; I go through one; three minus one is two: They are my first cousins twice removed. Now, this will continue on just like this, adding one removal for each generation. But now let’s talk about my grand aunts and uncles. Their children will be right down here. And we know it’s going to be a cousin relationship because the most recent common ancestor we share, our great grand parents, require journeys through at least one generation. I have to go up through one, two to get there. They only have to go through one, which means that they are first cousins: the degree is always the smaller of the two journeys. But, our journeys are of different lengths, which means we are removed in some way. I go through two generations; they go through one; so we are first cousins once removed. And at this point, you might say, whoa-wait, this is a little bit strange, because, my cousins’ children are my first cousins once removed. But my parents’ cousins are also my first cousins once removed. Heh. But don’t worry. That’s okay. In fact, it’s quite helpful, because if two people related to you in different ways have the same cousin name – same degree and removal – – well then they approximately share the same amount of DNA with you. Alright, now let’s move on. The children of my first cousins once removed through this pathway are gonna be here. We know they’re going to be cousins. But take a look at our paths up to our most recent common ancestor, what I call my great grandparents. I have to go through one, two generations to get there. And they have to go through one, two generations to get there. We both go through two; between two and two, the smaller is, well, still two, So they are my second cousins. And since we both go through the same number, the difference is zero, so there’s no removal at all. These are just my second cousins. But their children will be removed. They’ll still be second cousins because, although they go through one, two, three generations to reach the most recent common ancestor, I only go through two, and we always use the smaller of the two, so they’re second cousins. However, their journey goes through three generations; mine goes through two; three minus two is one: They are my second cousins once removed. Their children will be my second cousins twice removed, and so on. This pattern continues out, with my third cousins existing here, my fourth cousins off this way, and so on. But the interesting thing to ask is, how genetically similar am I to anyone else on this chart, and how can I use the chart to tell? Well, to do that, we need to know two facts: The first one is that children share half of their DNA with each parent. And, they share about half of their DNA with a sibling. So, knowing that, we can take a look at, for instance, how similar I am, genetically, to my aunts and uncles. But to be sure, before we go any further, we should remember that every individual alive today has a genome that is more than 99% similar to every other person alive today. So when we talk about DNA shared between two people, we’re talking about the proportion of that less than 1% that makes us all unique individuals. Okay, so, my aunt and uncle have some DNA. They share half of their DNA, approximately, with my parents. And I took half of my parents’ DNA, so we’ve halved this genome twice. 50%, 25%. I share about 25% of my genetic makeup with my aunts and uncles. But their children took half away from them. So although I share 25% of my DNA with my aunts and uncles, I only share 12.5%, about – really it ranges from like 7-14% – with my first cousins. The way you can tell is this: let’s take two different people. Let’s, for instance, choose me and my first cousins once removed. We take a journey up to the most recent common ancestor, and then go down… …halving the DNA shared every step of the way, except we skip the most recent common ancestor. So I share 100% of my DNA with myself. We go through one generation, so we’re gonna half once. There’s the most recent common. We half again, as we go down to my aunts and uncles – so that’s halving twice. Then we half a third time, and then a fourth time. So we’ve gone from 50%, 25%, 12.5%, somewhere around like 6%. Then 3%, then about 1.5%, and so on. This is where you will see that first cousins once removed, that are your cousins’ children, and first cousins once removed, that are your parents’ cousins, have the same, or roughly the same, genetic similarity to you. To get from me to these people, I go, as we already did, 50%-25%-12%-6%. To go from me to here, we go 50%-25%-12%-6%. So, both of these first cousins once removed are approximately maybe, you know, 6-some-odd% similar in DNA makeup to me. Now, CGP Grey also makes the funny observation that, if we continue this pattern of third-cousins (third cousins), second-cousins, first-cousins, Our siblings could, maybe arguably, be called our zeroth cousins. And I find that pretty good. I like that. But then, somewhat in jest, perhaps, he says that that might make you your own negative-first cousin. I think that’s pretty clever, but I will suggest that that doesn’t make any sense. Because, the degree of a cousin relationship tells us how many generations minimum must be passed through to reach a most recent common ancestor. In the case of your first cousin, you both pass through one. So, it’s a first cousin. In the case of your siblings, you both pass through zero to reach your parents, so zeroth cousins makes sense. But you and yourself have a most recent common ancestor that’s just your parents, so like your siblings, you’re zeroth cousins with yourself. You pass through zero generations to get to that most recent common ancestor. However, if we count traveling through a generation in the downwards direction negative motion, then I guess technically, if your own grandchildren gave birth to you, then you would be both your own zeroth cousin, and negative-first cousins with yourself. But if you remember just one thing from this video, keep this in mind: If we drew this graph out large enough, we would find that all of us, every person, every stranger on the street, me, you watching right now, every dog, every alligator, every rock, every molecule of oxygen, came from the same Big Bang. The same Everywhere Stretch. So I’ll see ya next time, family. And as always, Thanks for watching.

100 Comments

Add a Comment

Your email address will not be published. Required fields are marked *