Introduction to Mendelian Genetics
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Introduction to heredity
Well, before we even knew what DNA was, much less how it was structured or it was replicated or even before we could look in and see meiosis happening in cells, we had the general sense that offspring were the products of some traits that their parents had. That if I had a guy with blue eyes-- let me say this is the blue-eyed guy right here --and then if he were to marry a brown-eyed girl-- Let's say this is the brown-eyed girl. Maybe make it a little bit more like a girl. If he were to marry the brown-eyed girl there, that most of the time, or maybe in all cases where we're dealing with the brown-eyed girl, maybe their kids are brown-eyed. Let me do this so they have a little brown-eyed baby here. And this is just something-- I mean, there's obviously thousands of generations of human beings, and we've observed this. We've observed that kids look like their parents, that they inherit some traits, and that some traits seem to dominate other traits. One example of that tends to be a darker pigmentation in maybe the hair or the eyes. Even if the other parent has light pigmentation, the darker one seems to dominate, or sometimes, it actually ends up being a mix, and we've seen that all around us. Now, this study of what gets passed on and how it gets passed on, it's much older than the study of DNA, which was really kind of discovered or became a big deal in the middle of the 20th century. This was studied a long time. And kind of the father of classical genetics and heredity is Gregor Mendel. He was actually a monk, and he would mess around with plants and cross them and see which traits got passed and which traits didn't get passed and tried to get an understanding of how traits are passed from one generation to another. So when we do this, when we study this classical genetics, I'm going to make a bunch of simplifying assumptions because we know that most of these don't hold for most of our genes, but it'll give us a little bit of sense of how to predict what might happen in future generations. So the first simplifying assumption I'll make is that some traits have kind of this all or nothing property. And we know that a lot of traits don't. Let's say that there are in the world-- and this is a gross oversimplification --let's say for eye color, let's say that there are two alleles. Now remember what an allele was. An allele is a specific version of a gene. So let's say that you could have blue eye color or you could have brown eye color. That we live in a universe where someone could only have one of these two versions of the eye color gene. We know that eye color is far more complex than that, so this is just a simplification. And let me just make up another one. Let me say that, I don't know, maybe for tooth size, that's a trait you won't see in any traditional biology textbook, and let's say that there's one trait for big teeth and there's another allele for small teeth. And I want to make very clear this distinction between a gene and an allele. I talked about Gregor Mendel, and he was doing this in the 1850s well before we knew what DNA was or what even chromosomes were and how DNA was passed on, et cetera, but let's go into the microbiology of it to understand the difference. So I have a chromosome. Let's say on some chromosome-- let me pick some chromosome here. Let's say this is some chromosome. Let's say I got that from my dad. And on this chromosome, there's some location here-- we could call that the locus on this chromosome where the eye color gene is --that's the location of the eye color gene. Now, I have two chromosomes, one from my father and one from my mother, so let's say that this is the chromosome from my mother. We know that when they're normally in the cell, they aren't nice and neatly organized like this in the chromosome, but this is just to kind of show you the idea. Let's say these are homologous chromosomes so they code for the same genes. So on this gene from my mother on that same location or locus, there's also the eye color gene. Now, I might have the same version of the gene and I'm saying that there's only two versions of this gene in the world. Now, if I have the same version of the gene-- I'm going to make a little shorthand notation. I'm going to write big B-- Actually, let me do it the other way. I'm going to write little b for blue and I'm going to write big B for brown. There's a situation where this could be a little b and this could be a big B. And then I could write that my genotype-- I have the allele, I have one big B from my mom and I have one small b from my dad. Each of these instances, or ways that this gene is expressed, is an allele. So these are two different alleles-- let me write that --or versions of the same gene. And when I have two different versions like this, one version from my mom, one version from my dad, I'm called a heterozygote, or sometimes it's called a heterozygous genotype. And the genotype is the exact version of the alleles I have. Let's say I had the lowercase b. I had the blue-eyed gene from both parents. So let's say that I was lowercase b, lowercase b, then I would have two identical alleles. Both of my parents gave me the same version of the gene. And this case, this genotype is homozygous, or this is a homozygous genotype, or I'm a homozygote for this trait. Now, you might say, Sal, this is fine. These are the traits that you have. I have a brown from maybe my mom and a blue from my dad. In this case, I have a blue from both my mom and dad. How do we know whether my eyes are going to be brown or blue? And the reality is it's very complex. It's a whole mixture of things. But Mendel, he studied things that showed what we'll call dominance. And this is the idea that one of these traits dominates the other. So a lot of people originally thought that eye color, especially blue eyes, was always dominated by the other traits. We'll assume that here, but that's a gross oversimplification. So let's say that brown eyes are dominant and blue are recessive. I wanted to do that in blue. Blue eyes are recessive. If this is the case, and this is a-- As I've said repeatedly, this is a gross oversimplification. But if that is the case, then if I were to inherit this genotype, because brown eyes are dominant-- remember, I said the big B here represents brown eye and the lowercase b is recessive --all you're going to see for the person with this genotype is brown eyes. So let me do this here. Let me write this here. So genotype, and then I'll write phenotype. Genotype is the actual versions of the gene you have and then the phenotypes are what's expressed or what do you see. So if I get a brown-eyed gene from my dad-- And I want to do it in a big-- I want to do it in brown. Let me do it in brown so you don't get confused. So if I've have a brown-eyed gene from my dad and a blue-eyed gene from my mom, because the brown eye is recessive, the brown-eyed allele is recessive-- And I just said a brown-eyed gene, but what I should say is the brown-eyed version of the gene, which is the brown allele, or the blue-eyed version of the gene from my mom, which is the blue allele. Since the brown allele is dominant-- I wrote that up here --what's going to be expressed are brown eyes. Now, let's say I had it the other way. Let's say I got a blue-eyed allele from my dad and I get a brown-eyed allele for my mom. Same thing. The phenotype is going to be brown eyes. Now, what if I get a brown-eyed allele from both my mom and my dad? Let me see, I keep changing the shade of brown, but they're all supposed to be the same. So let's say I get two dominant brown-eyed alleles from my mom and my dad. Then what are you going to see? Well, you could guess that. I'm still going to see brown eyes. So there's only one last combination because these are the only two types of alleles we might see in our population, although for most genes, there's more than two types. For example, there's blood types. There's four types of blood. But let's say that I get two blue, one blue allele from each of my parents, one from my dad, one from my mom. Then all of a sudden, this is a recessive trait, but there's nothing to dominate it. So, all of a sudden, the phenotype will be blue eyes. And I want to repeat again, this isn't necessarily how the alleles for eye color work, but it's a nice simplification to maybe understand how heredity works. There are some traits that can be studied in this simple way. But what I wanted to do here is to show you that many different genotypes-- so these are all different genotypes --they all coded for the same phenotype. So just by looking at someone's eye color, you didn't know exactly whether they were homozygous dominant-- this would be homozygous dominant --or whether they were heterozygotes. This is heterozygous right here. These two right here are heterozygotes. These are also sometimes called hybrids, but the word hybrid is kind of overloaded. It's used a lot, but in this context, it means that you got different versions of the allele for that gene. So let's think a little bit about what's actually happening when my mom and my dad reproduced. Well, let's think of a couple of different scenarios. Let's say that they're both hybrids. My dad has the brown-eyed dominant allele and he also has the blue-eyed recessive allele. Let's say my mom has the same thing, so brown-eyed dominant, and she also has the blue-eyed recessive allele. Now let's think about if these two people, before you see what my eye color is, if you said, look, I'm giving you what these two people's genotypes are. Let me label them. Let me make this the mom. I think this is the standard convention. And let's make this right here, this is the dad. What are the different genotypes that their children could have? So let's say they reproduce. I'm going to draw a little grid here. So let me draw a grid. So we know from our study of meiosis that, look, my mom has this gene on-- Let me draw the genes again. So there's a homologous pair, right? This is one chromosome right here. That's another chromosome right there. On this chromosome in the homologous pair, there might be-- at the eye color locus --there's the brown-eyed gene. And at this one, at the eye color locus, there's a blue-eyed gene. And similarly from my dad, when you look at that same chromosome in his cells-- Let me do them like this. So this is one chromosome there and this is the other chromosome here. When you look at that locus on this chromosome or that location, it has the brown-eyed allele for that gene, and on this one, it has the blue-eyed allele on this gene. And we learn from meiosis when the chromosomes-- Well, they replicate first, and so you have these two chromatids on a chromosome. But they line up in meiosis I during the metaphase. And we don't know which way they line up. For example, my dad might give me this chromosome or might give me that chromosome. Or my mom might give me that chromosome or might give me that chromosome. So I could have any of these combinations. So, for example, if I get this chromosome from my mom and this chromosome from my dad, what is the genotype going to be for eye color? Well, it's going to be capital B and capital B. If I get this chromosome from my mom and this chromosome from my dad, what's it going to be? Well, I'm going to get the big B from my dad and then I'm going to get the lowercase b from my mom. So this is another possibility. Now, this is another possibility here where I get the brown-eyed allele from my mom and I get the blue eye allele from my dad. And then there's a possibility that I get this chromosome from my dad and this chromosome from my mom, so it's this situation. Now, what are the phenotypes going to be? Well, we've already seen that this one right here is going to be brown, that one's going to be brown, this one's going to be brown, but this one is going to be blue. I already showed you this. But if I were to tell you ahead of time that, look, I have two people. They're both hybrids, or they're both heterozygotes for eye color, and eye color has this recessive dominant situation. And they're both heterozygotes where they each have one brown allele and one blue allele, and they're going to have a child, what's the probability that the child has brown eyes? What's the probability? Well, each of these scenarios are equally likely, right? There's four equal scenarios. So let's put that in the denominator. Four equal scenarios. And how many of those scenarios end up with brown eyes? Well, it's one, two, three. So the probability is 3/4, or it's a 75% probability. Same logic, what's the probability that these parents produce an offspring with blue eyes? Well, that's only one of the four equally likely possibilities, so blue eyes is only 25%. Now, what is the probability that they produce a heterozygote? So what is the probability that they produce a heterozygous offspring? So now we're not looking at the phenotype anymore. We're looking at the genotype. So of these combinations, which are heterozygous? Well, this one is, because it has a mix. It's a hybrid. It has a mix of the two alleles. And so is this one. So what's the probability? Well, there's four different combinations. All of those are equally likely, and two of them result in a heterozygote. So it's 2/4 or 1/2 or 50%. So using this Punnett square, and, of course, we had to make a lot of assumptions about the genes and whether one's dominant or one's a recessive, we can start to make predictions about the probabilities of different outcomes. And as we'll see in future videos, you can actually even go backwards. You can say, hey, given that this couple had five kids with brown eyes, what's the probability that they're both heterozygotes, or something like that. So it's a really interesting area, even though it is a bit of oversimplification. But many traits, especially some of the things that Gregor Mendel studied, can be studied in this way.