Subtitles section Play video Print subtitles 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.