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  • 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