Fundamentals of Heredity and Chromosomes

Hello students! We're going to go ahead and kick off Unit 5 in this video. In Unit 5, we're going to be talking about heredity, which is how genes and traits are passed on from parents to their offspring. And this unit is going to be separated into two sets of notes. So there's the 5.1 notes and the 5.2 notes. As always, we're going to start in the first part with the 5.1 notes and Topic 1. In topic one, we're going to go over some introductory things related to sexual reproduction and heredity, some good background information that you guys need to know as we dive deeper into how all of this works. And so the first thing I want to kind of explain to you guys in topic one is the difference between diploid and haploid cells. And so most multicellular organisms, so organisms like us and plants and other animals that are made of many, many cells, most... multicellular organisms, we reproduce sexually. And sexual reproduction is when there's two parents contributing to the DNA that's producing the offspring. And so when there's an offspring being produced, it's actually getting DNA from two different individuals, just like you guys got DNA from your mom and your dad. So that's what the term sexual reproduction refers to is when you're creating an offspring using two people's DNA. And so for sexually reproducing organisms, For most of them, their somatic cells are diploid. And just to remind you guys, a somatic cell is a body cell. So all of the cells in your body, basically, except for your reproductive cells, your sperm and your egg cells. So when I say somatic cells, I'm talking about all the cells except sperm and egg. So your skin cells, hair cells, heart cells, muscle cells, all the cells. And for most sexually reproducing organisms, those cells, your body cells, are diploid. And what it means to be diploid is... That's a cell that contains two complete sets of chromosomes. So they have a set of chromosomes from their mom and they have a set of chromosomes from their dad. And that's what we see happening in most of sexually reproducing organisms and their somatic cells. They have two complete sets of chromosomes. And so inside, if I took one of your guys'cells from your body, like a skin cell, and I looked at the chromosomes inside that skin cell, I would see a complete set of human chromosomes. and then another complete set of human chromosomes. You would have two sets of chromosomes, and you got a complete set from your mom and a complete set from your dad. Now your reproductive cells, which are not somatic cells, we call them gametes. Gametes are your sperm and your egg cells. So these are just these special cells, the ones that are not the somatic cells. These cells are not diploid. These are haploid cells. And in a haploid cell, you only have one set of chromosomes. So that would be half the number of chromosomes as a diploid cell, as a body cell. So if I was looking at one of your sperm or egg cells, and I was looking at the chromosomes inside that sperm or egg cell, I would only see one complete set of human chromosomes. There wouldn't be a second set. So that's called haploid. And so in this example right here in this picture, I have six chromosomes, but that actually is just two sets of three. There's one of these chromosomes, one of those chromosomes, and one of these chromosomes. And then there's a second set, another one of these, another one of those, another one of those. So we call that diploid. Whereas if you just have one of each, like one of these guys, not both, one of these guys, not both, one of these guys, not both, that's a haploid cell. Now, every species has a characteristic number of chromosomes in their cells. So it's different for different species. Like for humans, humans have 46 chromosomes in their somatic cells. So in your guys'somatic cells, your diploid cells, you have 46 chromosomes. And we represent that with the variable 2n. So 2n represents the diploid number. For humans, that's 46. For other animals and other species and other plants, it's different. It can be as low as 1, it could be as high as hundreds. But for humans, it's 46, which actually is two sets of 23 for humans. So you've got a set of 23 chromosomes from your mom and another set of 23 chromosomes from your dad. So you have two sets of 23. So our diploid number is 46. 46 chromosomes but two sets of 23. Now the haploid number for humans is 23. So one set of chromosomes is just 23. We use the variable n for haploid. That's your haploid number. For humans, n equals 23. So if we were looking at a haploid cell, just a cell that contained one set of chromosomes, it would have 23 chromosomes. So again, if I was looking at your sperm or your egg cells, there would be 23 chromosomes inside of there. Whereas if I was looking at any of your other somatic cells in the rest of your body, you would have 46 chromosomes. So this will We'll hit on this a lot, so you'll start getting used to it. But for now, that's the quick description of both of them. This will come into play a lot, though, in these notes. And so then I want to also talk about sexual reproduction and how that happens. So in sexual reproduction, what's happening when you have two parents involved in creating an offspring is you're going to have... to have a gamete from each parent fused together to form a fertilized egg or what we call a zygote. And so basically there's going to be an egg cell and a sperm cell, one from each parent, and those are going to fuse together. And then you now will have DNA from both parents forming this the single fertilized cell, which we call a zygote. So that process is called fertilization when an egg and a sperm cell fuse together to create an offspring. zygote. And so the way that happens, if you guys are keeping track of these chromosomes, a sperm cell has 23 chromosomes, if it's a human sperm cell, and a human egg cell has 23 chromosomes, or haploid. But when those two haploid cells fuse together to make a zygote, that now is going, this cell is now going to have 46 chromosomes, and 23 of those came from your dad, and 20, the other 23 came from your mom. So now you have this unique cell that is a a combination of mom and dad, getting half of its chromosomes from mom and half of its chromosomes from dad. And then that process that creates sperm and egg cells. So there's a special cell division process that creates these gametes. And that's called meiosis. So meiosis is something we're going to talk about a lot in topic two. It's different than mitosis. So in unit four, we talked a lot about mitosis. And that's how cells make copies of themselves and divide and make more cells. Meiosis is also a way that cells divide, but this is special. This is when cells are gonna divide to make haploid cells, which are your gametes, your sperm and your egg cells. So your sperm and your egg cells, sex cells, they're made through a process called meiosis, which is basically gonna take normal diploid cells, diploid cells that you have in your body, and they're gonna divide in a special process that actually creates haploid cells. Which again, you'll see this very clearly as we go deeper and deeper into these notes. in topic two especially. But that's the process that creates sperminate cells. That's the gist of how fertilization takes place. And then that zygote, that fertilized egg, will then go through mitosis, lots and lots of mitosis. It will start dividing and dividing and dividing, making more and more copies of itself, becoming a clump of cells and a clump of thousands of cells. And those cells will start differentiating and developing. And then you form embryo that then grows and continues to develop into a baby and a fully functioning human being. So just remember though that these two types of cell division, mitosis and meiosis, they are different. Mitosis is going to basically take your body cells, which are diploid, and make more body cells, which are diploid. So taking skin cells, making more skin cells. Taking blood cells, making more blood cells. Whereas meiosis is going to take a special type of diploid cell that's found in your testes or your ovaries, and it's going to take these diploid cells and turn them into haploid cells. which are going to become your sperm and your egg, which only have half a set of chromosomes, or one set of chromosomes, okay? Half the number of chromosomes as the rest of your cells, which again, we'll go into a lot of depth on how that takes place in topic two of these notes. So during sexual reproduction, the offspring, like we talked about, is going to get DNA from both mom and dad. And in that DNA, there's genes. And genes are basically the unit of heredity. These are the... the pieces of DNA that actually are going to give you certain traits. And so heredity, the term heredity is basically just talking about the process of genes being or traits being passed on from one generation to the next, from your parents to you and to your children and their children. Now genes, these are the actual pieces of DNA that code for specific things that are going to be giving you your traits. And so a gene is any sequence has the instructions to produce a protein or some sort of functional piece of RNA. And those products of genes, those proteins and those functional pieces of RNA that genes code for, they are what ultimately give you all of your traits. And so if you guys have brown eyes, brown eyes is because of there's a gene, there's a sequence of DNA that codes for you having brown eyes. Or if you have a certain type of hair, there's a sequence of DNA that codes for that. That's that. that hair color, that trait. And those genes are located, those special sequences of DNA are located on your guys'chromosomes at specific locations. And the location that's located on a chromosome is called a gene locus. So this is the fake scenario, but here's the gene locus for eye color. And here's the gene locus for hair color. Plural, it's called loci. So this is where those different genes are located on a chromosome. Now in this picture, kind of Well, it gives you a little bit more description. visual of what's going on when I talk about a gene. A gene again is located at, it's a special sequence of DNA located at a specific spot in a chromosome and that sequence of DNA codes for a protein, usually a protein, which is made up of amino acids that are going to fold up into a specific shape and it's that protein that's responsible for doing something and your proteins are basically what's going to give you all of your traits and characteristics. In this case it's a protein that's going to produce green pigments in your eyes. And so because of this protein, you're producing these special green pigment molecules in your eye cells that are giving you green eyes. And so the reason you have green eyes is because of this protein. And the reason you have this protein is because you have a sequence of DNA that codes for that protein. And the reason you have this gene that codes for this protein that codes for green eyes is because it was on a chromosome that either your mom or your dad gave you. So that's what's going on here. And so we'll continue to talk about that a lot in that process, not only in Unit 5, but also in Unit 6. Anyway, that's the gist, though. And then the last thing I want to talk about on these notes is what homologous chromosomes are. And so this is where some vocabulary gets really important and things get a little confusing. And so, like I said earlier, somatic cells, you guys, are diploid. They have two sets of chromosomes, which basically means you have two of each type of chromosome. So on each type of chromosome, there are specific genes on it. So let's say that there's a chromosome that you have. and it has gene X on it and gene Y on it. Let's just come up with fake examples. Let's say it's eye color and hair color. So you have the eye color gene on this chromosome, and you have the hair color gene on this chromosome, along with like hundreds and hundreds of other genes. But let's just say there's these two that we're focusing on. Well, you actually have another chromosome that also has the gene for eye color and hair color. You have the exact same type of chromosome. You have a second one because you're diploid. You have two sets of chromosomes. And one of those was from your dad, and one of those is from your mom. Whenever you have two chromosomes that have the same set of genes on them, we call those homologous chromosomes. So these are homologous chromosomes. They're two different chromosomes that have the same set of genes. Same set of genes. One came from your dad. One came from your mom. Okay? And so you guys would have 23 pairs of these. Okay? You guys would have 23 chromosomes from your dad, each of them containing hundreds of different genes. And then you would have another 23 chromosomes from mom, each also containing those. hundreds of different genes, but if we're looking at one pair of them, those are called homologous chromosomes. Now what's important to point out starting now, but we'll talk about this a lot, is that homologous chromosomes are not identical. They're not exact copies of each other. And so when I say that they have the same genes, I'm saying that they have the instructions for eye color and this other chromosome has the instructions for eye color. But it's not necessarily the same DNA for eye color. The DNA that your dad gave you for eye color could be different than the DNA your mom gave you for eye color. And so, for example, your dad could give you DNA that codes for green eyes. And your mom, on the same type of chromosome that has the instructions for your eye color, could have given you slightly different DNA that codes for brown eyes. And so, your eyes'homologous chromosomes, the one you got from your dad and your mom, they're not genetically the same. Like, they're different. Like, your mom and your dad are different people. Yes, they have a chromosome for eye color and hair color, and your mom also has a chromosome for eye color and hair color, but the specific DNA in those chromosomes is different. And so they could code for different versions of that trait. So in this case, eye color, the gene for eye color, you could have two different sets of instructions that are different, that are not the same. And so just keep in mind, homologous chromosomes, they're not identical. They are different. They came from two different parents who are different people who have different DNA. But yes, they do have the same set of genes. So you'll find the instructions for certain things on this chromosome and on this other chromosome. You'll find the same sort of instructions, but those instructions might say something different. I hope that kind of makes sense. And so where this gets complicated is during mitosis and meiosis, your chromosomes duplicate before starting cell division. So before a cell divides. It's going to replicate its chromosomes. We saw this with mitosis. Before mitosis starts during interphase, all of the chromosomes make copies of themselves and those copies are still attached to each other at this location called a centromere. Those two copies of the chromosome are called sister chromatin. And so this is where vocab gets really important. If you look at this picture here, I have homologous chromosomes. A chromosome from your dad and a chromosome from your mom that has the eye color gene and let's say the height gene, whether you're tall or short. And we're just really simplifying this right now, right? Those are homologous chromosomes. But these are unduplicated. This is before they were duplicated during interphase. Now during interphase, what's going to happen is like during the S phase of interphase, they're going to duplicate this. All of the DNA in this chromosome is going to make a copy of itself and all of the DNA in this chromosome is going to make a copy of itself. But they're still going to be held together, those two copies. Those are called sister chromatids. Sister chromatids are exact copies of each other. Those are identical genetically. And so you can see that this chromosome from your dad that had the instructions for green eye colors and being short, you made a copy of it and it's exactly the same. Now you have, well, instructions again for green eyes and being short, but they're still connected together as one chromosome. This is still one chromosome. It's just duplicated now. But those two copies are the same. They're called sister chromatids. Whereas this other chromosome from your mom. who had the instructions for brown eyes and being tall, it made a copy of itself, it duplicated. And so now we have a duplicated chromosome with two copies of itself. So this is homologous chromosomes that are unduplicated. Here's what they would look like when they're duplicated after interphase before you start mitosis or meiosis. So now they're duplicated. And those, they're still homologous chromosomes. This chromosome has the instructions for eye color and your height. This chromosome has instructions for eye color and height. One came from your mom, one came from your dad. Those are homologous chromosomes, but now they're duplicated. They're each made up two sister chromatids. So this is where things get confusing when we start talking about meiosis in topic two. But hopefully this kind of makes sense so far. And so the last thing on this, these set of notes is the difference between your guys'between autosomes and sex chromosomes. And so this is called a karyotype. A karyotype is a nice little display of all your chromosomes. And you can see you have two of each type of chromosome. There's 23 pairs total. Now the chromosomes that have to do with sex, that determine your sex whether you're male or female, those are called sex chromosomes, and those are special chromosomes that for humans there's, and most mammals, there's an X and a Y chromosome that determine whether you're male or female. If you have two X chromosomes, you're female. But if you have an X and a Y chromosome, you're going to be male. So the X and the Y chromosome, those are your sex chromosomes. And you either have two Xs or an X and a Y if you're male or female, typically. And the X chromosome and the Y chromosome, though, those are not true homologous chromosomes. Those carry different genes. You can even see that they're very different in size. The Y chromosome is very, very tiny. It doesn't have a lot of genes on it. And then the X chromosome is a lot larger and has lots of genes on it, like the rest of your chromosomes. So they are a pair, but they're not truly homologous pairs, the X and the Y one. But if you have two Xs, like a girl, a girl would have two X chromosomes. And so a female with two X chromosomes, those X chromosomes are homologous chromosomes. Like they have the same set of genes, the same set of genes. You just have two of them. Anyway, we'll talk more about that later. But. All of the other chromosomes that are not your sex chromosomes, those are called autosomes. So autosomes are all these other chromosomes that are not sex chromosomes. So when I say autosome, I'm talking about a chromosome that has nothing to do with whether you're male or female. When I'm talking about a sex chromosome, I'm talking about these special chromosomes that have to do with whether you're male or female. Anyway, so you guys can see on this karyotype, whoever this person is that we're showing the chromosomes for a certain person, it looks like they're human because there's 23 pairs of chromosomes. And that's... Humans have 23 pairs of chromosomes. And you guys can see they have 22 pairs of autosomes that have hundreds of genes on them each. But then there's one pair, the 23rd pair is the sex chromosomes. And this person has an X and a Y chromosome. So this must be a male because they have an X and a Y chromosome. Anyway, that's it for topic one. Go ahead and try answering these questions. And then I'll see you guys later.

As always, we're going to start in the first part with the 5.1 notes and Topic 1. In topic one, we're going to go over some introductory things related to sexual reproduction and heredity, some good background information that you guys need to know as we dive deeper into how all of this works. And so the first thing I want to kind of explain to you guys in topic one is the difference between diploid and haploid cells. And so most multicellular organisms, so organisms like us and plants and other animals that are made of many, many cells, most... multicellular organisms, we reproduce sexually. And sexual reproduction is when there's two parents contributing to the DNA that's producing the offspring.

And so when there's an offspring being produced, it's actually getting DNA from two different individuals, just like you guys got DNA from your mom and your dad. So that's what the term sexual reproduction refers to is when you're creating an offspring using two people's DNA. And so for sexually reproducing organisms, For most of them, their somatic cells are diploid.

And just to remind you guys, a somatic cell is a body cell. So all of the cells in your body, basically, except for your reproductive cells, your sperm and your egg cells. So when I say somatic cells, I'm talking about all the cells except sperm and egg.

So your skin cells, hair cells, heart cells, muscle cells, all the cells. And for most sexually reproducing organisms, those cells, your body cells, are diploid. And what it means to be diploid is... That's a cell that contains two complete sets of chromosomes. So they have a set of chromosomes from their mom and they have a set of chromosomes from their dad.

And that's what we see happening in most of sexually reproducing organisms and their somatic cells. They have two complete sets of chromosomes. And so inside, if I took one of your guys'cells from your body, like a skin cell, and I looked at the chromosomes inside that skin cell, I would see a complete set of human chromosomes.

and then another complete set of human chromosomes. You would have two sets of chromosomes, and you got a complete set from your mom and a complete set from your dad. Now your reproductive cells, which are not somatic cells, we call them gametes.

Gametes are your sperm and your egg cells. So these are just these special cells, the ones that are not the somatic cells. These cells are not diploid.

These are haploid cells. And in a haploid cell, you only have one set of chromosomes. So that would be half the number of chromosomes as a diploid cell, as a body cell.

So if I was looking at one of your sperm or egg cells, and I was looking at the chromosomes inside that sperm or egg cell, I would only see one complete set of human chromosomes. There wouldn't be a second set. So that's called haploid.

And so in this example right here in this picture, I have six chromosomes, but that actually is just two sets of three. There's one of these chromosomes, one of those chromosomes, and one of these chromosomes. And then there's a second set, another one of these, another one of those, another one of those. So we call that diploid. Whereas if you just have one of each, like one of these guys, not both, one of these guys, not both, one of these guys, not both, that's a haploid cell.

Now, every species has a characteristic number of chromosomes in their cells. So it's different for different species. Like for humans, humans have 46 chromosomes in their somatic cells. So in your guys'somatic cells, your diploid cells, you have 46 chromosomes. And we represent that with the variable 2n.

So 2n represents the diploid number. For humans, that's 46. For other animals and other species and other plants, it's different. It can be as low as 1, it could be as high as hundreds.

But for humans, it's 46, which actually is two sets of 23 for humans. So you've got a set of 23 chromosomes from your mom and another set of 23 chromosomes from your dad. So you have two sets of 23. So our diploid number is 46. 46 chromosomes but two sets of 23. Now the haploid number for humans is 23. So one set of chromosomes is just 23. We use the variable n for haploid. That's your haploid number.

For humans, n equals 23. So if we were looking at a haploid cell, just a cell that contained one set of chromosomes, it would have 23 chromosomes. So again, if I was looking at your sperm or your egg cells, there would be 23 chromosomes inside of there. Whereas if I was looking at any of your other somatic cells in the rest of your body, you would have 46 chromosomes.

So this will We'll hit on this a lot, so you'll start getting used to it. But for now, that's the quick description of both of them. This will come into play a lot, though, in these notes.

And so then I want to also talk about sexual reproduction and how that happens. So in sexual reproduction, what's happening when you have two parents involved in creating an offspring is you're going to have... to have a gamete from each parent fused together to form a fertilized egg or what we call a zygote.

And so basically there's going to be an egg cell and a sperm cell, one from each parent, and those are going to fuse together. And then you now will have DNA from both parents forming this the single fertilized cell, which we call a zygote. So that process is called fertilization when an egg and a sperm cell fuse together to create an offspring.

zygote. And so the way that happens, if you guys are keeping track of these chromosomes, a sperm cell has 23 chromosomes, if it's a human sperm cell, and a human egg cell has 23 chromosomes, or haploid. But when those two haploid cells fuse together to make a zygote, that now is going, this cell is now going to have 46 chromosomes, and 23 of those came from your dad, and 20, the other 23 came from your mom. So now you have this unique cell that is a a combination of mom and dad, getting half of its chromosomes from mom and half of its chromosomes from dad. And then that process that creates sperm and egg cells.

So there's a special cell division process that creates these gametes. And that's called meiosis. So meiosis is something we're going to talk about a lot in topic two. It's different than mitosis.

So in unit four, we talked a lot about mitosis. And that's how cells make copies of themselves and divide and make more cells. Meiosis is also a way that cells divide, but this is special. This is when cells are gonna divide to make haploid cells, which are your gametes, your sperm and your egg cells. So your sperm and your egg cells, sex cells, they're made through a process called meiosis, which is basically gonna take normal diploid cells, diploid cells that you have in your body, and they're gonna divide in a special process that actually creates haploid cells.

Which again, you'll see this very clearly as we go deeper and deeper into these notes. in topic two especially. But that's the process that creates sperminate cells. That's the gist of how fertilization takes place. And then that zygote, that fertilized egg, will then go through mitosis, lots and lots of mitosis.

It will start dividing and dividing and dividing, making more and more copies of itself, becoming a clump of cells and a clump of thousands of cells. And those cells will start differentiating and developing. And then you form embryo that then grows and continues to develop into a baby and a fully functioning human being. So just remember though that these two types of cell division, mitosis and meiosis, they are different. Mitosis is going to basically take your body cells, which are diploid, and make more body cells, which are diploid.

So taking skin cells, making more skin cells. Taking blood cells, making more blood cells. Whereas meiosis is going to take a special type of diploid cell that's found in your testes or your ovaries, and it's going to take these diploid cells and turn them into haploid cells. which are going to become your sperm and your egg, which only have half a set of chromosomes, or one set of chromosomes, okay? Half the number of chromosomes as the rest of your cells, which again, we'll go into a lot of depth on how that takes place in topic two of these notes.

So during sexual reproduction, the offspring, like we talked about, is going to get DNA from both mom and dad. And in that DNA, there's genes. And genes are basically the unit of heredity. These are the... the pieces of DNA that actually are going to give you certain traits.

And so heredity, the term heredity is basically just talking about the process of genes being or traits being passed on from one generation to the next, from your parents to you and to your children and their children. Now genes, these are the actual pieces of DNA that code for specific things that are going to be giving you your traits. And so a gene is any sequence has the instructions to produce a protein or some sort of functional piece of RNA. And those products of genes, those proteins and those functional pieces of RNA that genes code for, they are what ultimately give you all of your traits.

And so if you guys have brown eyes, brown eyes is because of there's a gene, there's a sequence of DNA that codes for you having brown eyes. Or if you have a certain type of hair, there's a sequence of DNA that codes for that. That's that.

that hair color, that trait. And those genes are located, those special sequences of DNA are located on your guys'chromosomes at specific locations. And the location that's located on a chromosome is called a gene locus. So this is the fake scenario, but here's the gene locus for eye color. And here's the gene locus for hair color.

Plural, it's called loci. So this is where those different genes are located on a chromosome. Now in this picture, kind of Well, it gives you a little bit more description. visual of what's going on when I talk about a gene. A gene again is located at, it's a special sequence of DNA located at a specific spot in a chromosome and that sequence of DNA codes for a protein, usually a protein, which is made up of amino acids that are going to fold up into a specific shape and it's that protein that's responsible for doing something and your proteins are basically what's going to give you all of your traits and characteristics.

In this case it's a protein that's going to produce green pigments in your eyes. And so because of this protein, you're producing these special green pigment molecules in your eye cells that are giving you green eyes. And so the reason you have green eyes is because of this protein. And the reason you have this protein is because you have a sequence of DNA that codes for that protein. And the reason you have this gene that codes for this protein that codes for green eyes is because it was on a chromosome that either your mom or your dad gave you.

So that's what's going on here. And so we'll continue to talk about that a lot in that process, not only in Unit 5, but also in Unit 6. Anyway, that's the gist, though. And then the last thing I want to talk about on these notes is what homologous chromosomes are.

And so this is where some vocabulary gets really important and things get a little confusing. And so, like I said earlier, somatic cells, you guys, are diploid. They have two sets of chromosomes, which basically means you have two of each type of chromosome.

So on each type of chromosome, there are specific genes on it. So let's say that there's a chromosome that you have. and it has gene X on it and gene Y on it. Let's just come up with fake examples. Let's say it's eye color and hair color.

So you have the eye color gene on this chromosome, and you have the hair color gene on this chromosome, along with like hundreds and hundreds of other genes. But let's just say there's these two that we're focusing on. Well, you actually have another chromosome that also has the gene for eye color and hair color.

You have the exact same type of chromosome. You have a second one because you're diploid. You have two sets of chromosomes.

And one of those was from your dad, and one of those is from your mom. Whenever you have two chromosomes that have the same set of genes on them, we call those homologous chromosomes. So these are homologous chromosomes.

They're two different chromosomes that have the same set of genes. Same set of genes. One came from your dad. One came from your mom.

Okay? And so you guys would have 23 pairs of these. Okay?

You guys would have 23 chromosomes from your dad, each of them containing hundreds of different genes. And then you would have another 23 chromosomes from mom, each also containing those. hundreds of different genes, but if we're looking at one pair of them, those are called homologous chromosomes.

Now what's important to point out starting now, but we'll talk about this a lot, is that homologous chromosomes are not identical. They're not exact copies of each other. And so when I say that they have the same genes, I'm saying that they have the instructions for eye color and this other chromosome has the instructions for eye color.

But it's not necessarily the same DNA for eye color. The DNA that your dad gave you for eye color could be different than the DNA your mom gave you for eye color. And so, for example, your dad could give you DNA that codes for green eyes.

And your mom, on the same type of chromosome that has the instructions for your eye color, could have given you slightly different DNA that codes for brown eyes. And so, your eyes'homologous chromosomes, the one you got from your dad and your mom, they're not genetically the same. Like, they're different.

Like, your mom and your dad are different people. Yes, they have a chromosome for eye color and hair color, and your mom also has a chromosome for eye color and hair color, but the specific DNA in those chromosomes is different. And so they could code for different versions of that trait.

So in this case, eye color, the gene for eye color, you could have two different sets of instructions that are different, that are not the same. And so just keep in mind, homologous chromosomes, they're not identical. They are different. They came from two different parents who are different people who have different DNA.

But yes, they do have the same set of genes. So you'll find the instructions for certain things on this chromosome and on this other chromosome. You'll find the same sort of instructions, but those instructions might say something different. I hope that kind of makes sense.

And so where this gets complicated is during mitosis and meiosis, your chromosomes duplicate before starting cell division. So before a cell divides. It's going to replicate its chromosomes.

We saw this with mitosis. Before mitosis starts during interphase, all of the chromosomes make copies of themselves and those copies are still attached to each other at this location called a centromere. Those two copies of the chromosome are called sister chromatin.

And so this is where vocab gets really important. If you look at this picture here, I have homologous chromosomes. A chromosome from your dad and a chromosome from your mom that has the eye color gene and let's say the height gene, whether you're tall or short.

And we're just really simplifying this right now, right? Those are homologous chromosomes. But these are unduplicated. This is before they were duplicated during interphase. Now during interphase, what's going to happen is like during the S phase of interphase, they're going to duplicate this.

All of the DNA in this chromosome is going to make a copy of itself and all of the DNA in this chromosome is going to make a copy of itself. But they're still going to be held together, those two copies. Those are called sister chromatids. Sister chromatids are exact copies of each other. Those are identical genetically.

And so you can see that this chromosome from your dad that had the instructions for green eye colors and being short, you made a copy of it and it's exactly the same. Now you have, well, instructions again for green eyes and being short, but they're still connected together as one chromosome. This is still one chromosome. It's just duplicated now.

But those two copies are the same. They're called sister chromatids. Whereas this other chromosome from your mom.

who had the instructions for brown eyes and being tall, it made a copy of itself, it duplicated. And so now we have a duplicated chromosome with two copies of itself. So this is homologous chromosomes that are unduplicated.

Here's what they would look like when they're duplicated after interphase before you start mitosis or meiosis. So now they're duplicated. And those, they're still homologous chromosomes.

This chromosome has the instructions for eye color and your height. This chromosome has instructions for eye color and height. One came from your mom, one came from your dad. Those are homologous chromosomes, but now they're duplicated. They're each made up two sister chromatids.

So this is where things get confusing when we start talking about meiosis in topic two. But hopefully this kind of makes sense so far. And so the last thing on this, these set of notes is the difference between your guys'between autosomes and sex chromosomes.

And so this is called a karyotype. A karyotype is a nice little display of all your chromosomes. And you can see you have two of each type of chromosome. There's 23 pairs total. Now the chromosomes that have to do with sex, that determine your sex whether you're male or female, those are called sex chromosomes, and those are special chromosomes that for humans there's, and most mammals, there's an X and a Y chromosome that determine whether you're male or female.

If you have two X chromosomes, you're female. But if you have an X and a Y chromosome, you're going to be male. So the X and the Y chromosome, those are your sex chromosomes.

And you either have two Xs or an X and a Y if you're male or female, typically. And the X chromosome and the Y chromosome, though, those are not true homologous chromosomes. Those carry different genes. You can even see that they're very different in size. The Y chromosome is very, very tiny.

It doesn't have a lot of genes on it. And then the X chromosome is a lot larger and has lots of genes on it, like the rest of your chromosomes. So they are a pair, but they're not truly homologous pairs, the X and the Y one. But if you have two Xs, like a girl, a girl would have two X chromosomes.

And so a female with two X chromosomes, those X chromosomes are homologous chromosomes. Like they have the same set of genes, the same set of genes. You just have two of them. Anyway, we'll talk more about that later. But.

All of the other chromosomes that are not your sex chromosomes, those are called autosomes. So autosomes are all these other chromosomes that are not sex chromosomes. So when I say autosome, I'm talking about a chromosome that has nothing to do with whether you're male or female. When I'm talking about a sex chromosome, I'm talking about these special chromosomes that have to do with whether you're male or female. Anyway, so you guys can see on this karyotype, whoever this person is that we're showing the chromosomes for a certain person, it looks like they're human because there's 23 pairs of chromosomes.

And that's... Humans have 23 pairs of chromosomes. And you guys can see they have 22 pairs of autosomes that have hundreds of genes on them each.

But then there's one pair, the 23rd pair is the sex chromosomes. And this person has an X and a Y chromosome. So this must be a male because they have an X and a Y chromosome. Anyway, that's it for topic one.

Go ahead and try answering these questions. And then I'll see you guys later.

Transcript for:Fundamentals of Heredity and Chromosomes

Transcript for:
Fundamentals of Heredity and Chromosomes