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So for example, let's say I have a bunch of, well, this is the circle species. And one guy is that color, and then I've got a bunch more. Maybe some are that color. That's the same color. That one, and that one, and that one. And for whatever reason, sometimes there are no environmental factors that will predispose one... | Variation in a Species (4).mp3 |
That's the same color. That one, and that one, and that one. And for whatever reason, sometimes there are no environmental factors that will predispose one of these guys to be able to survive and reproduce over the other. But every now and then, there might be some environmental factor. And it makes maybe all of a sudd... | Variation in a Species (4).mp3 |
But every now and then, there might be some environmental factor. And it makes maybe all of a sudden, this guy is more fit to reproduce. And so for whatever reason, this guy is able to reproduce more frequently, and these guys less frequently. And some of them get killed or whatever, eaten by birds or they're just not ... | Variation in a Species (4).mp3 |
And some of them get killed or whatever, eaten by birds or they're just not able to reproduce for whatever reason. And then maybe these guys are something in between. And so over time, the frequency of the different traits you see in this population will change. And if they are drastic enough, maybe these guys start be... | Variation in a Species (4).mp3 |
And if they are drastic enough, maybe these guys start becoming dominant and start not liking these guys because they're so different or whatever else. We could see a lot of different reasons. This could eventually turn into a different species. Now, the obvious question is, what leads to this variation? In a populatio... | Variation in a Species (4).mp3 |
Now, the obvious question is, what leads to this variation? In a population, what leads to this? In fact, even in our population, what leads to one person having dirty blonde hair, one person having brown hair, one person having black hair, and the spectrum of skin complexions and heights is pretty much infinite? What ... | Variation in a Species (4).mp3 |
What causes that? And then one thing that I kind of point to, and we talked about this a little bit in the DNA video, is this notion of mutations. The DNA, we learned, is just a sequence of these bases. So adenine, guanine, let's say I got some thymine going, I have some more adenine, some cytosine. And that these code... | Variation in a Species (4).mp3 |
So adenine, guanine, let's say I got some thymine going, I have some more adenine, some cytosine. And that these code, if you have enough of these in a row, maybe you have a few hundred or a few thousands of these, these code for proteins or they code for things that control other proteins. But maybe you have a change ... | Variation in a Species (4).mp3 |
Maybe this cytosine, for whatever reason, becomes a guanine randomly. Or maybe these get deleted. And that would change the DNA. But you can imagine, if I went to someone's computer code and just randomly started changing letters and randomly started inserting letters without really knowing what I'm doing, most of the ... | Variation in a Species (4).mp3 |
But you can imagine, if I went to someone's computer code and just randomly started changing letters and randomly started inserting letters without really knowing what I'm doing, most of the time I'm going to break the computer program. Most of the time, the great majority of the time, this is going to go nowhere. For ... | Variation in a Species (4).mp3 |
But if I start getting rid of semicolons and start changing numbers and all that, it'll probably make the computer program break. So it'll either do nothing or it'll actually kill the organisms most of the time. Mutations. Sometimes they might make the actual cell kind of go run amok and we'll do a whole maybe series o... | Variation in a Species (4).mp3 |
Sometimes they might make the actual cell kind of go run amok and we'll do a whole maybe series of videos on cancer and that itself obviously would hurt the organism as a whole. Although if it occurs after the organism is reproduced, it might not be something that selects against the organism. But anyway, and it also w... | Variation in a Species (4).mp3 |
But anyway, I won't go too detailed into that. But the whole point is that mutations don't seem to be a satisfying source of variation. They could be a source or kind of contribute on the margin, but there must be something more profound than mutations that's creating the diversity even within, or maybe I should call t... | Variation in a Species (4).mp3 |
And the answer here is really, it's kind of right in front of us. It really addresses kind of one of the most fundamental things about biology. And it's so fundamental that a lot of people never even question why it is the way it is. And that is sexual reproduction. And when I mean sexual reproduction, it's this notion... | Variation in a Species (4).mp3 |
And that is sexual reproduction. And when I mean sexual reproduction, it's this notion that you have, and pretty much if you look at all organisms that have nucleuses, and we call those eukaryotes, maybe I'll do a whole video on eukaryotes versus prokaryotes. But it's the notion that if you look universally all the way... | Variation in a Species (4).mp3 |
In some organisms, an organism can be both a male and a female, but the common idea here is that all organisms kind of produce versions of their genetic material that mix with other organisms' version of their genetic material. If mutations were the only source of variation, then I could just butt off other cells. Mayb... | Variation in a Species (4).mp3 |
But that would, as we already talked about, most of the time we would have very little change, very little variation. And whatever variation does occur because of any kind of noise being introduced into this kind of butting process where I just replicate myself identically, most of the times it'll be negative. Most of ... | Variation in a Species (4).mp3 |
Now, when you have sexual reproduction, what happens? Well, you keep mixing and matching every possible combination of DNA in a species pool of DNA. Let me make this a little bit more concrete for you. So let me erase this horrible drawing I just did. So we all have, let me stick to humans because that's what we are. W... | Variation in a Species (4).mp3 |
So let me erase this horrible drawing I just did. So we all have, let me stick to humans because that's what we are. We have 23 pairs of chromosomes, and in each pair we have one chromosome from our mother and one chromosome from our father. So let me draw that. So I'll do my father's chromosomes in blue, so I have 1, ... | Variation in a Species (4).mp3 |
So let me draw that. So I'll do my father's chromosomes in blue, so I have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, and I'm running out of space. Let me do more here. 16, 17, 18, 19, 20, 21, 22. And then I'll throw another one here that looks a little bit different. I'll throw one here that looks like a Y. An... | Variation in a Species (4).mp3 |
16, 17, 18, 19, 20, 21, 22. And then I'll throw another one here that looks a little bit different. I'll throw one here that looks like a Y. And we'll talk more about the X's and the Y chromosomes. And I have 23 chromosomes from my mother. And not to be stereotypical, but maybe I'll do that in a more feminine color. Le... | Variation in a Species (4).mp3 |
And we'll talk more about the X's and the Y chromosomes. And I have 23 chromosomes from my mother. And not to be stereotypical, but maybe I'll do that in a more feminine color. Let's see. So I have 23 chromosomes from my mother. 1, 2, I just have to draw 1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 2... | Variation in a Species (4).mp3 |
Let's see. So I have 23 chromosomes from my mother. 1, 2, I just have to draw 1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23. So what's going on here? I have 23 from my mother. I have 23 from my father. Now, each of these chromosomes, and I made them right next to each other. | Variation in a Species (4).mp3 |
So what's going on here? I have 23 from my mother. I have 23 from my father. Now, each of these chromosomes, and I made them right next to each other. So for example, let me zoom in on one pair of these. So let's say we look at chromosome number 3. So let me zoom in on chromosome number 3. | Variation in a Species (4).mp3 |
Now, each of these chromosomes, and I made them right next to each other. So for example, let me zoom in on one pair of these. So let's say we look at chromosome number 3. So let me zoom in on chromosome number 3. I have one from my mother right here. And remember, actually maybe I'll do it this way. Remember, a chromo... | Variation in a Species (4).mp3 |
So let me zoom in on chromosome number 3. I have one from my mother right here. And remember, actually maybe I'll do it this way. Remember, a chromosome is just a big, if you take the DNA, it just keeps wrapping around. It actually wraps around all these proteins and it creates the structure. But it's just a big, you s... | Variation in a Species (4).mp3 |
Remember, a chromosome is just a big, if you take the DNA, it just keeps wrapping around. It actually wraps around all these proteins and it creates the structure. But it's just a big, you see it like that, you're like, oh, maybe the DNA, no, but this could have millions of base pairs. So maybe it'll look something lik... | Variation in a Species (4).mp3 |
So maybe it'll look something like that. It's a densely wrapped version of, well, it's a long string of DNA, and when it's normally drawn like this, which is not always the way it is, and we'll talk more about like that, they draw it as densely packed like that. So let's say that's from my mother and that's from my fat... | Variation in a Species (4).mp3 |
Now, these are both, we call them, I'll call them, they're the same, let's call this chromosome 3. They're both chromosome 3. And what the idea is here is that I'm getting different traits from my father and from my mother. For example, and I'm doing a gross oversimplification here, but this is really to just give you ... | Variation in a Species (4).mp3 |
For example, and I'm doing a gross oversimplification here, but this is really to just give you the idea of what's going on. This chromosome 3, maybe it contains this trait for hair color. And maybe my father had, and I'll use my actual example, my father had very straight hair. So let's say he had, some place on this ... | Variation in a Species (4).mp3 |
So let's say he had, some place on this chromosome, there is a gene for hair straightness. Let's say it's a little thing right there. And remember, that gene could be thousands of base pairs. But let's say this is hair straightness. So my father's version of that gene, or he had the allele for straightness. And remembe... | Variation in a Species (4).mp3 |
But let's say this is hair straightness. So my father's version of that gene, or he had the allele for straightness. And remember, an allele is just a version of a gene. So I'll call it the allele straight for straight hair. Now, this other chromosome that my mother gave me, this essentially, and there are exceptions, ... | Variation in a Species (4).mp3 |
So I'll call it the allele straight for straight hair. Now, this other chromosome that my mother gave me, this essentially, and there are exceptions, but for the most part, it codes for the same genes. And that's why I put them next to each other. So this will also have the gene for hair straightness or curliness. But ... | Variation in a Species (4).mp3 |
So this will also have the gene for hair straightness or curliness. But my mom does happen to actually have curly hair. So she has the gene right there for curly hair. So she has the version of the gene here is, let's see, allele curly. The gene just says, look, this is the gene for whether or not your hair is curly. E... | Variation in a Species (4).mp3 |
So she has the version of the gene here is, let's see, allele curly. The gene just says, look, this is the gene for whether or not your hair is curly. Each version of the gene is called an allele. Allele curly. Now, when I got both of these in my body, or in my cells, and this is in every cell of my body, every cell of... | Variation in a Species (4).mp3 |
Allele curly. Now, when I got both of these in my body, or in my cells, and this is in every cell of my body, every cell of my body except for, and we'll talk a little in a few seconds about my germ cells, but every cell other than the ones that I use for reproduction have this complete set of chromosomes in it, which ... | Variation in a Species (4).mp3 |
And I'm simplifying. Maybe they will on some other dimension. But let's say for simplicity, they won't matter in certain places. So certain genes are expressed in certain parts of the body, but every one of your body cells, and we call those somatic cells, and we'll separate those from the sex cells or the germ cells t... | Variation in a Species (4).mp3 |
So certain genes are expressed in certain parts of the body, but every one of your body cells, and we call those somatic cells, and we'll separate those from the sex cells or the germ cells that we'll talk about later. So this is my body cells. So this is the great majority of your cells. And this is opposed to your ge... | Variation in a Species (4).mp3 |
And this is opposed to your germ cells. And the germ cells, I'll just write it here just so you get it clear, for a male, that's the sperm cells. And for a female, that's the egg cells or the ova. But most of my cells have a complete collection of these. What I want to give you the idea is that for every trait, I essen... | Variation in a Species (4).mp3 |
But most of my cells have a complete collection of these. What I want to give you the idea is that for every trait, I essentially have two versions, one from my mother and one from my father. Now these right here are called homologous chromosomes. Chromosomes, homologous. What that means is every time you see the prefi... | Variation in a Species (4).mp3 |
Chromosomes, homologous. What that means is every time you see the prefix homologous, or if you see like homo sapien, or even the word homosexual or homogeneous, it means same. You see that all the time. So homologous means that they're almost the same. They're coding for the most part the same set of genes, but they'r... | Variation in a Species (4).mp3 |
So homologous means that they're almost the same. They're coding for the most part the same set of genes, but they're not identical. They actually might code for slightly different versions of the same gene. So depending on what versions I get, what is actually expressed for me. So my genotype, let me introduce another... | Variation in a Species (4).mp3 |
So depending on what versions I get, what is actually expressed for me. So my genotype, let me introduce another word. And I'm overwhelming you with words here. So my genotype is exactly what alleles I have, what versions of the gene. So I got like the fifth version of the curly allele. There could be multiple versions... | Variation in a Species (4).mp3 |
So my genotype is exactly what alleles I have, what versions of the gene. So I got like the fifth version of the curly allele. There could be multiple versions of the curly allele in our gene pool. And maybe I got some version of the straight allele. That is my genotype. My phenotype is what my hair really looks like. ... | Variation in a Species (4).mp3 |
And maybe I got some version of the straight allele. That is my genotype. My phenotype is what my hair really looks like. So for example, two people could have different genotypes with the same, but they might code for hair that looks pretty much the same. So it might have a very similar phenotype. So one phenotype can... | Variation in a Species (4).mp3 |
So for example, two people could have different genotypes with the same, but they might code for hair that looks pretty much the same. So it might have a very similar phenotype. So one phenotype can be represented by multiple genotypes. So that's just one thing to think about. And we'll talk a lot about that in the fut... | Variation in a Species (4).mp3 |
So that's just one thing to think about. And we'll talk a lot about that in the future, but I just want to introduce you into that there. Now, I entered this whole discussion because I wanted to talk about variation. So how does variation happen? Well, what's going to happen when I, so first of all, well, let me put it... | Variation in a Species (4).mp3 |
So how does variation happen? Well, what's going to happen when I, so first of all, well, let me put it this way. What's going to happen when I reproduce and I have a son? Well, my contribution to my son is going to be a random collection of half of these genes. I'm going to contribute either one. For each homologous p... | Variation in a Species (4).mp3 |
Well, my contribution to my son is going to be a random collection of half of these genes. I'm going to contribute either one. For each homologous pair, I'm either going to contribute the one that I got from my mother or the one that I got from my father. So let's say that the sperm cell that went on to fertilize my wi... | Variation in a Species (4).mp3 |
So let's say that the sperm cell that went on to fertilize my wife's egg, it just happened to have, let's say it happened to have that one, that one, well, I could just pick one from each of these 23 sets. And you could say, well, how many combinations are there? Well, for every set, I can pick one of the two homologou... | Variation in a Species (4).mp3 |
2 times 2 times 2, so it's 2 to the 23rd. So there's 22 to the 23 different versions that I can contribute to any son or daughter that I might have. We'll talk about how that happens when we talk about meiosis or mitosis. That when I generate my sperm cells, sperm cells are essentially, instead of having 23 pairs of ch... | Variation in a Species (4).mp3 |
That when I generate my sperm cells, sperm cells are essentially, instead of having 23 pairs of chromosomes in sperm, you only have 23 chromosomes. So for example, I'll take one from each of those, and through the process of meiosis, which we'll go into, I'll generate a bunch of sperm cells. And each sperm cell will ha... | Variation in a Species (4).mp3 |
So maybe for this chromosome, I get it from my dad. From the next chromosome, I get it from my mom. Then I donate a couple more from, I shouldn't have drawn them next to each other, I donate a couple more from my mom, then for the chromosome number 5, it comes from my dad, and so on and so forth. But there's 2 to the 2... | Variation in a Species (4).mp3 |
But there's 2 to the 23rd combinations here, because there are 23 pairs that I'm collecting from. Now, my wife's egg is going to have the same situation. There are 2 to the 23 different combinations of DNA that she can contribute, just based on which of the homologous pairs she will contribute. So the possible combinat... | Variation in a Species (4).mp3 |
So the possible combinations that just one couple can produce, and I'm using my life as an example, but you could use this, this applies to everything. This applies to every species that experiences sexual reproduction. So if I can give 2 to the 23rd combinations of DNA, and my wife can give 2 to the 23 combinations of... | Variation in a Species (4).mp3 |
Now, just to give an idea of how large of a number this is, this is 12,000, roughly 12,000 times the number of human beings on the planet today. So there's a huge amount of variation that even one couple can produce. And if you thought that even that isn't enough, it turns out that amongst these homologous pairs, and w... | Variation in a Species (4).mp3 |
And all that means is that when these homologous pairs during meiosis line up near each other, you can have this thing called crossover, where all of this DNA here crosses over and touches over here, and all of this DNA crosses over and touches over there. So all of this goes there, and all of this goes there. And what... | Variation in a Species (4).mp3 |
And the chunk that came from my dad now has a chunk that came from my mom. Let me do it in the right color. It came from my mom like that. And so that even increases the amount of variety even more. So you can almost now, instead of talking about the different chromosomes that you're contributing, where the chromosomes... | Variation in a Species (4).mp3 |
And so that even increases the amount of variety even more. So you can almost now, instead of talking about the different chromosomes that you're contributing, where the chromosomes are each of these collections of DNA, you can almost go to the different combinations at the gene level. And now you can think about an al... | Variation in a Species (4).mp3 |
And you can think about all of the variation that might emerge when you start mixing and mashing different versions of the same gene in a population. And you don't just look at one gene. I mean, the reality is that genes by themselves very seldom code for a specific. You can very seldom look for one gene and say, oh, t... | Variation in a Species (4).mp3 |
You can very seldom look for one gene and say, oh, that is brown hair. Or look for one gene and say, oh, that's intelligence. Or that is how likable someone is. It's usually a whole set of genes interacting in an incredibly complicated way. Hair might be coded for by this whole set of genes on multiple chromosomes. And... | Variation in a Species (4).mp3 |
It's usually a whole set of genes interacting in an incredibly complicated way. Hair might be coded for by this whole set of genes on multiple chromosomes. And this might be coded for a whole set of genes on multiple chromosomes. And so then you can start thinking about all of the different combinations. And then all o... | Variation in a Species (4).mp3 |
And so then you can start thinking about all of the different combinations. And then all of a sudden, maybe some combination that never existed before all of a sudden emerges. And that's very successful. But I'll leave you to think about it because maybe that combination might be passed on or it may not be passed on be... | Variation in a Species (4).mp3 |
But I'll leave you to think about it because maybe that combination might be passed on or it may not be passed on because of this recombination. But we'll talk more about that in the future. But I wanted to introduce this idea of sexual reproduction to you because this really is the main source of variation within a po... | Variation in a Species (4).mp3 |
And it's kind of a philosophical idea because we almost take the idea of having males and females for granted because it's this universal idea. But I did a little reading on it. It turns out that this actually only emerged about 1.4 billion years ago. That this is almost a useful trait because once you introduce this l... | Variation in a Species (4).mp3 |
That this is almost a useful trait because once you introduce this level of variation, the natural selection can start. You can kind of say that when you have this more powerful form of variation than just pure mutations, and maybe you might have some primitive form of crossover before. But now that you have this sexua... | Variation in a Species (4).mp3 |
So they started to essentially outnumber the ones that couldn't. So it became a kind of a very universal trait. But you could have imagined a world, and there are science fiction books written about this, where you have three genders, where you have gender one, two, three. You could have 10 genders. And it just happens... | Variation in a Species (4).mp3 |
This was based on some observations of meiosis and seeing how chromosomes behaved, and they seemed to behave in analogous ways to some of these inheritable factors, but they really didn't have good cellular proof that chromosomes indeed were the location for these inheritable factors. And we don't really start to get t... | Thomas Hunt Morgan and fruit flies.mp3 |
So why does he want to study fruit flies? Have you ever seen a fruit fly? They're very, very, very small, so you could actually put a ton of fruit flies in one jar. So that's convenient. You oftentimes don't think about the practical logistics of science, but you could put a lot in one jar. They were actually cheap, an... | Thomas Hunt Morgan and fruit flies.mp3 |
So that's convenient. You oftentimes don't think about the practical logistics of science, but you could put a lot in one jar. They were actually cheap, and that's another practical concern of science is you don't always have a lot of resources to do your work. And they had short lives, and they reproduced a lot. So yo... | Thomas Hunt Morgan and fruit flies.mp3 |
And they had short lives, and they reproduced a lot. So you could very quickly get many, many offspring in many, many generations if you wanted to study how the different traits were passed on or not passed on. And so he spent some time, he started this in 1908, working with the fruit flies, and he kept breeding them i... | Thomas Hunt Morgan and fruit flies.mp3 |
In general, when you look at traits in a species, the wild type, let me write this down, the wild type is the one that's typically seen, while the mutant trait is something that seems unusual. And after two years, he finally discovers a mutant trait in his fruit flies. He finds a white-eyed male. So this is the white-e... | Thomas Hunt Morgan and fruit flies.mp3 |
So this is the white-eyed male right over here. He says, okay, now this is interesting. Let me take this white-eyed male and begin to cross it with other, with, well, with the females. And you say, well, how does this actually occur? Well, what you do is you take a jar full of females and you put the white-eyed male in... | Thomas Hunt Morgan and fruit flies.mp3 |
And you say, well, how does this actually occur? Well, what you do is you take a jar full of females and you put the white-eyed male in there, and then the crossing happens. And what was interesting was the inheritance pattern that he saw for this white-eyed trait, because you have the parent generation here, but then ... | Thomas Hunt Morgan and fruit flies.mp3 |
And so just off of that first generation, it wasn't clear that anything interesting was going on. But then when he crossed these to each other, and I know what some of y'all are thinking, wait, aren't they all brothers and sisters being crossed to each other? Well, yeah, they're probably half brothers and sisters if th... | Thomas Hunt Morgan and fruit flies.mp3 |
But yes, that's what people are talking about when they're crossing the F1 generation. But when they crossed these with each other, he saw a pretty interesting pattern. He saw a three to one ratio of red eyes to white eyes. So for every four fruit flies, he would see, let me underline these, he would see three red-eyed... | Thomas Hunt Morgan and fruit flies.mp3 |
So for every four fruit flies, he would see, let me underline these, he would see three red-eyed, and he would see one white-eyed. So the white-eyed trait makes a reappearance, which in and of itself is interesting. It shows that this can be passed on genetically, and that's interesting because this was a mutant that j... | Thomas Hunt Morgan and fruit flies.mp3 |
But what was even more interesting about this three to one ratio, and that three to one is something that popped up a lot in Mendelian genetics. But what was even more interesting was that he only observed, he only observed the white eyes in the males in this F2 generation, in this second generation of the crosses righ... | Thomas Hunt Morgan and fruit flies.mp3 |
Well, he was a pretty astute guy, and he says, well, look, if I'm only seeing it in the males, and it's not like he only got four offspring, it was in the ratio, he might have had hundreds of them, but it was in the ratio of two red-eyed females for every one red-eyed male for every one white-eyed male. And so across t... | Thomas Hunt Morgan and fruit flies.mp3 |
And so what he was able to do is say, well, let's just assume that it is. Let's assume that that trait, that mutant allele, that mutant variation of the gene for eye color, let's assume it's carried on the X chromosome. And so the genotype for that first mutant fly, that white-eyed male that he found, we could call it,... | Thomas Hunt Morgan and fruit flies.mp3 |
The way that you would specify the genotype of that white-eyed male is, well, on his X chromosome, he had the white variation, he had the white allele, the white variation of that gene, and then on his Y chromosome, he had no variation for that gene. So we assume that it's only contained on the, only on the X chromosom... | Thomas Hunt Morgan and fruit flies.mp3 |
Well, this is a case where you hemizygous, you only have a version of the allele on one of your two chromosomes, one of the two that you've gotten from each of your parents. So this would be the genotype right here of the white-eyed male. The genotype for the red-eyed female is specified by, so it's on the X chromosome... | Thomas Hunt Morgan and fruit flies.mp3 |
So on each of the X chromosomes, we assume that the females start off with the red allele. And the red allele, the notation is the W plus, W plus. And you might say, well, why don't we just use the letter R? Well, we could have, but the general convention in genetics is to use the letter of the first mutant type discov... | Thomas Hunt Morgan and fruit flies.mp3 |
Well, we could have, but the general convention in genetics is to use the letter of the first mutant type discovered for that gene, and then to use this little plus type for the wild type. So the wild type is the red eyes, and then W, which is the first mutant discovered for this gene, is the first mutant allele, that ... | Thomas Hunt Morgan and fruit flies.mp3 |
So these are the genotype of the red-eyed female. And so when you cross that first generation, well, the white-eyed male, he can either, if he's with the, he'll either produce sperm that have the X chromosome in it, which is going to contain the allele, or sperm that have the Y chromosome in it, which is not going to c... | Thomas Hunt Morgan and fruit flies.mp3 |
And we can see how this crosses. You could get an X from both parents. If you get an X from both parents, you're going to be female, because you're going to be XX. And each of these females, since you got one wild type and one mutant type, and the wild type turns out to be dominant, they still show, their phenotype is ... | Thomas Hunt Morgan and fruit flies.mp3 |
And each of these females, since you got one wild type and one mutant type, and the wild type turns out to be dominant, they still show, their phenotype is still red eyes. They still have red eyes. But now they are heterozygotes. They are carrying the white allele. Now the male offspring right over here, well, in order... | Thomas Hunt Morgan and fruit flies.mp3 |
They are carrying the white allele. Now the male offspring right over here, well, in order to be male, they got the Y chromosome from their dad. So they're not able to get that white allele. And they get the red, the wild type, from their mom. And you could see it here. And this is why all of the males in that first ge... | Thomas Hunt Morgan and fruit flies.mp3 |
And they get the red, the wild type, from their mom. And you could see it here. And this is why all of the males in that first generation were red. They only got one copy of the allele from their wild type mother. But then what was interesting is the crosses that you see in that next generation. If you took these red-e... | Thomas Hunt Morgan and fruit flies.mp3 |
They only got one copy of the allele from their wild type mother. But then what was interesting is the crosses that you see in that next generation. If you took these red-eyed females that we already established, these are all going to be heterozygotes. And so you could see they have the red allele and they have the wh... | Thomas Hunt Morgan and fruit flies.mp3 |
And so you could see they have the red allele and they have the white allele. And you cross that with red-eyed males. You cross it with red-eyed males. What is going to happen? Well, the females in this generation, in order to be female, you have to get an X from your mom and your dad. And so they get an X from their d... | Thomas Hunt Morgan and fruit flies.mp3 |
What is going to happen? Well, the females in this generation, in order to be female, you have to get an X from your mom and your dad. And so they get an X from their dad, which has the mutant, sorry, which has the wild type there, the dominant red allele. And so regardless of which one they got from their mom, they're... | Thomas Hunt Morgan and fruit flies.mp3 |
And so regardless of which one they got from their mom, they're still going to be red-eyed females. Some of them might be homozygotes. Some of them might be heterozygotes. But now we see something interesting happening in the males. You could have heterozygote male flies here, where they got the X from, where they got ... | Thomas Hunt Morgan and fruit flies.mp3 |
But now we see something interesting happening in the males. You could have heterozygote male flies here, where they got the X from, where they got the red X from their mom. Or you could get the hemizygous white-eyed males, where they got the white allele, the white X from their mom. And this is the exact observation t... | Thomas Hunt Morgan and fruit flies.mp3 |
And this is the exact observation that Morgan made. So it was a very interesting thing that he was able to see. He started breeding these in 1908. He started breeding these flies in 1908. It wasn't until a couple of years that he finally found that first mutant white-eyed male. And it was in 1910 and 1911 that he publi... | Thomas Hunt Morgan and fruit flies.mp3 |
He started breeding these flies in 1908. It wasn't until a couple of years that he finally found that first mutant white-eyed male. And it was in 1910 and 1911 that he publishes these discoveries in Nature. And the reason why this is a big deal is he says, look, my observations are completely consistent with this eye t... | Thomas Hunt Morgan and fruit flies.mp3 |
And the reason why this is a big deal is he says, look, my observations are completely consistent with this eye trait, this gene, being on the X chromosome. So he was able to show a direct linkage between, in this case, sex chromosomes, and these heritable factors that Mendel first talked about. And he would go on, and... | Thomas Hunt Morgan and fruit flies.mp3 |
In fact, there are two key places that we have extensive regulation of the cell cycle. The first checkpoint is right here between the G1 and the S phase. So we regulate before we get to the point of DNA replication. The other major checkpoint is right here between G2 and the step where we jump right to mitosis. And the... | Cell cycle control Cells MCAT Khan Academy.mp3 |
The other major checkpoint is right here between G2 and the step where we jump right to mitosis. And there are a couple of proteins that regulate this process. Two main ones are called cyclin-dependent kinases. So cyclin-dependent kinases, which as you may recall, a kinase is something that adds a phosphate group. So I... | Cell cycle control Cells MCAT Khan Academy.mp3 |
So cyclin-dependent kinases, which as you may recall, a kinase is something that adds a phosphate group. So I'll put in parentheses, it'll plus a phosphate group. And it'll add a phosphate group on other enzymes or proteins to either activate or inactivate them. And these cyclin-dependent kinases will work together wit... | Cell cycle control Cells MCAT Khan Academy.mp3 |
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