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4. Genetic Mapping and Linkage

Mapping Overview


Mapping Overview

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Hi in this video, I'm gonna be talking about an overview of mapping. So mapping is exactly what it sounds like. So a map is, you know, it's this nice picture of where things are in the world or the state or the city depending on what kind of map you're looking at. Well in genetics, mapping is the same. It's usually though a picture of a chromosome and where the genes are located. So mapping is a process that determines the position of genes on a chromosome now. So from now on we're pretty much going to be focusing on things happening on a single chromosome for this video. And so all the genes that are found on the chromosome are called linked genes. And the reason that they're called linked is because they're on that same chromosome and therefore they are inherited together because of the fact that they exist on that same chromosome and during mitosis, during mitosis, the those chromosomes are separated. It's not that the chromosomes chopped up and some sales get parts of that chromosome and some get parts of another chromosome. No, the entire chromosome is replicated and divided into the individual daughter cells. So the chromosome is the unit of inheritance, not the gene, it's the entire chromosome. So if there are multiple genes on a chromosome, then they're all going to be inherited together. And we say these are linked genes. Now linked genes do not follow the rules of independent assortment and the reason that they don't because independent assortment said genes are divided into daughter cells completely independently. But that's not true for genes on the same chromosome because those are inherited together. So they're not independently a sorting their dependent lee a sorting essentially because they're dependent on that chromosome to get into the daughter cell. And so we say that we're going to talk about complete linkage and incomplete linkage, essentially. Complete linkage is genes that are always inherited together. They're always on the same chromosome, but they're also always inherited together. And so if you see F two ratios, remember, we've been dealing with these F two population generally from a hetero sickos cross. Now this is important. It's not from every cross, it's from a hetero sickos cross, you're gonna get a 1 to 2 to one ratio. And if you do a test cross, which first do you remember what a test crosses? Right. That's taking something that you're interested in and crossing it with a home as I guess recessive for all of the traits. So it's entirely homogeneous recessive. Test cross will be 1-1. So if you see these ratios show up in a test or quiz question, then you're starting to believe, oh, these might be linked. So here's an example of the linkage group. This line is going to represent a chromosome and you're gonna see this line a lot and you're gonna see jeans representative this way. Now remember because this is one chromosome. These are a little. So here we have a dominant Pluto we have a dominant allele and a recessive are real and they're all on the same chromosome, which means that they are a linkage group. They're going to be linked together. They're gonna be passed together. Now I defined complete linkage as something that is always inherited together. So they're always on the same chromosome, but sometimes a leal's on the same chromosome. So something like this are not inherited together. And you say, whoa, why would that be right? During mitosis? The entire chromosome is passed to the daughter cell. So why would a gene on the chromosome not be passed that with passed into the daughter cell? Well, the reason is because of a process called crossing over. Now, you're probably familiar with crossing over from some of your intro classes. Um, and we're going to talk about it a lot more. But essentially crossing over is the physical breaking and rejoining of homologous chromosomes. So these are chromosomes that have the same genes on them. They're Hamal against their pairs. Um, but one region breaks and replaces as they sort of switch out of wheels. This occurs during mitosis and it produces genetic recombination, which is a recombining of genetic material and that leads to a new combination of alleles and crossing over is the explanation that explains how genes on the same chromosome may not be inherited together. So here we have a homologous pair HP and um, you can see that they start to fold over each other and they undergo some different things. But eventually they replaced colors. You have this one chromosome that was white and one that was black. And now you have two mixtures. Because this region has switched across over. Between the two chromosomes. Crossing over is super important. We're going to talk about it a lot. But this is the overview. These alleles are switched and that is how a leo's on this chromosome that started out on this chromosome may not be entirely inherited with it at the end of crossing over. So back to napping. This is an overview of mapping, right? So chromosome mapping is performed by looking at the frequency of recombination recombination. So this is looking at the frequency of crossing over, which is why we have to talk about it before. So how frequently crossing over is can be directly correlated with where the gene is located on the chromosome now. Why is that? Well, this is because the closer the genes are together. So closer the alleles are together on a chromosome, the less likely they are to cross over. Whereas the farther the genes are for each other, the more likely they are to crossover highlight these. So you can get it now. Why? Therefore because of these? Because we know this and I'll explain how we know this in a second. But because of this, we can use the frequency of crossing over. So if it happens a lot or if it happens not that often to determine how close the genes are to each other. So why is this like this? Why is closer the genes are together? The less likely they are to cross over. And the best way that I can explain this is if you can just imagine you're in a really long hallway that high school or maybe somewhere in your college or somewhere that you've been in your life, you're in a very long hallway. And on each side Of the hallway there are individual chairs lined up there facing each other. So you have these individual chairs lined up down the hallway. Now you and your friend are on one side sitting on one side. So you've decided to sit maybe towards the front and your friend is sitting quite far away from you on the same side, maybe 30, 40 chairs. Okay, now there's one person, a stranger on the other side. So here we have these chairs are lined up. You and your friend are here and there's a stranger here. Let me back up. So this is you and this is your friend and this is a stranger. Now for some reason, this stranger is like, oh, I want to move over to the other side. So most likely what's going to happen is this stranger is going to just, you know, move across, right? He's just gonna sit somewhere in between you guys because there's so many, there's open spaces, there's tons of spaces here where this stranger just can come and do whatever you want, sit right in between you and it's okay because you guys are sitting so far apart. You know, it's not a weird thing for someone to do. But what would be weird is in this situation say if you and your friend are sitting here and there's one chair in between you. So now you have this stranger and the stranger wants to move sides. So it's very unlikely that the stranger out of all the empty chairs is going to choose that one chair in between you guys. Most likely the stranger will go over here or maybe he'll go over here or maybe even here or here. There's all these other chairs that the stranger could choose, very unlikely that it's going to choose this one open chair that are between you and your friend. Now, I'm not saying it couldn't happen. Right. There's always that weirdo person who may just decide out of all these empty chairs, I'm gonna sit right in between these two friends, it can happen, but it's fairly rare. Right? And so this is the same reason. So if you have two genes that are far apart from each other and you want crossing over to occur, well, it's probably likely it can happen between those genes, right? Because there's so much space in between them. But of course, you know, it may just happen there. But if you guys are sitting really close together and there's only this one area where it could happen. And it's a really small area. And it's kind of unusual and may be difficult to get to, then it's going to be very unlikely to be less likely that crossing over is gonna happen right here, It's going to be much more likely it's going to happen in any of these other spots. And so that is why genes that are closer together are less likely to cross over and jeans farther apart from each other are more likely to cross over. Because you know, there's if you're close together it's very unlikely that a crossover events going to happen in between you. Whereas if you're farther apart it's much more likely a crossover of it's going to happen in between. So that explains that hopefully. So when we I've added this here is the example, this is an example of a chromosome map. You can see here that you have your chromosome, you can think of these as like the chairs lined up against the hallway and you have a P P. L. E. L. Sitting in one a Dll sitting in another and a recessive. Our little sitting further away. And a map is gonna look like this. You're gonna have these numbers that represents distance. So let's say here these are five what are called map units. And I'll explain that later. But these are five chairs in between P. And D. And eight chairs between D. And R. And therefore there's 13 chairs between P. And R. Now chairs don't mean nucleotides, right? These are sort of arbitrary units. They're not feet, they're not nucleotides, they're not any kind of thing that I can measure with a ruler. For instance. They're just saying that, you know, these are arbitrary things. They're five chairs apart there, teach apart. And if we talk about it on a chromosome instead of saying chairs, we just say map units and it's exactly the same thing. But I'll talk about map units more and how you actually calculate these numbers in future videos. But with that let's now move on.