Mapping Overview - Online Tutor, Practice Problems & Exam Prep

Mapping Overview

Hi. In this video, I'm going to 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, you know, 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, from now on, we're pretty much going to be focusing on things happening on a single chromosome for this video. And 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're inherited together. Because of the fact that they exist on that same chromosome, and during meiosis, those chromosomes are separated. It's not that the chromosomes are chopped up and some cells 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, you know, 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 assorting, they're independently assorting, 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, F₂ ratios, remember we've been dealing with these F₂ population, generally from a heterozygous cross, now this is important, it's not from every cross, it's from a heterozygous cross, You're going to get a 1 to 2 to 1 ratio. And if you do a test cross, which first do you remember what a test cross is? Right? That's taking something that you're interested in and crossing it with a homozygous recessive for all of the traits, so it's entirely homozygous recessive. Test cross will be 1 to 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 a linkage group. This line is going to represent a chromosome. You're going to see this line a lot, and you're going to see genes, represented this way. Now remember, because this is one chromosome, these are alleles. So here we have a dominant P allele, we have a dominant D allele, and a recessive r allele. 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 going to be passed together. Now, I define complete linkage as something that is always inherited together, so they're always on the same chromosome. But sometimes, alleles on the same chromosome, so something like this, are not inherited together. And you say, woah, why would that be? Right? During meiosis, the entire chromosome is passed to the daughter cell, So why would a gene on that chromosome not be 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, 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 homologous. They're pairs, but one region breaks and replaces, and they sort of switch out alleles. And this occurs during meiosis, 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, you can see that they start to fold over each other, and they undergo some different things, but eventually they replace colors. So you have this one chromosome that was white and one that was black, and now you have 2 mixtures, because this region has switched. It has crossed over between the 2 chromosomes. Crossing over is super important. We're going to talk about it a lot, but that's this is the overview. These alleles are switched, and that is how alleles 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 mapping, this is an overview of mapping. Right? So chromosomal mapping is performed by looking at the frequency of recombination. So this is looking at the frequency of crossing crossing over, which is why we had 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 from each other, the more likely they are to cross over. Highlight these so you can get it. Now why, and 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