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4. Genetic Mapping and Linkage

Mapping with Markers


Mapping with Markers

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Hi in this video we're gonna be talking about mapping with markers. So mapping with markers attempts to link chromosome or regions without the need to link a little. So we've talked about linkage mapping before and that involved you know having um flies or plants or something. Some kind of organism with different um types making them together. Doing these crosses an attempt to understand the percentage of our competent phenotype in order to understand the distance of the aliens in the chromosomes. Well that's one way to do it. But it's not a way that we can easily do with something like humans right? Because we can't force these matings between humans and then follow the offspring. And that's just not ethical or a good thing to do. So mapping with markers attempts to overcome that in humans. But you can also do it in other organisms as well. So what is a marker? So a molecular markers, some kind of D. N. A segment that has a unique identifiable identifiable property. So it's not the same in every organism or um across across organisms and a species. It's different meaning that it's polymorphic. So it differs between individuals in a population. And so by looking at those markers we can really it's it's really useful to identify locations of unknown genes or unknown alleles which through normal crossings we would have to know what we're looking for at least the phenotype they cause. But markers help us to overcome knowing that and allow us to um map chromosomes are mapped genes or map locations in our chromosomes that otherwise wouldn't be able to be mapped through normal traditional mapping methods. So an example of this is through mapping restriction fragment length polymorphisms or RFLP s. And what they do um is you take restriction enzymes which if you remember are proteins that cut D. N. A. And so um when you take individuals and expose them to a restriction enzyme then this restriction enzyme is not going to cut DNA from every individual the exact same way because there are differences in the sequence that restriction enzyme will cut DNA from individuals differently and there will be different links. And so these different links are due to slight differences in the D. N. A. So things like mutations or single nucleotide polymorphisms. If you remember what those are. Deletions, duplications, these things that make us unique individuals. And so restriction enzymes are able to recognize those and cut DNA differently depending on the sequence that's present. And so when you take the D. N. A. You cut it with restriction enzymes. And then you compare the links of the D. N. A. On a gel. And so when you do that, that is a particular type of marker that you can follow through matings um whether force in a laboratory setting or for humans. Just meetings that occur and look for re combinations of these markers in offspring. So um RFLP s can be mapped through crosses. Like I just said an analysis of recombination. Ints phenotype which is just restating of exactly what I just said. So it's the same as linkage mapping. But instead of looking for phenotype for instance you're looking at the recombinant D. N. A. And so this generates what's called an R. F. L. P. Map. And this is the linkage map of these markers on the genome of an organism. So what this looks like. So we have a traditional pedigree here we have our parents and the offspring and siblings and here are the different Jenna types of them. Now the restriction fragment polymorphisms of different genotype looked like this. So if you have Hamas I guess dominant, you're going to get this one band. If you have a homicide it's recessive you'll get this band. If you're hunters I guess you'll get both bands. And so um just by doing just by taking the sequence, exposing it to restriction fragment length polymorphism to restriction enzymes. And then looking at the RFLP s on this um block. We can actually determine the genotype of the parents and the offspring. And this is solely because of restriction fragment length polymorphism. Not because we know the sequence and not because we actually can physically just see that there's that the parents are both heterocyclic right? Like if we didn't know that before, we could determine this solely using these molecular markers. So a second type of way that I'm going to talk about a second marker that I'm going to talk about less is a micro satellite microsatellites are these repetitive sequences that are over and over and over again. That can be used as markers to map the genome. So there's a see a repeat um it's repeated 5 to 50 times throughout the genome and it's repeated every 10,000 bases for instance. Um in humans. Now we can identify the C. A. Repeats through something like pcr southern blotting DNA micro raises a lot of different experimental techniques that aren't super important to know right now. But just know that we can identify the frequency of the sea A. Repeats and the length and the position of them in an organism and in their genome. Now every individual in this species is going to have a different number of repeats. It's going to have a different position of these microsatellites throughout the genome. And these can be used to identify different organisms. And if those organisms are made together determine the recombination rate and by having the nation of these different links and positions, we can then map them through the same way that we would do through linkage mapping through the same formula using rick competence. But instead of looking at phenotype. So we're looking at the position and number of these um D. N. A segments are these molecular markers in the genome. And so um the important part to get across here is that molecular markers mapping them were exactly the same as linkage mapping. Instead of just linkage mapping uses phenotype is whereas the molecular markers are looking at positions and links of DNA sequences. But both techniques used the confidence, use their combination frequency from matings and crosses to determine the position on the chromosome. It's just that the markers are different than the phenotype that you're traditionally used to working with and that we just learned about. So mapping with markers just uses DNA sequences. So with that let's now move on.

What is a molecular marker?


Which of the following markers is not useful for mapping genes?