Hi in this video, I'm gonna be talking about mapping genes. So finally we're this whole chapters on mapping. But finally this is the video where I'm going to tell you how you determine the locations of genes on a chromosome. So the this traditionally the best way to determine and map jean loh kai. So how you do this is you use recombination frequencies. So this is the frequency of recombinant offspring producing across. These are the offspring that are mixes of the parents. Not the exact the exact peanut ipic, not the not finna typically exact to the parents but the combinations. So you use this formula. Now this says map distance. You can also replace this with recombination frequency, whichever you prefer, whichever your professor uses use that. But essentially they're the exact same thing either R. F. Which stands for word combination frequency or map distance. And this formula says you have the number of re confident offspring over the number of of of total offspring And then you times this by 100 in order to make it into a percentage. So this gives you the ratio and then if you multiply by 100 that gets you the percentage. Now I'm gonna go over more more against cross again. So I went over this cross before and the crossing over section. Now, if you if you find yourself getting very confused over especially how morgan got these offering total. I suggest you go back and watch this video because this this video really explains this cross in every single detail. But essentially what happened if you already seen it, I'm going to just sort of briefly go over it. So what happened is morgue uncrossed Hamas. I guess dominant with homos I guess recessive. You got two gametes. This produced an F. One generation which was hetero zegas dominant for both traits. Then he took that F. One and he crossed it with a tester here and got these. Here's his results. So let me back up. So here are the jena types. Here are the phenotype is here the offspring totals in here are the types. Now the parental types are here because they resemble the parents. Right here we go. Do do these are recombinant because they do not resemble the parents. So this parent here was red long wing and this is purple vestigial, which is what they stand for. Again, if you find yourself confused, go back and check out the crossing over video. Um So here's the red long wing, here's the purple vestigial. These are the parents, here's the red vestigial, the purple long wing. These don't resemble the parents, their combinations of the two. So here the competence. So let's go back look at this formula number of competent offspring over the total number of offspring times 100. So we do that, we get the number of competence, which will be 1 51 plus 1 54 over the total number which have given it to you. But you get this from adding all of these 2839. And then you multiply this by 100 and I get you 10.7%. So this is your combination frequency but it's also your map distance. We measure them in units. So this is your map unit essentially. You can write it however you want. Now what is the significance of? 10.7? Well the significance of 10.7 is that it's 10.7% the length of the chromosome, meaning that the two genes are 10.7. Math units are in dot you a lot of times you'll see it like this apart, therefore physical distance. So 10.7 map units is directly correlated with recombination frequency. So frequency that you get the competence. So the closer. So here we go. Here's the map. So here are two genes and here's the 10.7 map units apart. Now the really important thing to know. And I'm highlighting this in blue and then I'm going to draw a little stars around it in all different colors so that you understand that you need to completely know this statement and you need to memorize it. And this needs to be what you sleep, talk and your to your roommate whenever you sleep talk at night. This is what you need to tell them. The closer the two genes are, the less likely they are to cross over and recombined. Now I explained this before in the overview. Right? This is that analogy of the chairs alongside of hallway, you and your friend are sitting on one side. So here we go, you and your friends are here, it's going to be very unlikely that a stranger on this side is going to sit right in between you, right? Because you guys are close together and so it's going to be less likely that a crossover event happens between you. But if you are far apart here and here then it's much more more likely that a crossover event is going to happen in between you because you guys are far apart. So this is important, make sure you know this because you will be asked questions on it and you need to understand why crossovers happen more likely in genes that are farther apart than closer together. So um The another important thing to realize here is that if you're given the recombination frequency, so let's say you just question and say, you know, the recombination frequency of these two genes is 33%. You can also determine whether that gene is linked or not. So the question would be like, well the recombination frequency is 33%. Is this gene linked? Yes or no, that's a very simple, you know, true false question or multiple choice question you could see on an exam. So it's important to know these two things to linkage is likely occurring. So genes are linked. If the recombination frequency is less than 50%,, It is the opposite. So linkages likely not occurring if there were combination frequencies are close or equal to 50%. Now you say, Okay, that gets 4-50, that's easy to remember. Less than 50 is linked. 50 is not linked, but what about greater than 50? And the answer to that is where combination frequencies are never greater than 50. And the reason is Because if they're 50%, they're not linked, meaning that they're undergoing independent assortment. An independent assortment equally assort alleles, but never can assort them more than 50%. So what I mean by this? So this is the case. You've seen this before twice already, pendant assortment. So these are two genes. These are sorting independently, meaning that they are on different chromosomes, those zones. Right? So a sorting independently means that the genotype of these gametes are going to be this right? 50% parental and 50% recombination. Now, how there's no possible way to make this larger than 50%. Right? The way you would have to make this larger than 50 is if for some reason, instead of this khalil, you've got this one and made it like this and that would make it 75 But that wouldn't be independent assortment, right? Because that's not independently a sorting that's not equally. A sorting the alleles, you got extra levels here, you got three of the dominant and only one of the recessive. So independent assortment says that they have to be equal. You have to have each type of allele represented equally. So the only way that's going to happen is if these two the components are 50%, So you cannot get never going to be greater than 50 because you're not going to get 75% because that is not a sorting equally. This isn't this would not be manned, alien genetics, it's not independent assortment. So that is why it's never gonna be greater than 50. So remember, linkage occurs if it's less than 50, and linkages not occurring if it's equal to 50 and nothing is going to be greater. So you don't have to worry about that. So with that, what's something fun?
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Okay. So now let's talk about modern mapping. So a lot of times I get asked you know, okay this is great. I know how to do this with fruit flies. Right. But is this actually real life? Do people actually still map things with fruit flies? And the answer is that they do? Sometimes I'm sure people do. But that there are new types of gene locus maps. Now the type you're familiar with these are the recombination maps. These use the recombination frequencies. These are the types that I explained to you. Now there are physical maps and this actually uses the genomic sequence. And how do you get the sequence where you have to sequence the D. N. A. Which is more advanced technology than they had during morgan's time. And so these are physical maps. You sequence the D. N. A. You have the nucleotides. And then you use those nucleotides to determine where the gene are located on the chromosome. So you have to sequence the entire chromosome or the genome of the organism to get the actual locations. Um Let me back up. And so um you can also map via certain genomic markers. These are also times where you use genomic sequencing. So you can use single nucleotide polymorphisms. These are single nucleotide changes. And you can say okay well I know there's some snips here and that's what you call them snips? Um and say I know that there's some snips here. Some snips here. So let me use these snip locations to determine where the gene is. You can use restriction fragment length polymorphism polymorphism RFLP S. And these are DNA sequences that restriction enzymes cut If you don't remember I've never heard of a restriction enzyme. All it is is a protein that cuts D. N. A. At a specific sequence. So you can take the D. N. A. You can cut it out specific sequences. You can then sequence that further to look at you know where the cuts were made and how you can use that to determine um jeans locations. And then you also have microsatellites. And these are short repetitive sequences found throughout the genome. There's a lot of repetitive D. N. A. In our genome and these are present in jeans and their present regulatory regions, their presence and sort of what's considered junk DNA. Or non coding regions. Um they're present throughout the entire genome. And you can use them to learn information about where genes are. Right jeans with these sequences can be very easily identified through gene sequencing because you just look for where the repeats are. So these modern mapping mapping methods are much more commonly used today um depending on what you kind of need and what the scientists are looking for. But understanding the traditional methods are super important to understanding the concepts of like how you identify where genes are on a chromosome and their components and parental types and all the things that you just learned. So with that that sounds fun
The genetic distances between three genes (ab, nm, kf) were determined using a two-point mapping cross. Determine the sequence of the three genes.
Ab – nm – kf
Ab – kf – nm
Kf – ab – nm
True or False:Recombination frequencies are never greater than 50%
Using the following data collected from a test cross, calculate the recombination frequency.