 Imagine, a long long time ago one of your forefathers decided to hide away some treasure and you just found out about it. So you've decided that you're going to go out on an adventure and retrieve this treasure for yourself. But before you get started, there's one thing that you're going to need in order to find this treasure and that is a map because this map will tell you exactly where the treasure is hidden and it will give you the directions to actually get there because isn't that what all maps do? It's going to give you the exact information where it tells you the precise location of a certain thing or a person or a place. I mean, if we didn't have maps, exploring the world would be a pretty difficult job. Now, imagine it this way, we have thousands of cities and towns and all of these other places which is easier to explore because we have a map. Now, similarly, there are thousands of genes in your body. Now, wouldn't it be handy if you had a map for that as well? I mean, if you know exactly where each and every gene is, that would make research and experiments so much more easy, so much more efficient. And for the best part about this map is that we already have a map. We have a map that tells us the location of each and every gene perfectly in our entire system. Now, how did we manage to do something like that? Did we actually get a huge microscope and decided to look into every tiny chromosome in our body? Well, not really. And also, who even had this idea of coming up with something so amazing? Well, we'll be answering all of those questions in this video. And we'll be talking all about what gene mapping is, who came up with it, and how can we possibly find out the location of something so tiny? Gene mapping was established by this dude called Alfred Sturtevant. He was this brilliant, brilliant scientist who was also a student of Thomas Morgan. Now, Morgan and his team, along with Sturtevant, they were studying this fruit fly cross. And while he was studying it, he realized that if he collected all the data from this cross, he will be able to pinpoint exactly where each and every gene is on the chromosome. And that's how Sturtevant ended up mapping a whole chromosome. But wait, how did he even do that? Let's bring up the cross that he was actually working on. Morgan and his team, including Sturtevant, were working on this fruit fly cross, which included two major genes. One of the genes was controlling the body color and the other gene was controlling the length of the wing. Now, the thing about this cross was that it stumped Morgan and his entire team because unlike Mendel's perfect 50-50 population of his peas, Morgan got a fruit fly population in which most of them, about 83% of those offspring, looked exactly like the parents. While only a mere 17% were hybrids. Now, this was a huge, huge problem back in the day because so far back then, all they knew was that it should have been a 50-50 thing and not such a weird, bizarre number like that. So eventually, Morgan and his team figured out why this was happening. And the reason behind these numbers was something called linked genes. Linked genes. Now, what are these linked genes? Well, sometimes in your chromosome, you will find two genes which are so close to one another that it makes it very difficult for them to separate out. So when they end up crossing over, they cross over as a package. Let me quickly draw two chromosomes and show you exactly how these genes look like. These two genes, right over here, they're pretty close to one another. So when something like that happens, when the genes are really, really close to one another, then the chances of crossing over to happen between them, so in this space right here, it's very difficult for crossing over to happen in this distance between the genes because it's so small. Because of that, the chances of recombination also drops. So when two genes are super close to one another, then crossing over chances or recombination chances drop like crazy. At the same time, if these genes, let's assume that they were super far away from each other instead of being so close, then in that case, the chances of crossing over or recombination is much, much higher. Because you have all these points or so much of space to work with, right? We don't have that in this close situation. So in this case, where the genes are far away, we are going to have a much higher chance of recombination to take place. Now in this case, the ones which are super close to one another, this BNL gene over here, these are the linked genes. Now, I've been telling you that if the genes are closer to one another, then the recombination is less or crossing over is less. But if they're far away, then they're not. And be saying that it's closer or higher or less is a very vague way of saying things. So if you had to quantify or let's say you have to put a number to this degree of linkage between two genes, if you had to do that, then you would be able to do that with the help of something called recombination frequency. So it's recombination and there we have frequency. Now, your first question is probably that what is this? What is a free recombination frequency? Well, let's translate this term word by word. So recombination is the event of getting recombinants, right? So it's the way, it's the event that's going to give you more hybrids, technically. So and frequency refers to a number or a percentage. So the number or the percentage of hybrids in a population is the recombination frequency, that is technically it. That is what we refer to as RF or the recombination frequency. Now, how will this help us in figuring out the degree of linkage? And again, how is this RF going to help us map out an entire chromosome? Let's figure that out now. So essentially, we said that the recombination frequency is nothing but the number or percentage of hybrids in a population, right? So from the cross, from this Morgan's cross that the one we have been talking about, I have separated out four types of offspring which we got. And I have also put down the exact numbers that they had gotten. So we just need the exact number of offsprings that came from this cross. So in order to figure out, this is exactly what we're gonna calculate. If we have to figure out how to calculate the RF, we are going to do exactly what we just said. We will find out the percentage of hybrids in this cross. So the first thing we're gonna do is we are going to add up the total number of hybrids that we got. So the ones marked in blue, they are the hybrids. So if we calculate this, we are going to get 391. So that's the total number of hybrids that we have. And also notice that the number of hybrids are pretty low as compared to the parent to lookalikes, I mean they are in 900s and these are barely making it to 200s. So now we are going to add up the entire population because if we have to find out the percentage of hybrids, we need to divide the total number of hybrids with the whole population, with the total number of all the offsprings that you have. So we are going to quickly sum this up, 944 and then this is the hybrid. This is going to give us 23, 2300. So this is the total number of population that we have. Now we are going to quickly find out the percentage of hybrids. So we have the hybrids divided by the total population times 100 and that is going to give us 17%. Now let's go back quickly and isn't that what exactly we had seen over here? 17% of hybrids. So that's how this number came. This is the actual percentage of whatever hybrids that they had gotten. And this is the RF. But we're still not quite sure how this RF is going to help us map out the gene, right? So let's clear this out. So we have gotten 17% as our RF between B to L. And this is the RF that we have. Now what does this technically mean? So if I draw a chromosome over here, let's use another color. Okay, so here we have a chromosome. And these are our genes over here. So we have the B gene and over here a bit far away, we have the L gene. So this is B and this is L. Now when I say that the RF between B and L or the RF of B to L is 17%, it means that this piece between them, there is a 17% chance of recombination to happen between these two genes. Now, this chance of recombination happening is also giving us an approximate idea of how far the genes could possibly be from one another. So between this particular distance over here, wait, I'll use a different color. So between these two genes right over here, this is the part where you have a 17% chance of recombination to take place. And also, arbitrarily, there is a distance of about 17% between them. So it's not the exact. So always remember that RF is not the exact value or it doesn't tell you exactly where a gene is. It's not a true value, but an approximate estimate of how far the genes can be. So when I say that there's a 17% chance of recombination to happen between them, it means that these two genes, B and L, are somewhat 17 units away from one another. So this 17% can be written as 17 arbitrary units. So you can say that that's the distance between B and L. Now that we know that how the units are done, so now we go on to the most important part about RF. And that is that in this chromosome, this entire distance from one end to another cannot go beyond 50%. Can you guess why? That's because the chances of recombination to take place between two genes which are absolutely far away, like on the farthest end of the same chromosome or if they are on different chromosomes. In that case, there will always be a maximum of 50% chance of recombination to take place. So you can say that the frequency maxes out at 50%, it cannot go beyond that. And a very easy way of figuring that out is if you draw out an actual punnett square. Let's say that you put the parents on either side and you did the whole math. Then you'll see that out of the four types of offspring that you're getting, two of them will always look like the parents and the other two will be completely new combinations. And isn't that what happened with Mendel anyway? Like 50% of his peas looked absolutely new while the remaining 50% looked absolutely like the parents. So that is exactly what I mean when I say that the recombination frequency maxes out at 50%. So that is why the maximum distance between two genes can never exceed beyond 50% or 50 units because that's the highest chance of recombination to take place. So RF will always max out at 50% and which means that the total distance of this entire chromosome from one end to the another, that will be 50 units. So we now know the idea, right? So if it's 17%, then this is how the distance will be. So if this wasn't 17% and it was, let's say about 45%, then this L gene would have been somewhere around here. And if it wasn't, if it was lesser than 17%, let's say it was about 10%. Then in that case, this L gene would be much closer to B. So that's how we are able to figure out how far away or how close the genes are to one another. Now let's add another gene to this mix and try to figure out where these other genes could be. So we have an extra gene this time called C, which is short for cinnabar. It's a gene for eye color also in fruit flies and also something that Morgan worked with. But all of that details doesn't matter. So we have three genes now, and we have their respective RFs as well given right next to it. So what we're going to do is that we'll quickly draw a chromosome first, and then we are going to place these genes on these chromosomes. So my chromosome is a little crooked, but we'll make do with it. So we have our RFs given. So the first thing we're going to do is we will go ahead and plot B and L on this chromosome, which is going to be super easy because it's the highest. And we'll always start with the highest because those two genes are the farthest from each other. So we are going to go ahead and place those two genes right over here like that. So now we have these two genes. There is a 17% or about 17 units of distance between them, which is the relative distance or the approximate relative distance that we just talked about. And now we have to figure out where we can place this third gene, which is C. So we have two different relationships that we can go with. One is with L, one is with B. So let's pick out this C to B ratio first. And so we can either place the C before B, oops, yeah. So we can either place it before B or we can place it after B like this. So in that case, we'll have to figure out which one makes more sense. So now in this case, now we have to consider the distance between C to B. So in that case, we know that C to B is 9%. But then we also have C to L, which is 8%. So if you put C over here, then the C to L distance becomes a whole lot more than 17%. And that's just not correct. So C will definitely not be here. Instead, C will be here. Because if it is placed over here, then the C to B distance makes sense, which is about 9%. And the L to C distance is about 8%. So again, it makes sense. So that is how you can figure out the order or the sequence of genes on a chromosome. And this is how you can effectively map an entire chromosome, which is basically what Sturtevant and Morgan had done all those years ago. Now, the only thing to remember is that you need multiple frequencies in order to find out this order. Because if you don't, then it becomes really difficult. At present, we don't use these methods anymore because we have more accurate, fancier ways of sequencing a whole genome in a much less amount of time. And they actually tell us the exact location, unlike this approximate value that Morgan and Sturtevant had gotten. But the underlying principle remains the same. They are still an exact match to what they had done back then and which is why both Morgan and Sturtevant and their entire team are known to be the pioneers of gene mapping.