 We've been trying to solve Morgan's mystery cross for a while now and we're almost there. And this is all that we have figured out so far. We know that the genes which are involved in this cross are present on the same chromosome because if they weren't then we wouldn't be getting this proportion of parental look alike and hybrids that we have gotten in this cross. Second we found out that randomly arranging these genes aren't just enough because if we were just doing that we wouldn't get any hybrids at all. And we know that that's not the case. So along with the random arrangement of genes, these genes are also undergoing an event called crossing over. And this crossing over is what is giving us these hybrids. However, a perfect crossing over or a successful crossing over will give us a 50-50 proportion and not this bizarre number that we have over here. So we know that the crossing over which is happening over here might not just be successful. But why is that? Why is crossing over not a successful event sometimes? When is it not successful? And how does it even fail in the first place? These are some of the questions that we will be answering in this particular video. So let's go. Now before we find out what is an unsuccessful crossing over, we need to know what exactly makes a crossing over successful. Well, you see this crossing over event has one major job to do. It needs to go ahead and separate the genes so that they can swap or exchange with one another. If the crossing over event fails to separate the genes, then it is not considered to be a successful crossing over event. So if you look over here, we have some blue genes and some pink genes over here on two different chromosomes. You can see that they have crossed over but then again the genes have not separated at all. So these genes are not separated. And that is what makes them a very flopped version of the crossing over event, making them completely unsuccessful or we can say that this crossing over event did not carry out the way it should because the genes never got separated and then they never got exchanged or they never got swapped. When something like this happens, this is what we're going to call an unsuccessful crossing over event. But why is it so? Why are these genes not getting separated in the first place? Well, let me ask you something. You have a chromosome over here and there are a few genes. Let's quickly label those. All right. So we have three genes which are placed on this chromosome in which A and B are super close to one another and B and C are pretty far away from one another. Now, tell me something. Do you think the genes would be separated if the crossing over was happening right between them? For example, if the crossing over happened right between A and B, then do you think they would get separated? Yes, they would definitely get separated. And if it was happening between B and C, then B and C would get separated right. So when we say that two genes are not getting separated, it means that crossing over is not happening between them because they're so close to one another. That is why they don't get separated. But now the question arises, why is crossing over not happening between these two genes just because they're close? Well, sort of. Let's do one extra thing over here. Let's plot all the points where crossing over can possibly happen. And then we're going to see. So I'll just quickly go ahead and mark all those points. There we go. Now, we have all those possible points at which crossing over can take place. Now, I've numbered them out as well so that it's easier for us to follow. Now, can you tell me how many points can you see between A and B where crossing over can take place? Just this one over here, right? This lonesome one point over here. Meanwhile, how many points can you see between B and C? There's like 13 points over here. Quite a lot, right? I mean, compared to one. So you see crossing over between B and C is much, much simpler and easier as compared to A and B. Because between B and C, there are 13 points, 13 options, or 13 chances for crossing over to take place. Meanwhile, there's only one between A and B. So this kind of teaches us two very important things. First, if the genes are far away from one another, then the chances of crossing over or recombination or the chances of getting more hybrids increases. Because if more crossing over happens, then more recombination is going to take place and we're going to get more hybrids. So farther the genes from one another, like B and C over here, higher are the chances of crossing over to take place. Meanwhile, if the genes are super, super close to one another, like A and B over here, then the chances of crossing over decreases or they go down. So it's less likely for crossing over or recombination to take place, which means that we're not going to get too many hybrids. We're just going to get a very limited number of them. This, by the way, over here, that is the chances of crossing over to take place or the chances of recombination to take place. This is called the recombination frequency, by the way. All right, so now that you have an idea of what crossing over really looks like when two genes are close to one another or if they're really far away from one another, let's go ahead and do one thing. Let's go ahead and make an actual crossing over. Let's say that we want to cross over at this point, number five over here. Let's see what happens if we go ahead and do that. Okay then, so crossing over has taken place at our favored number five spot over here, and this is the outcome of the crossing over of the chromosomes that has happened. Now, why don't you pause the screen for a few moments here and tell me the first thing that comes to your mind when you look at this picture? Well, you see the pink genes, that is capital A and B, and the blue genes, small a and small b, they kind of moved, but they moved together, kind of like as a package, as if they're stuck together. Now, we know they didn't get separated because the crossing over didn't happen between them, like between A and B. Instead, it happened between B and C somewhere at five. Meanwhile, you can see that the C gene right below, they kind of stayed the same, nothing really happened. Now, we know that crossing over has happened because you can see the change in the color of the chromosomes, but the genes, they remain together, they never got separated. So you can say that they kind of remained stuck together, like best friends. And these best friend genes, which stay together as a unit or they move together as a unit or a package, they are called linked genes. And the most fascinating thing about these linked genes is that they don't separate, it's not that they cannot. It's not impossible for these genes to be separated out. You can do that if the crossing over point happens to be right between them. But since it is so difficult for that crossing over point to pick out that very specific point, we saw how difficult it was. So for that to happen is a very, very rare event. So most of the time, these genes, they stay together as best friends and that is exactly what we called the linked genes. Now, why am I telling you about these linked genes? What do they have to do with Morgan's Mystery Cross? Well, everything, they have to do everything with that cross. Let's circle back and take a look at that cross for a bit. In Morgan's Mystery Cross, the one thing that stumped us all was the bizarre proportion of offsprings that we were getting. There were a lot more parental lookalikes as compared to the hybrids. Now, does that remind you of something? Let's go back to our linked genes once. Now, one thing that we just learned about linked genes is that they're very difficult to separate because they are really, really close to one another. Now, what happens when genes are super close to one another? The chances of crossing over drops. And if there are lesser chances of crossing over to happen, that means recombination or the chances of recombination also drops. Which means lesser number of hybrids. And isn't that something that we have been dealing with in Morgan's Cross since forever? This is the connection that we've been looking for all this time. Now, keeping these linked genes in mind, let's take a look at what the production of gametes is gonna look like. This time when we look at the production of gametes, there is one major, major change in the diagram itself. You can see that the genes which were involved in Morgan's Cross, the body color and the length of the wing genes, both of these genes are placed super close to one another. So you can see B and L placed really close. And again, B and L are placed really close on the chromosomes right here. And because of this, B and L, they become linked genes. So we already know that if a crossing over needs to happen right between them, it's gonna be a super rare event. And over here in this diagram, that's the event that I've chosen. I have chosen a crossing over point that is right between B and L. And that crossing over has happened, as you can see right over here. Then the chromosomes, they get separated out and then the different types of gametes are formed. Now you can see that the gametes that we are getting from this combination are exactly similar to the type of gametes that we were getting in Morgan's Cross. Except, since this is an extremely rare event, the number of times this event to happen is less. And we will only get hybrids if this rare event happens. So if this rare event happens, we will get hybrids or else if this doesn't happen, we won't. So because this rare event is happening, we are getting our hybrids, but the number of times this happening is really low. So we are going to get a very low number of hybrids because of how rare this event is. Because of how difficult it is to separate the linked genes B and L. Most of the time, this separation is not gonna happen. Most of the time, B and L will stick together and crossing over or the point of crossing over will be somewhere anywhere else, anywhere on this chromosome except for between B and L. So most of the time, we are gonna get the capital BL and the small BL gametes over and over again, but every now and then, every once in a while, we are going to get the hybrids too. Except this time, the number of hybrids is gonna be less. Meanwhile, the number of parental lookalikes are gonna be so much more. And that is why we have this bizarre proportion of numbers that we have where 83% of them are the parental lookalikes. Meanwhile, only 17% are the hybrids. So you see, it was these linked genes all along that caused all the confusion. But if you're thinking that this is where the story ends, then think again. Because studying these genes gave us the power to find out the location of each and every gene in our entire genome. Now, how did that happen? Well, we're gonna find that out in another video.