 Thomas Hunt Morgan's life turned upside down when he discovered a die-hybrid cross between fruit flies that didn't follow Mendel's law of independent assortment. The cross should have given him an offspring population in which half of them look exactly like the parents and the remaining half look completely brand new, a.k.a. they were the hybrids. However, instead of this 50-50 population, Morgan got way more parental look-alikes compared to the hybrids. There were about 83% parental look-alikes which were carried out by these two gametes over here and the hybrids which were about a mere 17% they were taken care by these two gametes. Now these gametes are super important for our video over here. We're going to be referencing them a lot throughout this entire thing. And a very easy trick to remember which gamete is responsible for what is to look at the case of the letters of the gametes. So you can see that each gamete is represented by two letters over here, right? So if both of them are uppercase or lowercase letters, then that gamete is going to be responsible for the parental phenotypes. Meanwhile, if it's a combination of both, then it's definitely going to be a hybrid. So that's a very easy way to remember. Remember if it's all uppercase or all lowercase, it's going to give you parental phenotypes. If it's a combination of both, then it's going to give you a hybrid. So now we had to figure out why this happened, why didn't Morgan get this 50-50 population and got a much different one compared to that. So to answer this, we looked into the gamete production of these fruit flies and we discovered a couple of very important things. First, we found out that the genes which are responsible for the traits of this cross, they are present on the same chromosome. Why? Because if they were present on different chromosomes, then they would end up following the law of independent assortment. And if that happened, then it would give us the perfect 50-50 proportion that we've been looking for all this time. In that case, we would have 50% parental lookalikes and 50% hybrids. And we know for a fact that this did not go down in Morgan's case. So we can be sure that the genes are on the same chromosome. However, randomly arranging these chromosomes ended up giving us only parental lookalikes and no hybrids. All of them looked exactly like the parents. And we know for a fact that that also didn't happen in Morgan's case. Even if the numbers were pretty low, we did have some hybrids. So we know for a fact that even though the genes are on the same chromosome, randomly arranging them isn't going to cut it. So we need to find a way to keep these genes on the same chromosome and also come up with hybrids. So that is going to be the goal here of this video. We are going to find out a way to have hybrids while having these genes on the same chromosome. So what do you think we can do about this? How can we achieve a feat of this sort? However, before we move on, there's one thing that I would like to do which is going to make things super simple for us. Let's get rid of this extra pair of chromosomes right here. Now why did I do that? Because all the genes that we're going to work with, they are already here because they are present on the same chromosome. We don't need any extra chromosomes to work with. Whatever genes we need, they're already here. We needed those chromosomes earlier because we were considering a situation where the genes could have been on different chromosomes. But that's not the case anymore. We are pretty sure that the genes are present on the same chromosomes. So why do we need to keep these chromosomes and confuse us in the process, right? So let's get rid of all the extra chromosomes. Great. So we have all of the extra chromosomes removed from our entire diagram. Now we're going to dive deep into how this process actually works out. Let's go. Now all this time we have been playing around with the chromosomes only, right? We haven't really looked at the position of the genes or tried to do anything with them. So this time, why don't we try exchanging the genes instead of the chromosomes? So we are going to exchange these two genes, the capital B and the small b genes and let's see what happens if we try to do that. So if we exchange the positions of these two genes, then the chromosomes would look something like this. Now notice carefully earlier we had these two combinations to only work with. We had capital B, capital L, small b and small l which were the committees that were responsible for the parent to look-alikes, right? This time we have two new combinations to work with. We have small b, capital L and then we have capital B and small l. These two combinations are ones that we didn't have before. We have these for the first time now. So if we exchange the genes first and then we randomly arrange them into their respective gametes, then we just might have a chance at solving out this mystery. Now the best part about this assumption is that this is not really an assumption. This is reality. This exchange really does take place and this is the solution to our problem. This exchange affects the genes which are present on the same chromosome and it gives us newer combinations or hybrids to work with at the same time. But how exactly does this exchange happen? It's not like we just want them to exchange themselves and it just so happens to be like this. No, there is a method to this madness. Let's figure that out. During gamete production, there is this one step in which the chromosomes duplicate. An identical copy of this existing chromosome is produced with the same genes and everything. So now we have this new copy of the existing chromosome that has been produced and these two copies, they are attached together at this one point in the middle over here. So now we have two pairs of chromosomes which came from these existing chromosomes. So we have two X-looking chromosome pairs, something like this. Now each strand of this chromosome pair is called a chromatid. And strands of the same chromosome pair, they are called the sister chromatids. Meanwhile, the strands which belong to different chromosome pairs are called non-sister chromatids. So this strand and this strand, these two will be the non-sister chromatids. Now why am I telling you about all of these things? Well, the exchange that we were talking about earlier, the exchange of the genes, they take place between these chromatids, mainly between the non-sister chromatids. Now they do take place between the sister chromatids but they all have the same genes. The sister chromatids have the same genes. So even if exchange takes place, we won't get anything new out of it because this B will exchange and give another B, like another capital B. So that doesn't really give us anything new to work with. But if that exchange takes place between non-sister chromatids, where we know for a fact that there are different genes present over here, then that's what we're really aiming for. Now for this exchange to take place, the chromatids of these chromosome pairs, they're going to cross over each other something like this. And boom, they exchange their positions. Now this event where this exchange takes place because the chromatids go ahead and cross over each other, this event is called crossing over. Quite literally. So now that the genes have exchanged their positions and crossing over is done, the next question is what happens after this crossing over? Well, after crossing over, the chromosome pairs that we had over here, they are going to completely come apart. They will break apart or they will separate from their pairs and they will become single chromosomes once again. So we had these chromosome pairs over here, they are going to now come apart or they're going to separate or break apart and they're going to give us the single chromosomes that we're going to work with. So now that our chromosomes have separated out and they're into single files once again, this cell will now go ahead and give us the gametes that we need. And in true gamete production style, this cell will divide into four daughter cells or four gametes. And we know that all of our gametes have only one set of chromosomes, right? So each of these chromosomes will have a chance of getting expressed in each of these four gametes that we're going to get out of the cell. So we're going to get one cell with just this chromosome, then another one with just this chromosome and so on and so forth. So that way now we have four different types of gametes that we can go ahead with. Now will you look at these four types? Aren't they the same gamete types that we had in Morgan's Cross? Does that mean that we've solved this mystery? We finally know what went down in Morgan's case. Well, not really. You see there's a problem with this gamete production over here. You see the chances of these four different types of gametes and them having each one of these types of chromosomes or this combination, all of them have an equal chance of auguring. So all of them have a 25% chance of being produced. So that means that combining the gametes which are responsible for the parental lookalikes, which is this one and this one, if we combine them, then both of them will have a 25-25% chance. So the parental lookalikes will have a 50% chance of occurring, 50% P, let's put it here. And our hybrids, which are these two over here, they'll also have a 25-25% chance. So they're also going to have a 50% chance of occurring, which means that this will give us a 50-50 proportion and that means that it's going to follow the law of independent assortment. And we know for a fact that Morgan's fruit flies did not do that. So this isn't exactly what is happening. We're on the right track, we're pretty close to it, but this isn't what is really happening. So where are we going wrong then? We seem to have everything that we need and yet we cannot solve this mystery because our numbers are not right. For some reason, we are not getting the 83-17% population that Morgan had gotten. We are just not getting it, instead we are stuck with this 50-50 thing. So here's my next question to you. In this entire gamete production diagram that we have, which event is directly affecting these numbers, like which event in this thing or which cell over here is going to determine that we're going to get 50-50 population, the exchange thing, the gene exchange, the crossing over event, this is the one event that is determining everything. So there has to be something wrong with this thing. Something must be going wrong here that is affecting these numbers. So we are only getting these numbers if crossing over is a very successful event. So what if this crossing over is not a successful event? What if crossing over happens but the genes don't separate out like they should? What do you think will happen? Let's say that the chromatids, they cross each other but there is no separation of genes. The genes stay stuck together and they don't separate out and hence there is no exchange of genetic material at all. What do you think an unsuccessful crossing over will look like? Now that's a question we're going to explore in another video but till then keep thinking.