 This is Thomas Hunt Morgan, a scientist from the 1900s who was obsessed with fruit fly genetics. He would perform multiple crosses in order to understand what kind of traits and genes are there in these fruit flies. One fine day he performed a cross that did not follow Mendel's law of independent assortment. Mendel's piece had given amazing results but that wasn't the case with Morgan's fruit flies. So let's dig deeper and take a closer look at what exactly went down in this cross. Now this cross that I keep telling you about were dealing with two very specific traits. One of them was the body color which could either be brown or black encoded by a capital B and a small b and the other one was the length of the wing which could either be long or short encoded by capital L and small l. To perform this cross, Morgan had taken a fruit fly which showed all the alleles of both of these genes. So it had a capital B, a small b, a capital L and a small l meaning that this fruit fly was a heterozygote and since it had the dominant alleles in it of course those are the ones that got expressed. So on the outside this fruit fly had a brown body with very long wings. He crossed this heterozygote with another fruit fly which was black in color and had really short wings. This fruit fly didn't show any dominant alleles only the recessive ones. So this one was a homozygote that showed recessive features only. Now I may have drawn this homozygote in white color for my own convenience because I couldn't use a black color on a black background but I assure you this one was black in color and had really short wings. Now that we have the parents for this cross, the next step is to find out what type or what kind of gametes these fruit flies are going to form. Mendel's law of independent assortment states that each and every gene and their alleles are completely independent of each other. So they can come together and form any kind of combination they really want. So in this case this capital B can combine with this capital L but at the same time this capital B can also combine with the small L. Similarly the small B can combine with the capital L and this small B can combine with this small L to give this gamete. So that means this heterozygous dominant fruit fly, the brown colored one with the really long wings, this one can produce four possible types of gametes and each of them has an equal chance of farming. It's not like that only this gamete will have all the chances of forming and the remaining three gametes will suffer. No, all of them will have an equal chance. So if the total percentage is 100 and we have four possible types of gametes then each of them will have a 25% chance of farming. Now let's see the type of gamete this black fruit fly is going to form. You can see that in its genotype there are no dominant genes at all. None, only recessive ones. So no matter what kind of combination it tries to make because the genes are independent of one another right. So no matter what kind of combination it tries to make it will always form one type of gamete which is a small B and a small L. So if this B tries to combine with this L it's going to be a BL then again this with this a BL, again this small B tries to combine with these two small Ls BL. So no matter what kind of combination you try to carry out with the genes in this black fruit fly it will always give this specific gamete. Great. So we have our gametes now. It's time to figure out the Punnett square. Working out the Punnett square will give you four different types of offspring and each of them will also have a 25% chance of occurring because all of these gametes have that very same chance. Now let's take a closer look at these types of offspring that we have. If you notice really closely you will see that two of these types look exactly like the parents. They are also brown in color with long wings and then black in color with really short wings. So this portion of the offspring population constitutes about 50% and these parental lookalikes are called the parental phenotypes where they don't just look like their parents but they also share the same genotypes. On the other hand the other two types look nothing like the parents. They have completely new trait combinations altogether. This one has a brown body with short wings and this one has a black body with long wings. Completely new combinations that we have never encountered in this entire cross. So these two, they constitute the remaining 50% of the population and they are called the hybrids or the recombinants. This is what Mendel had observed with his pea plants and this is exactly what Morgan was expecting too. But this is not the outcome that he had seen. When he performed this cross he found that there were about 83% of parental phenotypes in the offspring population and only 17% of hybrids in that very same population. So that means for some weird reason these two gametes were getting most of the attention. That means they were more likely to form compared to the other types of gametes. But why? Why didn't they follow Mendel's law? Why were these two more likely to form compared to these two? In order to answer that question we're gonna have to look into the formation or the production of these gametes and how these genes are transferred during that production. Let's take a look. Now you already know that genes are found on the chromosomes. In fact that is something that Morgan himself had established. So over here I have drawn out the sex cell of the heterozygote fruit fly from our previous cross. And here I have drawn out the chromosomes with the genes on different chromosomes as well. So I've placed the genes on different chromosomes. Now you must be wondering why there are two colors. That's because one of the set of chromosomes comes from the mother and the other set comes from the father. So when gametes are formed they undergo different types of events and processes and the final result gives us a combination of different genes. So over here as well we are going to randomly arrange these chromosomes that is something that happens during gamete production and see what kind of combinations we can come up with them. So we're just going to randomly arrange these chromosomes and see what combinations we can come up with. So one is going to be this where we have the capital B and the capital L. Then we're going to have one with the capital B and the small L. So this will be pink and then we have the capital B here, small L here. Then we're going to have small B and capital L. This is small B and capital L. And finally we're going to have small B and small L. Done. We have randomly arranged all our chromosomes and these are the different combinations that we have gotten. Now this is very strikingly similar to the gametes that were being formed if we go ahead with Mendel's law. Mendel had said that the genes are not dependent on one another and if the genes are present on different chromosomes then they would have no problem making these combinations. So over here you can see that we have a BL gamete here, a B small L gamete here, small B capital L gamete and a small L gamete. So over here you can see that all of them have an equal chance of occurring because that's what Mendel had stated and that's what looks like is happening over here because this random combination is going to give every single one of them equal chances. So we have all of our four gametes and in the desired combinations that we want but it doesn't really match with the results of Morgan's fruit flies. Morgan had received more parental look-alikes than hybrids and that is why we said that this gamete and this gamete are getting the maximum preference but over here that's not what is happening. All of them are getting equal preferences, all of them have an equal chance of getting formed. So one thing we can definitely conclude from this is that this isn't what is happening in Morgan's case. Over here if this had worked out then we wouldn't have to prove this entire issue about Morgan's fruit flies not following Mendel's law. So definitely this is not what is happening. Then what can happen? How else can we arrange these genes? They're on different chromosomes now but what if we place them on the same chromosome this time? Not different ones but the same ones. So over here again I've drawn out a gamete and I've placed the genes on the same chromosomes. Now let's see how the combination turns out. We're going to randomly arrange these chromosomes once again. We have another set here so let's do that. Now we just saw that the capital B and capital L, they are getting the maximum preference and again the small b small l, again that gamete is also getting a lot of preference and if you notice one thing here that's all this has. If this is what had happened, if the genes were on the same chromosome and we randomly arrange the chromosomes then we are getting all those gametes which were responsible for the parental phenotypes and this is like all of them. So if you carry out the cross with these gamete combinations then we are going to get 100% of parental lookalikes and that is definitely not what happened in Morgan's case. So we can also rule this one out. That means even if the genes are present on the same chromosome something is different about them or they are going through some other process which is making sure that they can combine to form hybrids at some point because we are not getting any hybrids over here so and Morgan did get some right? It's not like he didn't get any. So this isn't what is happening. The genes might be on the same chromosome but there has to be something else involved too. Now I wonder if and only if we could exchange these two genes wouldn't that be great? Like we replace b we put b over here and this b over here. Now wouldn't that be something? Wouldn't that give you some more newer combinations to work with? Till then we can be sure about at least two things. One, the genes are definitely on the same chromosome and two, some other mechanism fuels their combination. That means newer combinations are still being formed is just that we don't know exactly what mechanism it really is but we will definitely look into it and we're going to talk about this mechanism even more in another video.