 Hi there, this is Gregor Mendel. Hi Mendel, what's up? I love to grow pea plants. Yes, he's obsessed with pea plants and today we'll see his obsession led us to discover the law of independent assortment. What is this independent assortment you ask? Well, let's take an example. Mendel this time starts playing with two characters of a plant. For example, let's say we take a pure tall plant with yellow seeds and cross it with a pure short plant with green seeds. See, over here we're dealing with two characters, the height, there's a difference in the height and there's a difference in the color of the seeds as well. So two things to take care of. And so we'll take it really slow because it's very easy to get confused I used to get confused a lot over here. And what does this pure mean? Well, just to remind you, it's pure means that if you were to self-fertilize this plant then all its kids would be tall and yellow. None of them would be short none of them would be green. And similarly, if you were to self-fertilize the other one, then all its kids would be short and green. None of them would be tall, none of them would be yellow. That's what we mean by pure. Okay, so let's get rid of this. And so another big question is what if we cross them? And what kind of options do we get? Let's see. Well, in the first generation we get all tall plants with yellow seeds. None of them are short or green. Does this look surprising to you that we didn't get any short plants or any green peas? Or do you think this is familiar? Think about this for a moment. Can you recall the law of dominance and see if you can answer why this is happening? And if you need a refresher, we've talked about them in previous videos on dominance and segregation laws. So get ready to go back and check that. Okay, so let's see. Let's first write down the traits for the parents and then we'll see what happens to the children. So for this parent, this one, we'll write in yellow, it must have tall, tall, yellow, yellow. And the one this one is having must be short, short, green, green. And just to be clear, what are these things? Well, these things are called genes. Don't worry too much about what they are. Think of genes as something that decides what your characters are. You know how your characters look. So this is the height gene you can think of. These are the height genes. And this is the gene for color. And why there are two copies? Well, for every gene you have two copies, one you get from your dad and one you get from your mom. And why am I writing capital and small letter? Well, the reason for that is again, because you can have a dominant type and you have a recessive type. So the dominant ones are usually written capital and recessive ones are usually written small. So this one has both dominant type of genes, both from the parents it got that. And this one got both from its parents, both recessive. Same is the case with the color. So yellow is dominant that we can see over here and that's why it's dominating. And the green color must be recessive. Alright, so what's going to happen when you cross them? Well, now to think about that, I like to think in terms of sperm and the egg. So let's say this is the sperm from the dad. Let me call this as the dad. Let me call this as the mom. And here's the egg from the mom. So how do these genes pass along? Well, the law of segregation tells us not all of them pass along. One copy of each gene passes along. So one copy of height will come. And since both copies are tall, anyone if it comes, it doesn't matter. It'll be tall. And one copy of color will come. And again, both are yellow. So both are, any one of those will come. And similarly in the egg, one copy of height which is recessive and one copy of the color will come. Okay. And so now when the sperm goes and fertilizes that egg, what will be the genes of that fertilized egg? Can you write that? Can you pause the video and see if you can write that? Alright, let's see. It now gets the tall copy from the dad, from the sperm, and gets the short copy from the mom. This is the height gene now. And similarly it gets the yellow copy from the dad and it gets the green copy from the mom. And this is now the color gene. And so as a result, can you see that these kids are what we call hybrids. They're hybrids because the copies of the genes are not the same. These are pure because they have identical copies of the genes. But these are hybrids because they have different copies of the genes. One dominant, the dominant one gets expressed. So it's tall because tall is dominant. This is becoming, this is getting hidden. And again yellow is dominant. That's why it's being seen. The green is getting hidden. So it's a hybrid. And for that reason, this experiment is called as a dihybrid cross. Why dihybrid? Because it's a hybrid for two characters. There are two characters they're dealing with and it's a hybrid of them. That's basically why it's called a dihybrid cross. Now here is the big question that Mendel is trying to ask over here. So what is trying to ask is, when these genes pass from the parent into its sperms or into its gametes, the sperm cells and the excels, are these genes passing as a package or they are independent? So for example, what I mean is are the tall gene and the height gene and the color gene, is this a single package? Is this one single package? Or are these are two individual or independent packages? Same question over here. Are these two a single package that gets passed on from here to here eventually? Or these are individual? That's what Mendel is trying to figure out because at this point we don't know what even genes are or what exactly are the concept of DNA chromosomes yet to come. So how do we answer that question? Does this experiment answer our question? Not really. At least not this part of it. And the reason is, think about it. Even if these things were packaged let's say over here, this was packaged. Then when it fertilized and gets passed on, this becomes one package. And this becomes one package. Kind of makes sense right? The way I'm writing it over here. But the plans would still look the same. They would still look the same. On the other hand, let's say they were not packaged. Let's say they were individual. These traits were individual. Then over here inside, these things would be individual. But again they would look the same. So how do I know inside these plans whether they are packaged together or they are individual? How do we know that? This is where Mendel decided and maybe you can guess it now. We need to go one step further and see what happens to their children. So let me just get rid of this. And so that's what he does next. He takes one of the F1 generation plant which has this. It takes a hybrid and then self fertilizes it. Okay? And then he takes the seeds and he sows them. And you know what he gets? He gets a lot of tall plants with yellow peas which may not be all that surprising. He gets some short plants with green peas. Again may not be surprising. But this is the surprising part. He also gets some tall plants with green peas and some short plants with yellow peas. This is interesting because now we are getting new variations. We are getting tall plant with green peas and short plants with yellow peas. What could this mean? Mendel looked at these F2 kids F2 generation kids and he got the answer for the question he was looking for. So how does this answer our question? Well let's see. Let's take it slowly. Okay? Remember for what follows. Remember the yellow ones remember they are the ones that comes from the dad and the green ones comes from the mom. Okay? And the question is whether they are packaged or not. Okay? So let's now look at the sperms and the eggs of this particular plant. Okay? What could be the different possible sperms that this plant might get? Let's first assume that they were packaged together. The genes of the dad and the genes of the mom were packed together. What would have happened? Well let's see. If that was the case then the sperm could get the tall, the high gene from the dad but that sperm must also get the color gene from the dad if they were packaged together. They should travel into the sperm as a single package. Right? And another possibility is the sperm will get both of the traits from the mom. Again if that was a package. T and Y. And same would be the case for the eggs as well. These will be the only possibilities if these were a package. You should either get everything from your dad or everything from your mom. Okay? Now let's see what would have happened in this situation. If this sperm fertilizes this egg, we would get tall yellow. Does that make sense? So we'll get this plant. If this sperm fertilizes this egg, again we'll get tall yellow because it will dominate on this. And so again we'll get this plant. What happens if this sperm fertilizes this egg? Again we will get tall yellow. Right? So these three possibilities will only give me tall yellow. And in the last possibility, if this sperm goes and fertilizes this egg, then I would get short green because they're both recessive characters. Then I would only get short green. Now think about this. This is the important part. Okay? If this was true, then we saw that the only possible children we should have gotten is tall yellow or short green. Nothing in between. That means our children should have looked exactly like their parents. But the experiment shows otherwise. In the experiment we got these two others as well, these two variations as well. How is this possible, Mendel asked? How can you have a tall with green? How is that possible? Mendel thought that can only be possible if these traits are not packaged together. Because if they are not packaged together, if these are not packaged, then we can have other possibilities. For example, I can now have a sperm which gets the high trait from the dad but might get the color trait from the mom if they are not packaged. This is a possibility. And similarly, another sperm might get the other way around. It might get the high trait from the dad sorry, from the mom, from the mom and it might get the color trait from the dad. And of course the same for the ex as well. And so you see, in order for the new variations to occur, this has to happen. Which means the traits that get passed along for different characters can pass independently. The height character does not depend on what color character passes or the color character does not depend on what height character passes. They pass on independently. And this is what we call the law of independent assortment. And so Mendel said in order to be absolutely sure that this is what's happening, let's count the number of plants and see what ratio we end up getting. And then let's see theoretically if we get the same ratio. So when he counts them, he finds that this comes is 9. This is about 3, 3, 1. I'm writing the ratios okay, not exactly the number of plants. There were definitely hundreds of plants, but the ratio turns out to be 9 is to 3 is to 3 is to 1. And that's true for all the experiments that he does. So he does this with maybe color and shape. He does that maybe with color of the flower and maybe the color of the piece, all the different experiments that he does, he gets the same ratio. And now let's work out what are the different possibilities we might get over here and see if we get the same ratio. And so just like before we are going to make a box to nicely represent all the possibilities. And since this time we have four different possibilities for X n's perms, there are total four times four 16 different possibilities. So again, great idea to pause the video and see if you can fill up this square yourself. All right, hope you have filled it yourself. This is what we will end up with. Now let's see what these plants look like and the way I will do it, I'll first do the color. Let's see which is yellow and which is green and then I'll do the tall and the short. So whichever has at least one Y, one yellow definitely has going to be yellow. So this is yellow, this is yellow, this is yellow. Remember yellow is dominant. So one Y is enough. One yellow color Y is enough yellow, yellow, yellow, yellow, yellow, yellow, yellow, yellow. And these don't even have that yellow wise. These are the ones that will be green. Green, green, green, green. All right, now let's see which is tall and short. Again, if there is a capital T, it's going to be definitely tall, at least one capital T tall. So T, this is tall, tall, tall. This is tall, tall, tall, tall, tall, tall, tall, tall, I think rest all are short. So rest all would be short, short, short, short. Now let's count. How many tall yellow? One, two, three, four, five, six, seven, eight, nine. That's what we got over here. How many tall green? One, let me use a different color. One, two, three. That's what we got here. How many short yellow? Short, yellow, one. Okay, different color. One, two, three. And how many short green? Only one. Perfect. Mendel was spot on on his theory. So indeed this conclusively proves that the traits passed on are sorted independently as not as a package. So that's pretty much it. Let's see if we can recall what we learned in this video. Can you recall what a dihybrid cross is? What is the law of independent assortment? And finally, can you see if you can work out what will be the F2 generation of spring ratio? And finally, if we cross two short plants, one which has round green seeds versus one which has wrinkled yellow seeds. So the two traits over here is the shape of the seeds and the color of the seeds. And by looking at the letters you can kind of figure out which is dominant and recessive. If you have difficulties answering these questions, no worries. You can always go back and re-watch the video.