 My friend Marcus has beautiful blue eyes, but you know what's strange? Both his mom and dad have brown eyes How is that possible? Well in this video Mendel and his peas are going to help us answer this question So in a previous video, we had seen one of Mendel's experiment He took a pure tall pea plant and cross fertilized it with a pure short pea plant and Saw that the offsprings were all tall. He called them the F1 generation This was shocking because we believe that we know crossing them would give us some medium-sized plants But we didn't we got all tall no short no medium all tall And so Mendel was wondering whether these tall plants were the same as these pure tall plants as well And what do we mean by pure tall plants? How do we find out with the pure or not? What is the meaning of pure? Pure means if you were to self fertilize it So if you self fertilize a pure tall plant then all its offsprings would be tall Similarly, if you self fertilize a pure short plant all its offsprings would be short That's what we mean by pure. So Mendel wanted to know whether This these tall plants were also pure. So what he did is he said let me take these F1 Generation tall plants and self fertilize and see what we would I get and what he got was again a surprise Now he got both tall and short plants in a specific ratio roughly three is to one And so the two important questions to answer over here is in first case, why did we get all tall? Why wasn't the short trade seen at all? And another question is in the second case suddenly where did the short trade come from? Why it's in the ratio threes to one always? It's as if the short rate has skipped one generation and then appears back in the next generation. What's going on? How do you explain all of this? All right, so to answer both these questions Mendel comes up with two laws The first one is called the law of dominance So what does it say? So Mendel says by looking at this he guesses that when you cross the pure tall and the short plant Both the tall and the short trade got passed along So these plans have gotten both the traits. Let me just write that down They got the tall trait which I'm gonna represent with the capital T and they also got the short trait from this plant Which I'm gonna write as small t and I'll tell you why I'm writing is a small t and not s Okay, so these plants contain both the traits the tall trait and the short rate But then you might be wondering well if they contain both the traits then why are they all appearing tall? Why not the short tree? Why why don't they appear short? Well, this is where Mendel guessed that maybe that the presence of tall trait is dominating Maybe the short trait is hidden in the presence of the tall trait again. Let me just write that down So what is trying to say is that maybe the tall trait? This trait is Dominant that's what this is called the law of dominance. Okay, he believes that this trait is dominant and The short trait he calls it recessive recessive so the meaning of this is that recessive means the trait gets hidden Hidden in presence of dominant rate. So in presence of Dominant rate that trait gets hidden and that's why it's not being seen And that's why what you see is only the dominant rate. And so we say the dominant rate is getting expressed the word expressed means That's what we end up seeing Okay, so in short to summarize he's saying that both the traits are passed along It's not a mix nothing like that both are being passed along is just that one of them is dominant And that's what we see the other trait is there but it's hidden in the presence of dominant the tall trait The short it gets hidden Now, of course at this point you might have some questions as to why the tall trait is dominant Why not the short trait? Why isn't that dominant? Well, look Mendel is just trying to guess what might be happening So at this point, we don't know why one of the traits is dominant. What makes it dominant? Okay, of course later on we'll study more about genes and how these traits gets passed and about DNA That's where we'll get some clues about why certain traits become dominant and other becomes recessive or other becomes hidden But as of now, let's not worry about why one of them is dominant. Okay, let's just accept this All right, and if you're wondering why I wrote over small t and not s for short This is the notation that we write today for dominant rates We use a capital letter and for recessive traits the hidden traits We use the small letter of that same alphabet. Okay, so we use the same alphabet to represent both the traits That's how we basically write it now Okay, so even this plant is tall has a tall and a short trait in it Fine. So the next question is how do we explain this? Why is it that when we self-pollinate this plant? We get both tall and short plants and why is it we get in that same ratio three is to one What's the specialty of the threes to one? Where does that come from? Well to answer this question? He comes up with another law. So let's move up. Let's make some space and write down that law So the second law that he comes up with to answer this is called the law of segregation Segregation and the word segregation over here you can think of it as separation separation Okay, so what's being separated so mental guesses that when the gametes are formed in this plant Meaning the sperm cells and the exels are formed in the plant. They do not carry both the traits He says that maybe when the traits pass into the gametes, they get separated Meaning some of the sperms will get a tall trait some of the sperms will get a short trait Similarly when the eggs are formed in the ovary of the flower They don't get both of them. They get separated meaning some eggs will get tall traits Some eggs will get short traits. So let me just write that down this way So imagine this is one of the sperms which is found inside the pollen grains Of course, the sperms may not look like this. You know different plants have different kinds of sperms Let me just write that way. So when that sperm is formed some sperms will get a tall trait But some sperms Might get a short trait and that's what he means by separation the traits get separated when these sperm cells are formed and Similarly when the exels are formed inside the ovary So imagine this is an egg and the XL so when the XL is formed again Some excels will get the tall trait and some excels will get the short trait Okay, love segregation All right now, how does this explain what happens and now think about it when we self-pollinate what's happening Basically, the sperms are fertilizing the eggs now depending upon which sperm fertilizes which eggs We might end up with a tall plant or a short plant For example, if this sperm which has with the tall trait goes and fertilizes this egg With also a tall trait and the fertilized egg will have both tall traits And so when that plant grows, it's gonna be a tall plant Okay, but there are other possibilities as well, right? So, you know what I want you to do at this point I want you to pause the video and see if you can find out how many such possibilities are there and Out of those possibilities, how many do you think will be tall and how many do you think will be short? And see if you can get that magic ratio 3 is to 1 so great idea to pause the video and see if you can figure this out now Okay, hopefully you have tried. Let's see. So one possibility is this sperm fertilizes this egg That's one or maybe another possibility is this sperm goes and fertilizes this egg That's two Another possibility is third possibility is this sperm goes in fertilizes this egg and The last possibility would be this sperm goes in fertilizes this egg So total we have four different possibilities All right, and the way we'll usually like to write down that possible those four possibilities all the possibilities over here We like to draw a a box. So here's how we like to do. So let me just redraw those sperm cells This is a tall sperm and this is a short sperm. Okay, it's not tall It's just that it's a sperm that contains tall rate and the sperm that contains short rate and let me draw the egg Which contains tall rate here and the egg which contains short rate over here And usually what we like to do is we like to draw a box like this Alright, now, let's look at all the possibilities if this sperm fertilizes this egg Then we get the fertilizer has both tall rates. So TT But if this sperm goes in fertilizes this egg, then we'll have one tall trait coming from the egg and one short trait coming from the sperm Okay, if this happens Then we get a tall trait coming from the sperm and the short trait coming from the egg And finally if these this fertilizes then the fertilized egg will have both short rates And these are the only four possibilities we have Right. So now let's look at what will happen when these plants grow So when this plant grows this one, it's going to be tall because both its traits are tall When this plant grows look the law of dominance tells us that it is still going to be tall because this one will dominate This is a recessive character recessive trait. It'll it'll it'll hide. So this will dominate. So this is still going to be tall What about this one even this is going to be tall Remember it doesn't matter from where the traits came whether it comes from the egg or whether it comes from the sperm That doesn't matter as long as you have one tall trait. It's going to dominate the plant is going to be tall But what about this last one? Well, notice it doesn't have any tall traits. Both traits are recessive When both traits are recessive, there is no tall trait to dominate then and only then the plant is going to be Short and so right in front of you you can now see what are the chances of getting tall? It is three out of four and What are the chances of getting short? It is one out of four and That's the reason we get the three is to one ratio and Just to be clear this does not mean that if you grow four seeds and the four plants then you'll get three Out of them will be tall and one will be short You know just that chances doesn't work that way It just means that if you grow lots of plants maybe hundreds of plants and then you count them Then you will see about roughly three to one ratio will turn up for tall and short plants Now one question you might be having is let's see what happens if you apply the same logic to the first generation Let's apply the same logic to these these the sperms and the exels over here. What happens then? Well, let's see this particular plant was a pure tall plant Which means it must have had both tall traits Does that make sense? Why it must have had both tall traits? Let me use the same color why it must have had both tall traits because we know that if we self pollinate this one All its offsprings are going to be tall We have already seen that and that's only possible if both of them were tall if one was tall and one Was short we would have hot we would have had this now We just saw and similarly we same logic we can argue this one must have both short rates Right, that's the only reason it's looking short if it had any tall trait It would have been tall by the law of dominance and so now think about it now when a sperm from here Goes and fertilizes an egg from here. The sperm will carry the tall trait Right and the egg will definitely carry the short rate And so when they fertilize you'll get one tall one short does that make sense? And even if it's the other way around even if the sperm from here goes and fertilize and I go over here Same thing one tall one short make sense, right? And now you can use the same logic to explain what happens with the flowers also So when you cross a pure purple flower with a pure white flower in the first generation, we got all purple flower Now, can you guess what was the reason for that? Well, we can guess that when they crossed it both the purple and the white trade got passed along But maybe the purple trade is dominating and that's why we see all purple over here and no whites And then later on when you take one of this purple and you self-pollinate by the law of separation Now we'll get the same ratio three is two one Make sense, right? And so finally can you now see how a recessive trait can skip one generation? But then come back in the next generation Ooh, so Can you now explain how my friend got blue eyes from his brown-eyed parents? Think about what traits might be dominating and what traits got passed along Okay, let's see since we know that the blue trade got skipped at least one generation We can guess that the blue trait must be a recessive trait and so we can guess that my friend has both blue And I'm writing small letter because it's recessive if we had even one brown trait Then his eyes would have looked brown because brown. I'm guessing must be dominant. So what must his parents be having? So his parents must be having one brown trait That's what makes them brown eyes, but they must also be carrying a blue trait. The blue trait is hidden It's recessive it's hidden and that's why there it's not being expressed And so when the zygote was formed basically when fertilization happened we can guess now maybe the sperm from the dad carried the blue trait and It went and fertilized the egg from the mom which also carried the blue trait The chances of happening is pretty small as we saw it's a three-to-one ratio We just saw the chances but can definitely happen and that's the reason why My friend ended up with both blue traits giving him those beautiful blue eyes That's it. So let's see if we can recall what we learned. Can you recall what dominant and recessive traits are? What the law of segregation is and Can you work out what would be the ratio of the offsprings if we cross a short plant with a tall plant? Which has both the tall and the short trait in it? And if you get stuck at any point no worries you can always go back and rewatch that part of the video