 Let's explore how Mendel used pea plants to uncover the secrets of traits and inheritance. So first of all, why did he choose pea plants? Well, there are a few reasons. First of all, you can control the fertilization cycle because it's a plant, so you get to choose when to use the pollen grains and to pollinate it. Secondly, it has short life cycle, which means you can have multiple generations in short time, and so you can experiment and see how the traits are transferred from one generation to another. And finally, you have a lot of characters to experiment with. For example, you can experiment with the height. Pea plants come with tall versions, and they have short ones. They also have round seeds and wrinkled seeds. What about the pea color? Peas can be yellow, or they can be green. The flower itself can be white or purple. Because of that, we can now experiment with so many things, and that's why Mendel chose to experiment with pea plants. Alright, but now let's see exactly what experiment did he do. For the first experiment, he decided to cross a homozygous tall pea plant with a homozygous short pea plant. Now, you may be wondering, what is this homozygous? Well, today we know that all genes that we have have two copies. One copy we get from our mom, another copy we get from our dad. These copies are called alleles. So when both the copies have the same version, and this will make a little bit more sense as we proceed, but if they have the same version, in this case both have the capital T and capital T version, then we say it's homo. Homo means same, so that's why it's called homozygous tall. Similarly, here also you have two copies, two alleles of the gene of the same version, small t and small t. And again, if you're wondering why we're using small t for short, it'll make sense a little bit later. But anyways, we're crossing this with this, and what this means is we're taking polygons from one and pollinating it into the flowers of the other. The question was, what will we get? Well, I would think that maybe we'll get some short and some tall plants. But to our surprise, we get every plant to be tall. So we call this the F1 generation. And in the F1 generation, we get 100% tall plants. And mental was shocked, how does that make any sense? Why is everything tall? So what's happening is that these plants are getting one copy of the gene from this plant and another copy from this plant. And so if you look at their alleles, you'll have one capital T, one small t. Now they're all tall. Therefore, this means that this tall one is a dominant allele. That means that when tall allele is there, it doesn't matter what the other allele is, it gets expressed. It gets shown over here. And the other one is called the recessive allele. It is hidden in the presence of dominant allele. So whenever you have two different alleles, one will be dominant, the other will be recessive. In the pea plants, it turns out that the tall allele is dominant and the short allele is recessive. This is what we call the law of dominance. One allele will be dominant, another one will be recessive. And now you can understand why we write it this way. The dominant allele is always written as capital. And the recessive one is written with the same letter but with the small. That's why we didn't use SS for it, we use T because T is tall is dominant. But we're using small letters because the short is recessive. Finally, what do we call this combination of alleles? It is not homozygous now, it is heterozygous because we have two different copies. But it is tall because the tall one is dominant. Now the next thing Mendel does is that he takes one of these plants and makes it self-pollinate. So he takes one of the plant from the F1 generation, the heterozygous tall, and self-pollinates it, takes the pollen grains from the flower and puts it in the same plant itself, pollinates in the same plant itself. The question is what do we get? This time he finds that we get both short and tall plants. But we get it in the ratio three is to one. That means we get about three out of four, that is 75% tall, and one out of four, which is about 25% short. We will call this the F2 generation, the second generation over here. But again, why is this happening? To answer this question, Mendel came up with the second law called the law of segregation, in other words, separation. What does that mean? Well, he says that whenever gametes are formed, meaning sperm cells or excels are formed, each of these cells don't get both the copies of the alleles. Each of them will get one copy of the allele. For example, if you look at the sperm cell, some sperm cells will get the capital T from here, and some sperm cells will get the small T from here. Similarly the excels, some excels will get capital T from here, and some other excels will get small T from here. And now when fertilization happens, depending upon which sperm goes and fertilizes which egg, we will get different, different combinations. So we can actually draw a table to write that down. So if this sperm unites with this egg, we'll get capital T, capital T. If this sperm unites with this egg, we'll get capital T, small T. If this one unites with this one, again we'll get capital T, small T. And if these two unite, we'll get small T and small T. And now look, this will be tall. This will be tall, and this will be tall. And so we get three out of four, which is 75% tall. And we get one out of four, which is 25% short. And the reason for that is again, the law of dominance. Wherever you have capital T, the tall allele, you will end up with, it'll dominate and you'll get tall plants. So look how beautifully the law of dominance and the law of segregation explain all of this.