 We will continue our discussion of Mendelian genetics in this module. The next question Mendel wanted to address was what is the influence of one trait on another trait when it is transmitted from one generation to the next generation. Let's look at that. In order to understand some of these genetic events, we also should have some understanding of probability. Say if we flip a coin, coin toss, what is the probability we will get a head or a tail? It's 50%. 50% of the time we will get head, 50% of the time we will get a tail. Now let's talk about two coins. Now we have the coins shown on your screen are actually American penny, the brown coin is American penny and the silver coin is an American dime. So we flip a penny and a coin together. What is the probability that we will have two heads? So the probability getting one head is half. Probabilities are always the range between zero and one. If an event is absolutely certain to happen, the probability is one. If an event cannot possibly happen, the probability is zero. Everything else is a number between or a fraction between zero and one. So we have said that probability of a single coin landing in head is 0.5 or one and a half. So if we flip two coins, the probability of both of them happening at the same time is half for one coin and the other half for the other coin. So when you are combining two probabilities, independent events happening together, you basically multiply both the probabilities. So half times half is one over fourth. Now let's look at the other two cases where one is head, other is a tail. When the two events can happen in two different independent ways, we add the probabilities. So in this case, one head, one tail, we can achieve this result by two different ways. In one case, the penny has a head and the dime is a tail. The other case, the penny has a tail and the dime has a head. So when we add these probabilities, it is one fourth or a quarter. Quarter plus quarter, 0.25 is 0.5. So 50% of the time we will have this situation that one of the coins will be head, other will be a tail. Similarly, as we can see, the probability of having two tails is also 0.5. So I wanted to introduce you to the probability concept because you can also use probability concepts to calculate the genetic combinations. Now Mendel wanted to see what is the question Mendel framed was, what is the influence of transmission of one allele, one trait to the next generation or the progeny? How does it influence the other traits being transmitted to the next generation? So basically, he started looking at two traits at the same time. We are already familiar with capital less and smallest, the shape of the seed. He also took into account the color of the seed. The yellow color is dominant over the green color. So these two traits, these two characters are located on different chromosomes. So S is located on homologous chromosomes and Y is located on a different homologous chromosome. So in a diploid state, a heterozygous will have a capital Y, small Y, capital S, small S. When these individuals form gametes, these will separate and they can, if they're independently divided between the gametes, it will have absolutely no bearing on what, whether a gamete gets S, in what combination the gamete gets S's and Y's, whether it gets capital S always comes with capital Y or whether capital S can also come with small Y. So this is basically what meant, this is the experiment, Mendel did and let's look at that experiment. So he crossed again true breeding, capital S, capital Y plants with small S, small Y, true breeding plants. Now we are going to get a progeny which will be capital S, capital S, small S, capital Y, small Y. This plant, if it is, this plant, if it is, if these two genes are divided in the gametes independent of each other, which is also called assortment, if they're independently sorted, so we can have these four combinations. Since we are looking at two different, two traits, now this cross will be dihybrid as opposed to the monohybrid cross, which we looked at earlier. So let's make the gametes, this F1 generation will make four types of gametes in which these four combinations are possible. Capital S can go with capital Y, capital S can go with small Y, small Y, small S can go with capital Y and small S can also, of course, go with small Y. So again, we have already, we are familiar with pundit square. We write the gametes, two gametes on one either side of the pundit square and look at the combinations, what we get. So we will get basically nine spherical yellow seeds and three spherical green seeds, three wrinkled yellow seeds and one wrinkled green seed. So if these genes were linked, we could not have possibly gotten a small capital S with a small Y. If capital S was linked with this capital Y, we would not get this result. Additionally, I would like to draw your attention to the components. The components are these two categories, the wrinkled seed with yellow color and green seed with the spherical, green seed with a spherical shape. These combinations were not present in either the parents or in the F1 generation. These combinations will only be possible if genes were allowed to separate from each other during meiosis, independent of each other. Therefore, with this data, Mendel came up with his second law, law of independent sortment. Alleles of different genes assort, meaning they are divided into gametes independent of one another during gamete formation. The segregation of allele A is independent of segregation of allele B. Here, again, you can see the pundit square in which Mendel did the dihybrid cross to show that genes are assorted independently. We will continue our discussion of genetics in the next module.