 In this module, we will continue our discussion of Mendelian experiments. We have talked about the separation of two traits. Let's look at the physical nature of it. We've also studied meiosis, the process in which gametes are formed. So here on your screen, you can see both representations, the punnett square and also the meiosis. So in meiosis, you know that we have two chromosomes, two homologous chromosomes, which have not identical but similar genetic information. So in our heterozygous plant, we had two chromosomes, one containing the capital S gene, the other containing the small S gene. Capital S, the smooth seed, is dominant over the recessive wrinkle seed trait. When this plant, capital S, small S, is going to make its gamete, we will have the meiosis. We know the chromosomes will replicate, they will duplicate. We also know the tetrides will form in meiosis 1 and two homologous pair, two homologous chromosomes will separate. They will separate and in meiosis 2, the chromatids, the sister chromatids will also separate, resulting in production of haploid gametes. Gametes have half the number of chromosomes as compared to their parent. So this is what, how we understand it today. But Mendel did not have this information available when he was doing his experiments. He was solely relying on the mathematical model and also the statistics he had accumulated. So Mendel's interpretation of this data was the law of segregation. When any individual produces gametes, the two alleles, alleles we have already defined, two alleles separate so each gamete receives only one member of the pair of the alleles. Here on the screen, you can see that Mendel did not restrict himself to just the seed shape. He did similar experiments with other traits, other characters of the plant. But he always got the same result. When he crossed F1 generation, the F2 generation had the same ratio. The ratio was 3 is to 1, 3 from the dominant, whether it was the seed shape, seed color, flower color, the shape of the pod, the color of the pod, whether it was the position of the flower, exhale or terminal flowers, or the size of the plant. So Mendel did not draw his conclusions just from one experiment. As you can see, he did thousands of these breeding experiments over several years and then he came up with his law of segregation. So in order to prove that the F1 generation had a recessive gene, small S gene, Mendel performed a test cross. Test cross is performed to ascertain the genotype of an unknown individual. In this case, our individual is a plant with round seed. We know that this plant could be heterozygous or homozygous, meaning it can have two capital S genes or one capital S and small S gene. So how would we know what is actual genotype of this plant? In order to determine that, we do the test cross. Test cross is between an individual of unknown genotype with the recessive, homozygous recessive of the same trait. So we have two, we can split it into two, we can do two experiments. So if we cross this S question mark with SS, if the question mark is a capital S, then the result of the cross, cross, this cross will produce all spherical seeds. None of them will be wrinkled because it has only, one of the parents only has capital S. It cannot contribute small S to the progeny. So all the progeny will have spherical seed. On the other hand, if the unknown genotype was capital S, small S, half of the gametes of that particular plant will have capital S, the other half gametes will have small S. When we do this cross, as seen in the Punnett square, you can see 50%, half of the progeny will have wrinkled seed and the other half will have the spherical seed. We will continue our discussion of Mendelian genetics in the next module.