 Hello again and welcome to our third lecture. If you'll recall, we mentioned earlier that we would eventually be discussing the Punnett Square. The Punnett Square was created by Reginald Punnett, who is a British geneticist in the early 1900s. The Punnett Square is meant to be a teaching tool, and it's a beautiful way to illustrate some of the principles that Mendel discovered in his work. Now if you'll recall, we mentioned that as Mendel was conducting some of his 28,000 crosses, he used some very specific conditions to make sure that his science was conducted properly. First of all, he used plants that would have traits, sweet peas to be exact, that would have traits that could be categorized into one group or another. For example, he would cross a plant that happened to have a purple flower or a white flower. Those were the two choices. If in fact he wanted to cross these two individuals, he then used those that were considered to be true breeding. Recall, true breeding means purebred. We said that Mendel picked plants that could either be self-fertilized, in which case that's what he used to create his true breeding lines, or manipulated to carry out certain crosses, which is where he got to conduct the crosses he wanted to. So using his pure breeding lines, he crossed a purple flower plant with a white flower plant. Now in sweet peas, individuals have two copies of every gene. The purple flower plant, being true breeding for purple flowers will then have two copies of the purple flower gene, which we're going to show with a big pea, big pea. The white flower plants then will have two copies of the white flower gene, which we're going to show as a little pea, little pea. Using a pun and square, let's actually work through this and figure out what the genes of the next generation will look like. Now let's go ahead and figure out what our gametes are going to look like. If you recall, according to Mendel's law of segregation, as gametes are created, they're going to have half the number of chromosomes as the individual. We said this is essential if you're going to maintain the proper number of chromosomes in the next generation. So our purple flower plant is going to give half of its genes to its gametes. In this particular case, it can only give a big pea. Now there's a half a chance as far as which big pea it gets, but you know, it's only going to give a big pea. The white flower plants are only going, because they're pure bred, are only going to be able to give a little pea. So in this particular case, they will be giving either a little pea or a little pea. So let's put these in our Punnett square. We said our purple flower plant could either be a big pea or a big pea. We said our white flower plant could only be a little pea or a little pea. Now let's go ahead and carry this out and figure out what the genotypes of the next generation are going to be. So according to this, these individuals are going to have a big pea here, little pea there, big pea here, coming from there, a little pea there, okay, big pea, little pea, and big pea, little pea. 100% of our offspring are going to be big pea, little pea. Let's start using some of the terminology that geneticists use. Our initial generation, when we first conducted our cross, is known as the parental generation, which we'll show that with a big pea, not to be confused with our genotype, but that's the way they show it. Our offspring here are known as the first filial generation, and this is shown with an F1, filial is a Latin term, and this is how Mendel denoted the first children. So all of the individuals in this particular generation are going to be big pea, little pea. Now when Mendel conducted his cross, what he found was in fact that these were all purple flowered plants. Interesting, isn't it? What was even more interesting is the fact that Mendel then took this generation, the big pea, little pea, and crossed it with itself, again, his ability to manipulate the plants. So let's work through this generation, this particular cross. We have an F1, again, where we're crossing a big pea, little pea with a big pea, little pea. First thing you have to do, of course, is work through the gametes. What are the two different type of gametes you could get? Take a second and think about it. If you believe that they could get either a big pea or a little pea, you would be correct. And according to Mendel's law of independent assortment, you'd have half a chance of either one. So you'd have half a chance of getting a big pea, half a chance of getting a little pea. This individual, same thing, could either be a big pea or a little pea. All right, let's put them into our punent square and we'll figure out what our next generation, known as the second filial generation, or F2, will look like. We have our big pea here, little pea here, big pea here, little pea there. Now, we have a big pea, big pea from this individual, a big pea, little pea, a big pea, little pea, and a little pea, little pea. What was interesting was when Mendel conducted this cross, he found that three of his plants would be purple for every one of his plants that would be white. And initially, he didn't have these punent squares to work through it or to work it out. But what he discovered was that because the white allele was back, in this particular case, it must be considered recessive. And in fact, it had to be present in this particular generation all along, even though you couldn't see it. So this helped him conclude that the purple was in fact dominant to the white allele. Over time, as he began to really work this out and additional geneticists contributed, they realized that this was made up of one big pea, big pea, which is pure bread for purple, to two big pea, little pea, otherwise known as heterozygous, to one little pea, which would be white. So this is where our three to one ratio came from. What an individual looks like is known as its phenotype. What an individual has with regard to genes is known as its genotype. And so this is genetic makeup. So in this particular case, our phenotypic ratio of our F2 offspring would be three purple to one white. Now that's made up of a genotypic ratio of one big pea, big pea, two big pea, little pea, to one little pea, little pea.