 What if we could predict the odds of a particular gene being passed on to offspring? In genetic studies, we often use a tool called Punnett Squares to do so. In this video, we'll learn how to use this tool. As you may recall, Gregor Mendel was well known for his experiments with pea plants. Through his experiments, he proposed that observed traits were due to the interaction between dominant and recessive factors, or alleles. After studying flowering pea plants, Mendel concluded that the purple flower color allele was dominant, while the white allele was recessive. He also found that these phenotypes occurred in specific proportions in the offspring, 100% purple in the first generation and 75% in the second. But what was the reason for this? We can now use a tool known as Punnett Squares to predict and make sense of his experiment. Let's take a closer look at his experiment using this tool. First, we'll look at the parental generation from Mendel's experiment. He first crossed over homozygous or purebred purple and white flowers. A Punnett Square is drawn as a grid. Each gamete is written along the top and the left side of the Punnett Square. In this instance, we'll place the purple up on the top and the white flowers gametes on the side. But either way around is fine. Next, the alleles from the first flower are written out onto their corresponding squares. So, we can do this by writing the dominant pea alleles into the squares below. This is then repeated for the second parental gene, so this time the recessive peas are written out. Also note, when writing the parental alleles in the Punnett Squares, conventionally the dominant alleles is written first, followed by the recessive. These four squares indicate the possible genotypes from crossing over these two genes. Each of these individual squares also represent a 25% or 1 in 4 chance of that genotype being found within the offspring. In this case, there's a 100% chance of the offspring being heterozygous, or big pea little pea, meaning that they will all be purple. This was consistent with Mendel's observations, where all of his F1 flowers were purple. Now let's see what happens to the white trait if we cross over two heterozygous plants. In other words, what if we crossed over two offspring from the previous generation? Following the same process as before, we start by writing the two gametes along the sides. We can then fill in the Punnett Square, just like last time. Now we can predict the genotype and phenotype ratios. In this example, the possible genotype includes one big pea big pea, two big pea little pea, and one little pea little pea, which results in a genotype ratio of one to two to one. However, the predicted phenotype ratio is three to one, which is for purple to white. This helps explain Mendel's observation, which is roughly 75% purple in the second generation. Now that we know how Punnett Squares work, let's work through an example with cystic fibrosis. This is a recessive condition, so it is only expressed if both copies of the CFTR gene are inherited. If both parents are carriers of the CFTR gene, what is the chance that the offspring will inherit the condition? Here we will represent the CFTR gene with a small C, shown in red, and the big C is a dominant non-diseased gene, shown in the black. Each of the parents being carriers will be heterozygous or big C, little C. We can then write out their gametes and then fill in the Punnett Square. If we draw out the Punnett Square, we get a genotype ratio of one to two to one, as shown below. Since cystic fibrosis is a recessive condition, inheritance of the genotype, little C, little C, would cause the development of the condition. This means that there is a one in four or 25% chance of the offspring inheriting the condition. There is also a one in two or 50% chance of them being a carrier. Just one last thing I promise, Punnett Squares are a helpful tool for determining the probabilities that can occur with cross-breeding genes. However, it's important to remember that these are only the possible outcomes that can occur, and are not definite outcomes. So for instance, if the couple from the previous slide were to have four children, it doesn't mean that one of the four kids will definitely have cystic fibrosis. Instead, each child has the same odds of one in four chance of inheriting two cystic fibrosis genes. The more offsprings there are, the closer the actual ratios will be to the probabilities. For instance, Mendel bred thousands of pea plants before proposing his offspring ratio. This is known as the law of large numbers. In summary, within this video, we covered that Punnett Squares are a predictive tool used to calculate the probabilities of different genetic outcomes from cross-breeding. From these Punnett Squares, we can also generate genotype and phenotype ratios. We also looked at the law of large numbers, which demonstrates that the more offsprings there are, the closer the predicted ratios match the actual ratios.