We're first going to focus on gamete formation. And this is super important, because people get really confused with mapping and gametes, and what the genotypes of the gametes are. And it's crucial, because if you don't understand the genotypes of the gametes, you're not going to understand anything about mapping. So we're going to go through this very slowly in the beginning. So first, we're going to talk about independent assortment. Now, we've talked about this and you've actually seen this before in a different chapter when we talked about independent assortment. But I want to refresh it here, so that when we go on and talk about crossing over and mapping, you understand how it's different. So Mendel's Law of Independent Assortment says the alleles of 2 genes assort independently. So what that looks like is here. We're looking at these 2 genes, a and b. There's 2 alleles for each of them, and they create certain phenotypes. So if I were to ask you what are the genotypes of the gametes, you hopefully can fill this out. Because this is normal Mendelian inheritance. So take a second, fill out what are the genotypes of the gametes, and then you can come back, press play again, and see if your answers are correct. They should be, hopefully.
So, if we're determining the genotypes of the gametes, you're going to take one of each allele, so a big A, a little a, and you need to repeat it because there are 4. Then you make the other combinations with the b. You do big B, little b, little b, big B. And this gets you 4 gametes and these are the genotypes. Right? And eventually, when they create an organism, they'll combine with other gametes, and then be dihybrid again. But for now, they're haploid, because they contain one allele for each gene, a and b. Now, this is independent assortment. So we're saying that these genes assort independently, so they are not on the same chromosome. But what happens if these 2 a and b genes are on the same chromosome? They're physically linked and but they still have to be sorted in the gamete. So what do you get? Well, first, you can see this looks different. Right? So each one of these lines is representing a chromosome. Remember, they come in pairs. So here you have your dominant allele and recessive allele on the different chromosomes, and your dominant for b. Now you're never gonna see this like this. Why? Why is this wrong? You know? Right. The reason is that this is not heterozygous. These are if you write it like this, you're saying that these are 2 different genes. Instead, the other allele must be written over here because this is on its homologous pair. This is not. This is a separate gene location on the same chromosome. Homologous pairs are written on different chromosomes. So here, we have it written like this, a and b. Now, you'll notice that, before, remember an independent assortment, we wrote it like this. Well, when we know that things are linked, we write it differently. Right? So this is a a−b and a−b, and this is representing different homologous chromosomes. The dash. So, it's a little different notation, but you do need to understand this notation. And then it's something you cannot just blow off because if you get asked a question, and you need to know if it's written like this or like this, because this represents 2 different things. This represents independent assortment and this represents linkage. But the question is now, what are the genotypes of the gametes?
Okay. So now we're talking about dividing the chromosome. So what we get is we get the 1st chromosome, and it has a and b. And we get the 2nd chromosome, and it has a and b. Now, because these are linked they're physically linked what are the other 2 gametes? Now, they would be a and b a and b. This is if no crossing over is occurring. We'll get the crossing over in the next example. Right? So these are very different gametes than what you saw above. Right? Because the genotypes are these, is this. You have 2 genotypes and they're just repeated again for the other 2. So this is if 2 genes are on the same chromosome. And the reason that it's like this, right, is because the full chromosome goes into the daughter cell. They don't split and cut up the genes. The entire chromosome has to go. So these two alleles are linked together. They have to be sorted into these gametes this way. But we know that crossing over occurs, and crossing over results in different gametes. And the reason is that homologous chromosomes break and rejoin. So notice here, that the genotype and the notation is written exactly the same as above. The only thing that's different is I made this one red, and this one blue. So let's follow what happens now if there's crossing over. So crossing over can occur here. Right? And switch these around. So what happens is you get 2 genotypes which are normal. You get the a, b, and the a, b, which is what you're expecting. Right? Actually, it's, let me write this correctly. You're not gonna care, but I don't wanna lead you astray. So, it's written like that. Right? It suggests there are 2 separate chromosomes. But crossing over can occur and what you get is you get a connected to b and you get a connected with b. And so now you have 4 genotypes. You have the a b, you have the uppercase a and little b, and the lowercase a and big b. Now this looks similar to independent assortment. Right? I mean, it looks exactly the same as independent assortment. Right? The genotypes are exactly the same. But what's different here is the independent assortment, these are 2 chromosomes. Clearly, 2 different chromosomes where this is the result of crossing over. Now, you may ask yourself, okay, if I'm just given the gametes, I know the genotypes, but I know nothing else about the problem, How do I tell if it's crossing over or independent assortment? Well, I'm going to explain it, but how you know is by your ratios. And I'm going to go through the ratios. So normal independent assortment with 2 genes is going to have an f two ratio of 9 to 3 to 3 to 1. But you're gonna see is that if it's crossing over, it's not going to be this ratio. It's going to be very different. So, with that, let's now move on.