Okay. So now, let's talk about other types of gene interactions. The first one I want to mention is the complementary gene action. This is when two genes interact because they are in the same pathway. So you have a single pathway that includes, you know, six or seven dozen genes, that produce proteins that interact in this pathway. Well, if you need one to start the other, this is complementary union action. The ratio here that you need to definitely know is 9 to 7. That's the super important one.
Let's look at an example here. A breed of flower comes in two colors, purple and white. Coloration is determined by two genes, c and p. So here we have, if you have a dominant c and a dominant p, you're going to get purple. If you have a dominant c but a recessive p, you'll get white. The same in the reverse with the recessive c and the dominant p, you'll get white, and recessive for both, you'll also get white. So, you have to be dominant in both the c and the p in order to get this color. The genotypic ratio will be 9 to 3 to 3 to 1. Don't believe me, do a Punnett square. But the phenotypic ratio will be 9 to 7 because these add up to seven and they're all white.
So, we say that these genes are complementary, and we're working through complementary gene action because both genes need to have a dominant allele in order to have that being a purple phenotype. This is complementary gene action.
There's another type, the second type called Suppressors, and these are mutant alleles. So now we're dealing with mutants, that dominance mutants. And the mutant of one gene will actually reverse the effect of a mutation in a second gene. So, now we're working with two mutations. There are two phenotypes that you can get here. The first, and this is the most common, the one that I'm going to present to you, and the one that you're most likely to be tested on is this. And, this is when the suppressor causes the phenotype to be like wild type. And it'll have a 13 to 3 ratio. The second, I'm not giving you an example of. It does exist. A couple of your books mention it, not all of you will even hear about it. But essentially, it's different because the suppressor causes the phenotype to be mutant, it has a different ratio. Feel free just to throw that into your memory just in case you're asked about it. But most of the time, if you're asked about a suppressor, it's going to be the 13 to 3 case.
So, an example of this is a breed of flower comes in two colors, the wild type red and the mutant purple. Coloration is determined by two genes, p and r. So, what we're dealing with here is if you have the wild type p and the dominant r, you get red. If you have the wild type p and the recessive r, you also get red because the wild type p is making it red. If you have the recessive p and the dominant r, you get purple. And the recessive in both, you get red. So in this case, the wild type p allele and the dominant r allele both are causing the plant to be red. So the only time that you get it to be purple is the mutant p allele. And this is because the recessive suppressor, which in this case is here, is suppressing the purple phenotype. Suppresses the purple phenotype. And so, that's why it's red. Because normally these both would cause it to be purple, but because this is a suppressor, it says, no, you're not going to be purple. I'm repressing you, and therefore, I'm going to be red, turns the plant red. Now, the genotypic ratio, again, is 9 to 3 to 3 to 1. However, the phenotypic ratio is going to be 13 to 3 because you have red, red, and red, and you have 3 purple. So, there's where that ratio comes from.
So, that's super important for a suppressor. And then, finally, the very or no, we have two more. So, this is a little different, though. These are modifiers, and this is when a mutation in one gene changes the degree of expression, so kind of how much it's expressed, of a mutated second gene. So here we go. So if you have, wild type at both genes, it's going to be wild type. If you have wild type at one and mutant at another, it's going to be defective in some way. So an example of this is it's defective. It has low transcription. If you have mutant at one and at the other, it's going to be also be defective, but in a different way. So, So now you have this mutated protein, and it does something different that doesn't have anything to do with transcription, but is a different pathway. And if you're mutated in both, they're extremely defective. So this is a modifier because the mutation at one gene affects the degree of expression of a mutated second gene. So these are modifying each other and, causing the, degree of expression to be defective or extremely defective. And, these are sort of a rare case. You many of you may not even be asked about these, but I wanted to throw it in there just in case that you were.
And then finally, synthetic lethals, and this is, lethal means dead, of course. So this is when two viable single mutations result in death when found as a double. So I'm not even giving you an example here, but here's just two dominants that are going to be purple, one dominant will be cyan, the other dominant will be white. But if you have, mutations in both, it's dead. So the genotypic ratio will be 9 to 3 to 3 to 1, but the phenotypic will be 9 to 3 to 3 because this one you won't see because it's dead. So, this is a unique case too. It's you may be asked about it, you may not. But just in case, I would memorize the ratios because that's how you're going to tell all of these different things apart. So if you see a 9 to 3 to 3 ratio, you know that this is because one of the alleles is a synthetic lethal. Meaning that when you have double mutants, so when you have mutated in this gene, the c gene, and the p gene, that causes it to be dead.
So, with that, let's now move on.
