Okay. So now we're going to talk about epistatic genes. This is going to be a long video, so you're going to have to stick with me. The reason is that I'm going over dominant epistasis and recessive epistasis, which are different and have different phenotypic ratios at the end. So just bear with me, and I'm going to give you some really great examples of both of these cases that you will need to know because you're going to see them.
The important thing about this is that with each type of epistasis and everything that I present from now on, I'm going to be giving you a ratio. It's not going to be 9 to 3 to 3 to 1, but it's going to be something different. And you'll need to know what ratio goes with what topic. So, be sure that you're writing these ratios down because you will see these ratios in a test situation, and you're going to have to know which one goes with which.
The first thing that I'm talking about is dominant epistasis. This occurs when a dominant allele of one gene is masking the effect of a second gene. Remember, epistasis is talking about two gene interactions. There are two genes here; one of them is dominant, and because that dominant allele is present, it covers up or masks the phenotype of the other allele, whether or not it's dominant or recessive. Here, we say that the dominant allele is epistatic, and the phenotypic ratio of a cross from a heterozygous cross is not 9 to 3 to 3 to 1 but instead 12 to 3 to 1. Memorize this and associate it with dominant epistasis because 12 to 3 to 1 always means dominant epistasis.
We're dealing with a certain breed of squash that comes in 3 colors: white, dark red, and light red. Coloration is determined by two genes, d and w. For example, if you have dominant d and dominant w, it doesn't matter whether they are homozygous dominant or heterozygous. If there’s at least one dominant allele, you get a white phenotype. If you are recessive for the d and dominant for the w, you also get a white phenotype. However, if you have a dominant d and recessive w, you get a dark red, and if recessive for both, you get light red. Here, we say the dominant w allele is epistatic because, anytime it's present, it determines the phenotype you will see.
Therefore, in the presence of dominant w, the phenotype will always be white, regardless of the other allele’s state. This information leads us to the phenotypic ratio of 12 to 3 to 1 because both cases involving dominant w results in white. The genotypic ratio remains 9 to 3 to 3 to 1, which can be confirmed via a dihybrid Punnett square or a branch diagram.
However, there is an alternate form called recessive epistasis, where the recessive allele masks the phenotype of the second gene with a different ratio of 9 to 3 to 4. For example, consider a breed of flower with colors of blue, magenta, and white, determined by two genes. If both genes are wild type, you get blue; if wild type for w but mutant for m, you get magenta; if mutant for w and wild type for m, you get white; and if mutant for both, you also get white. Here, the mutant w allele is epistatic and recessive because it must be present in two copies to affect the phenotype.
This phenomenon also has real-life implications, such as the Bombay Phenotype in humans, which involves blood types and is a type of recessive epistasis. This deals with the I and h gene families where the rare h mutation masks the expression of any I allele leading to a perceivable blood type O regardless of the I genotypes present. Thus, recessive epistasis can play significant roles beyond academic tests or breeding studies, highlighting its importance in fields like genetics.
Understanding these concepts, their phenotypic and genotypic ratios, and being able to distinguish them is vital for anyone studying genetics. It’s important to know the differences between dominant and recessive epistasis, their implications, and their applications, which can be decisive in various biological and genetic contexts.
