In this video we're going to introduce epistasis. And so epistasis is another type of inheritance pattern where one gene's product affects the phenotype of another gene. And so in other words, epistasis is when one gene affects another gene. And so as an example we're going to take a look at epistasis in blood type. And so in human blood type there is an h protein, and this h protein serves as a connector molecule and its role is literally to connect or attach A and B molecules to the surface of red blood cells. And so the recessive allele for this h protein, the lowercase h, it actually encodes an inactive form of the h protein that does not work and it does not connect a or b to the blood cells as it's supposed to. And so what this means is that even if person has the alleles capital IA or capital IB, they will actually have blood type o if they are homozygous recessive for the h protein, because again the h protein is what's necessary to attach the A and B molecules. And so without a functional H protein, the person will have type O blood regardless of the alleles that they have. And so in other words what we're saying here is that one gene which is the H gene is going to affect the expression of another gene which would be the capital IA or capital IB genes. Now although epistasis applies with both IA and IB, in our image down below we're only going to be looking at the example as it applies to IA. And so in this example down below we can see how epistasis can cause inconsistencies in inheritance of blood types. And so what you'll notice is we have the genotype here in the first column of this image and the phenotype over here in the second column of this image. And previously what we had seen is that any individual that has capital Ia in their genotype, regardless of what this second allele is even if it's another capital I a or even if it's a lowercase I or even if it's a capital IB, regardless of what this one is, if the individual had a capital IA, then they would have a molecules on their surface. And this is true but it requires the a functional h protein, and that means that the individual must have at least one capital H for their H protein so that that connector molecule works properly. And so in this scenario when they have this particular genotype, that means that they will have an active h protein that works properly. And again, the h protein is, basically represented by these black, connector lines, which are again required in order for the a molecule to be expressed on the surface. And so if you have an active h protein then the, the a molecules will have no problem being attached to the surface, and that means that the individual will have blood type a as we already have discussed in our previous lesson videos. Now what we're introducing new here is that if the individual has 2 lowercase h's, if they are homozygous recessive for the h protein, that means that they are going to have an inactive form of the protein that does not connect a or b molecules. And so over here with the 2 lowercase h's, now instead of having an active h protein, it has an inactive H protein that does not work. And so notice that the connector molecules are no longer here, and without these connector molecules the A molecules cannot be attached to the surface phenotype of these individuals, even though they do have capital I a just like these individuals do, they actually end up having type O blood. And that is because again, one, because of epistasis, because of the fact that one gene which is the h gene for the h protein is affecting the expression of another gene which is the capital IA gene here in this scenario. And so ultimately epistasis is just when one gene's product affects the phenotype of another gene. And so this here concludes our introduction to epistasis and we'll be able to get some practice as we move forward in our course. So I'll see you all in our next video.
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Problem
Problem
Which of the following statements best describes epistasis?
A
An allele that changes the genotype of another allele.
B
A gene that changes the genotype of another gene.
C
A gene that controls or masks the expression of another gene.