Epistasis - Online Tutor, Practice Problems & Exam Prep

Epistasis is an inheritance pattern where one gene's product influences the phenotype of another gene. For example, in human blood type, the H protein connects A and B molecules to red blood cells. A recessive allele for the H gene results in an inactive protein, leading to type O blood, regardless of the presence of IA or IB alleles. This demonstrates how the H gene affects the expression of other genes, highlighting the complexities of genetic inheritance and the importance of understanding gene interactions.

Epistasis

In this video, we're going to introduce epistasis. Epistasis is another type of inheritance pattern where one gene's product affects the phenotype of another gene. In other words, epistasis occurs when one gene affects another gene. As an example, we're going to look at epistasis in blood type. In human blood type, there is an h protein, which serves as a connector molecule. Its role is literally to connect or attach A and B molecules to the surface of red blood cells. The recessive allele for this h protein, the lowercase h, encodes an inactive form of the h protein that does not work and does not connect A or B to the blood cells as it's supposed to. This means that even if a person has the alleles IA or IB, they will actually have blood type O if they are homozygous recessive for the h protein, because the h protein is necessary to attach the A and B molecules. Without a functional H protein, the person will have type O blood regardless of the alleles that they have. In other words, the H gene affects the expression of another gene, which would be the IA or IB genes.

Although epistasis applies to both IA and IB, in our image below, we're only going to look at the example as it applies to IA. In this example, we can see how epistasis can cause inconsistencies in the inheritance of blood types. We have the genotype in the first column of this image and the phenotype in the second column. Previously, we had seen that any individual with IA in their genotype, regardless of the second allele (whether it's another IA, a lowercase i, or IB), would have A molecules on their surface. This is true, but it requires a functional H protein, meaning that the individual must have at least one capital H for their H protein so that the connector molecule works properly. When they have this particular genotype, it means that they will have an active H protein that works properly. The H protein is represented by black connector lines, which are required for the A molecule to be expressed on the surface. If you have an active H protein, the A molecules will have no problem being attached to the surface, meaning that the individual will have blood type A, as we have already discussed in our previous lesson videos.

What we're introducing new here is that if the individual has two lowercase h's, if they are homozygous recessive for the h protein, it means that they are going to have an inactive form of the protein that does not connect A or B molecules. With the two lowercase h's, instead of having an active H protein, they have an inactive H protein that does not work. Notice that the connector molecules are no longer here. Without these connector molecules, the A molecules cannot be attached to the surface phenotype of these individuals, even though they have IA just like the others; they actually end up having type O blood. This is because of epistasis, where one gene (the H gene for the H protein) affects the expression of another gene (the IA gene in this scenario). Ultimately, epistasis is when one gene's product affects the phenotype of another gene. This concludes our introduction to epistasis, and we'll be able to get some practice as we move forward in our course. I'll see you all in our next video.

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.
• D. A gene that changes the genotype of the organism.
• E. None of the above.

Here’s what students ask on this topic:

What is epistasis and how does it affect genetic inheritance?

Epistasis is an inheritance pattern where one gene's product influences the phenotype of another gene. This interaction can mask or modify the expression of the second gene. For example, in human blood type, the H protein is necessary to attach A and B molecules to red blood cells. If an individual has a recessive h allele, it results in an inactive H protein, leading to type O blood, regardless of the presence of IA or IB alleles. This demonstrates how the H gene affects the expression of IA or IB, highlighting the complexities of genetic inheritance and the significance of gene interactions in determining phenotypes.

Can you explain epistasis using the example of human blood types?

In human blood types, the H protein acts as a connector molecule that attaches A and B molecules to the surface of red blood cells. The recessive h allele encodes an inactive form of the H protein, which fails to connect A or B molecules. Therefore, even if a person has IA or IB alleles, they will have type O blood if they are homozygous recessive (hh) for the H protein. This is because the inactive H protein prevents the expression of A or B molecules on the red blood cells, illustrating how one gene (H gene) can affect the expression of another gene (IA or IB).

How does the H protein influence blood type in the context of epistasis?

The H protein is crucial for attaching A and B molecules to the surface of red blood cells. In the context of epistasis, if an individual has a recessive h allele, it results in an inactive H protein. This inactive protein cannot attach A or B molecules to the red blood cells, leading to type O blood, regardless of the presence of IA or IB alleles. Thus, the H protein's functionality directly influences the expression of blood type, demonstrating how one gene's product can affect the phenotype of another gene.

What is the role of the H protein in determining blood type, and how does it relate to epistasis?

The H protein serves as a connector molecule that attaches A and B molecules to the surface of red blood cells. In cases of epistasis, a recessive h allele results in an inactive H protein, which fails to connect A or B molecules. Consequently, individuals with IA or IB alleles will have type O blood if they are homozygous recessive (hh) for the H protein. This scenario illustrates how the H gene affects the expression of IA or IB alleles, emphasizing the role of gene interactions in determining phenotypes.

How does epistasis cause inconsistencies in the inheritance of blood types?

Epistasis can cause inconsistencies in the inheritance of blood types by altering the expected phenotypic outcomes. For instance, the H protein is necessary to attach A and B molecules to red blood cells. If an individual has two recessive h alleles, the resulting inactive H protein cannot attach A or B molecules, leading to type O blood, even if IA or IB alleles are present. This interaction between the H gene and IA or IB alleles demonstrates how one gene can mask or modify the expression of another, leading to unexpected phenotypic results.