Epistasis: Definition & Examples

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  • 0:02 Epitasis
  • 2:08 Epistasis Examples
  • 3:14 Epistasis in Coat Color
  • 6:15 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

Labradors come in three different colors due to two different genes. In this lesson, find out how epistasis works as one phenotype is controlled by the products from two or more genes.

Epistasis

Exceptions to your day are just a part of life. I'm sure there are times in your day when you say to yourself, 'Well, I was going to do this but then this other thing happened and now my plans are changed,' right? It probably happens a lot; what happens at the beginning of your day will affect what happens at the end of your day. This cause and effect is a normal part of life. In this lesson, we're going to talk about a genetic concept that involves occasions when the phenotype of one gene depends on a second gene because these two genes control a common phenotype.

To better understand what we're about to discuss, let me tell you a story. You are out of milk. This is a problem, because you need milk in your coffee. So you decide to head out to the store and buy some. Simple enough, right? But to get to the store you need to do a few things first. First, you need to get your keys. Second, you need to drive your car to the store. And next, you need to buy the milk.

Now, let's say you can't find your keys. Ugh. So if you can't find your keys, this makes it impossible to start your car and then you can't go to the store to get your milk. Your car is in the driveway; it's ready to go, but you are keyless. Therefore, you don't end up getting any milk.

In genetics, epistasis occurs when two or more different gene loci contribute to the same phenotype, but not additively. Epistasis is often described as occurring when one gene locus masks or modifies the phenotype of a second gene locus. The term epistatic describes the relationship between the genes in epistasis.

In our out-of-milk example, we can think of having milk as the final phenotype. There are actually three things that need to happen that influence having milk, which are like our 'genes' in epistasis. These are epistatic to each other. First, we have to find our keys. Second, we have to drive our car to the store, and third we need to buy the milk. If the first step never happens because there was a mistake (in this scenario, we can't find our keys - so we can call this a mutation) then the second step and anything after it also cannot happen. Therefore, the mutation in the first step masked or modified the second step from ever happening. No milk in your coffee this morning!

Epistasis Examples

Epistasis can occur in scenarios other than a step-wise progression, but this is the most common example. Another way to look at this is that a product, like coat color in some animals, is controlled by a pigment (P). Different genes contribute to the steps needed to make P from a precursor molecule. In order to get to P, all these steps have to be fully functional. If there is a mutation in one of these genes, the reaction cannot take place and the phenotype, or in this case, the production of pigment resulting from coat color, is affected.

Another common epistatic interaction between genes can occur when two genes produce proteins that have the same role. In this case, a precursor is converted by two different gene products into a phenotype of colored wheat kernels. Here, if there is a mutation in one gene so that the resulting protein wasn't functional, the other protein from the second gene could take over. However, if you have mutations in both genes so that both were not functional, the wheat kernels are colorless. Here, mutations in both genes would modify the phenotype of color, making this another example of epistasis.

Epistasis in Coat Color

I started this lesson by saying that life is full of exceptions. I'd like to show you that epistasis is an example of why you might get an exception to a basic genetic principle.

You've probably seen, petted, or even owned a Labrador before. These dogs come in three colors: Black, chocolate brown, and yellow. The color behind these dogs' coats is due to epistasis.

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