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Types of Epistasis

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we will explore what the term epistasis means as well as what role it plays in our phenotypic expression of traits. We will discuss several different types of epistasis.

Genes Work Together

Imagine you, Bob and Susan are doing the dishes. You wash the dishes, Bob dries them, and Susan puts them away. But now, imagine you walk away. All of the sudden, Bob isn't drying the dishes and Susan isn't putting them away.

You, Bob, and Susan all have a job to do, just like the genes in your DNA have a job to do. And just like the genes in your DNA, you're not working in isolation. Similarly, your genes have effects on one another. When one gene depends on another gene to do its job, the way Bob depends on you to wash dishes before he can dry them, we call that epistasis.

One simple example of epistasis is the gene for baldness. You might have the gene for brown hair or the gene for blond hair, but if you also have the gene for baldness, it doesn't matter which of those other genes you have, does it?

Additive effects

Epistasis is often called an antagonistic effect because in most cases one gene will block or mask the effect of another. Indeed, epistatic effects are often described in contrast to additive effects, where two genes work in tandem with each other. Remember that in humans and most organisms, everyone has two copies of each gene. Sometimes these copies have different alleles (alternative forms of the same gene). When you have two different alleles and only one expresses, we call the one that expresses dominant, and the one that does not express recessive.

An example of an additive effect might be two different genes that both determine the size of an organism. Say, for example, leaf length of a certain type of plant. Say the dominant allele for Gene A might increase the leaf length by one centimeter, while the recessive allele does not. Now say that the dominant allele for Gene B also increases leaf length by one centimeter while the recessive allele does not.

Say you're measuring leaves for your plant, and you notice that all plants that have dominant alleles for both A and B have 10-centimeter leaves. All plants that have dominant alleles for A and recessive alleles for B have 9-centimeter leaves. All plants that have recessive alleles for A and dominant alleles for B have 9-centimeter leaves. All plants that have recessive alleles for both A and B have 8-centimeter leaves.

In that case, Genes A and B have a simple additive effect. You can predict that a dominant gene for A will increase leaf size by 1 centimeter regardless of what B is doing, and vice versa. Thus, this example is not epistasis.

Epistasis

But suppose something stranger happens. Suppose that when you have dominant alleles for both A and B, your leaf is much larger than you would expect - say, 15 centimeters. Then, these genes would have a synergistic effect. Or maybe having dominant alleles for both A and B gives you a leaf that is only 9.5 centimeters long - still large, but not as large as you'd expect. Then, we would say that the genes have an antagonistic effect. Or maybe something even weirder happens, like Genes A and B interact in such a way that they change the shape or color of the leaf. Now we have epistasis!

Types of epistasis include dominant epistasis, duplicate dominant epistasis, recessive epistasis, duplicate recessive epistasis, dominant-and-recessive epistasis, and duplicate interaction. We won't have time to cover all of those types in detail, but we'll focus on a few.

Dominant Epistasis

Primroses make a pigment called malvidin that produces blue flowers. The gene responsible for making malvidin is called K. But there is another gene called D, which is located nowhere near K. The gene D is capable of suppressing K so that it can't make malvidin; thus, the primrose flowers won't be blue.

This is epistasis because D interferes with K. It's dominant epistasis because D is dominant over K. So, although each plant will have two copies of D, any plant that has one copy of the K-suppressing allele at D will not have blue flowers.

Let's return to our dishwashing analogy. Bob might have an 'allele' for drying dishes with a blue towel or drying them with a pink towel. If you storm off in a rage, it doesn't matter what Bob's allele is. He won't be drying any dishes at all. That would be dominant epistasis.

Primroses can be blue or other colors.
Primroses can be blue or other colors.

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