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What is Epistasis? Causes and Examples

Joseph Comunale, Giulietta Spudich
  • Author
    Joseph Comunale

    Joseph Comunale obtained a Bachelor's in Philosophy from UCF before becoming a high school science teacher for five years. He has taught Earth-Space Science and Integrated Science at a Title 1 School in Florida and has Professional Teacher's Certification for Earth-Space Science.

  • Instructor
    Giulietta Spudich

    Giulietta has taught college students, graduate students and researchers in scientific topics from genomics to biochemistry. She has a PhD in Molecular and Cell Biology.

Learn the definition of epistasis and understand what epistasis means. Discover various epistasis examples in humans. Learn the types of epistasis and understand its causes in humans. Updated: 01/27/2022

What is Epistasis?

Genes are sequences of DNA which give instructions to particular cells on what molecules to make and when. However, a single gene can also often come in various forms called alleles. Additionally, a single individual can contain multiple alleles or versions of the same gene. The individual might only express one of the alleles physically or a mixture of the two depending on the type of trait. How do genes work exactly, though? Does one gene control and result in one trait?

There are circumstances in which one gene might have sole control of a single physical trait such as in the case of monogenic traits. Whether or not an individual has cheek dimples, for example, is a monogenic trait. Many other traits are polygenic, or controlled by the interaction of multiple genes, such as in the case of eye color.

In the case of monogenic traits, a single gene or set of alleles determines the phenotype of the individual. Phenotype is the physical expression of a gene or allele. Polygenic traits are therefore phenotypes which are determined by multiple genes or multiple sets of alleles. Even more, and the focus of this lesson - there are epistasis traits, but exactly what is epistasis? What does epistasis mean for genes or traits?

Epistasis Definition

A simple epistasis definition is the phenomenon of one gene affecting the physical expression of another gene. That is, epistasis occurs between genes of polygenic traits and when the phenotype or physical expression of one gene interferes with the phenotype of another gene. Epistasis specifically means "to stop" or "stand upon." It is important to note that not all polygenic traits demonstrate epistasis all of the time. For a polygenic trait to demonstrate epistasis, there must be an epistatic gene present.

Epistatic Gene

An epistatic gene is a gene which if present will suppress or interfere with the effect of another gene. Epistatic genes are therefore often called inhibiting genes because they inhibit the expression or phenotype of another gene. Epistasis is therefore caused by the presence of an epistatic gene among other genes which all determine a polygenic trait or phenotype. What are some epistasis examples?

Definition of Epistasis

Often when we learn about genes, a simple model is described. One gene is said to be a recipe that codes for one feature (phenotype). We can make an analogy to a baker baking: one baker makes one cake.

Epistasis describes the phenomenon when one gene affects the phenotype of another gene. Sometimes, despite what one gene says, another gene might come in and change things, like a second baker adding or subtracting from the first baker's recipe to make a totally different cake.

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  • 0:04 Definition of Epistasis
  • 0:35 Example: Red Hair
  • 1:43 Another Example: Albinism
  • 2:49 Another Example:…
  • 3:18 Other Definitions of Epistasis
  • 3:48 Lesson Summary
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Epistasis Example

Some polygenic traits, for example hair color, eye color, and skin color, require the presence of an epistatic gene. Not all polygenic phenotypes result from epistasis, though. For example, blonde, light-brown, and dark-brown hair do not require the presence of an epistatic gene. However, red hair does require an epistasis gene in order to be expressed in humans.

Not all occurrences of epistasis are as benign as red hair, however. Other occurrences of epistasis have more influential phenotypical outcomes such as in the case of albinism. Either way, understanding the interactions of the genes as mechanisms which determine the physical traits for red hair or albinism better helps one understand the phenomenon of epistasis.

Epistasis in humans: Red Hair

Hair color is a polygenic trait determined by multiple genes. For example, for a human to have dark-brown hair or hair which appears black as a physical trait, large amounts of a pigment called eumelanin would need to be present. However, multiple genes are involved in the production of and distribution of eumelanin throughout a strand of hair.


Hair color is a polygenic trait. Dark hair is NOT the result of epistasis.

Which of the following provides an example of epistasis, red hair or dark hair. Dark hair is not epistasis.


Eumelanin is made through the amino acid tyrosine going through multiple chemical reactions or transformations. The gene called melanocortin 1 receptor (MC1R) is responsible for one of these transformations. In order for eumelanin to be produced, the MC1R gene creates the MC1R protein which reacts with tyrosine, transforming it into eumelanin. At least 10 other genes are involved in hair color. Two more examples include OCA2, which plays a role in the production of the P protein involved in melanin production, and HERC2, which is a protein coding gene involved in binding proteins, resulting in pigments. However, not all genes for hair color have been mapped to the extent that scientists know exactly what each gene does. A general idea of what genes are involved in is present through association with physical traits.

However, red hair color in humans is a physical trait that has been mapped out and the epistatic gene that causes it has been determined. In order for an individual to have red hair, a buildup of a pigment called pheomelanin must be present. This pigment is part of the transformation process of tyrosine. If the MC1R protein is introduced, the pheomelanin is consumed in a chemical reaction which produces eumelanin.


Red hair occurs due to epistasis because it involves the MC1R gene inhibiting other genes.

Red hair meets the epistasis definition and is an epistasis example.


There is a variant or allele of the MC1R gene which causes it to not produce the MC1R protein. If an individual has this MC1R allele, then none of the pheomelanin pigment will be consumed in chemical reactions to produce eumelanin because no MC1R protein is being produced to fuel the chemical reaction. This causes a buildup of the pheomelanin pigment rather than eumelanin. So, this MC1R allele is an epistatic gene because it has an epistatic effect on production of eumelanin. That is, it doesn't matter that every other gene is working to produce the dark pigment eumelanin: The presence of that MC1R variant stops the transformation of pheomelanin (red hair pigment) into eumelanin (dark pigment) and therefore results in the buildup of pheomelanin over eumelanin and emerges as the phenotype of red hair. It is also possible for two black-haired parents to have a child with red hair simply due to epistasis. The parents may be carriers for the broken MC1R gene.

Epistasis in humans: Albinism

Albinism also occurs because of the presence of a epistatic gene. Again, skin color, hair color, and eye color are all polygenic traits and therefore involve the interactions of multiple genes to determine varieties of phenotypes. Albinism can have slightly different versions of phenotypes because the mutations of several different genes can cause different kinds of albinism. However, the most common type of albinism is oculocutaneous albinism (OCA) which involves the color of the eyes, hair, and skin.


This image shows a little boy who exhibits the albinism phenotype.

The little boy on the left is exhibiting an epistasis example.


In order for the body to produce the pigment melanin, which is responsible for eye color, hair color, and skin color, an enzyme protein called tyrosinase must be present. The gene called TYR codes and produces tyrosinase. Tyrosinase is then transformed into melanin. However, an individual can have a mutated version of the TYR gene, which instead produces a version of tyrosinase which is inactive. This inactive version of tyrosinase cannot react to produce melanin and the individual who possesses this epistatic gene will not be able to produce melanin. So, regardless of what kind of pigment or version of melanin all other genes are coded for in regards to eye color, hair color, or skin color, the individual will not produce pigment at all and will appear pale.

Example: Red Hair

Let's look at the example of red hair color in human beings. Blond, brown, and black hair in humans is determined by the amount of a pigment called eumelanin. Without much eumelanin, you get blond hair; an intermediate amount will be brown; a lot will get you black hair. A second pigment called pheomelanin makes red hair. But usually people don't build up a lot of that because of a totally different gene called MCR1.

MCR1 allows the conversion of pheomelanin into eumelanin, making redheadedness rare. The genes for eumelanin and MCR1 interact to produce one single phenotype. So why do we ever get redheads? Well, a certain variation of the MCR1 gene will stop the conversion of pheomelanin into eumelanin, allowing the build up of pheomelanin in the hair, which leads to red hair. This is epistasis.

Let's go back to our baking analogy. Let's say our baker Jacques has three differently shaped icing tips to make hearts, flowers, and stars out of icing. However, the second baker, Michel, blocks Jacques from using the flowers and heart icing tips, so Jacques simply covers the cake in stars alone.

Another Example: Albinism

Albinism, a lack of pigment resulting in pale, white individuals, is another example of epistasis. The albino condition occurs due to an entirely different gene than the genes that encode skin color and tone. If the albinism gene is present, the organism will not have any pigment, no matter what skin color is encoded by other genes.

An important protein named tyrosinase is necessary for the production of the pigment melanin. A gene named TYR codes for tyrosinase. However, a variation of the TYR gene has a mutation that codes for a non-functional tyrosinase that doesn't work. If a person only has non-functional tyrosinase, then no melanin will be made in the body. Even though that person's genetic code might have genes for dark skin, if there is no pigment to make it with, the person will be an albino.

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Video Transcript

Definition of Epistasis

Often when we learn about genes, a simple model is described. One gene is said to be a recipe that codes for one feature (phenotype). We can make an analogy to a baker baking: one baker makes one cake.

Epistasis describes the phenomenon when one gene affects the phenotype of another gene. Sometimes, despite what one gene says, another gene might come in and change things, like a second baker adding or subtracting from the first baker's recipe to make a totally different cake.

Example: Red Hair

Let's look at the example of red hair color in human beings. Blond, brown, and black hair in humans is determined by the amount of a pigment called eumelanin. Without much eumelanin, you get blond hair; an intermediate amount will be brown; a lot will get you black hair. A second pigment called pheomelanin makes red hair. But usually people don't build up a lot of that because of a totally different gene called MCR1.

MCR1 allows the conversion of pheomelanin into eumelanin, making redheadedness rare. The genes for eumelanin and MCR1 interact to produce one single phenotype. So why do we ever get redheads? Well, a certain variation of the MCR1 gene will stop the conversion of pheomelanin into eumelanin, allowing the build up of pheomelanin in the hair, which leads to red hair. This is epistasis.

Let's go back to our baking analogy. Let's say our baker Jacques has three differently shaped icing tips to make hearts, flowers, and stars out of icing. However, the second baker, Michel, blocks Jacques from using the flowers and heart icing tips, so Jacques simply covers the cake in stars alone.

Another Example: Albinism

Albinism, a lack of pigment resulting in pale, white individuals, is another example of epistasis. The albino condition occurs due to an entirely different gene than the genes that encode skin color and tone. If the albinism gene is present, the organism will not have any pigment, no matter what skin color is encoded by other genes.

An important protein named tyrosinase is necessary for the production of the pigment melanin. A gene named TYR codes for tyrosinase. However, a variation of the TYR gene has a mutation that codes for a non-functional tyrosinase that doesn't work. If a person only has non-functional tyrosinase, then no melanin will be made in the body. Even though that person's genetic code might have genes for dark skin, if there is no pigment to make it with, the person will be an albino.

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Frequently Asked Questions

How is albinism an example of epistasis?

Albinism is an example of epistasis because it can be caused by the presence of a broken gene which inhibits the expression of physical traits associated with other genes. Physical traits like hair color, eye color, and skin color require the presence of melanin in order to produce the variations of colors. If the gene which allows for the production of melanin is broken, then the other genes and pigments don't matter and the individual will exhibit albinism.

What is the definition of epistasis in biology?

The definition of epistasis in biology is the genetic phenomenon in which the presence of one gene inhibits the expression of a phenotype encoded in another separate gene. The gene which does the inhibiting is the epistatic gene.

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