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? Causes and Examples
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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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.
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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.
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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.
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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.
Alzheimer's Disease (AD)
Some examples of epistasis are more dramatic than others. Some studies have found that Alzheimer's disease may be a result of the influence of an epistatic gene. Alzheimer's disease is not only the result of polygenic interactions, but also environmental factors. This makes Alzheimer's disease a multifactorial inheritance disorder, or one that is caused by both polygenetic and environmental factors.
At least 27 genes have been found to be somehow involved in the outcome of Alzheimer's disease. One variation of the gene APOE has been found to have an epistatic effect which contributes to the worsening of Alzheimer's disease.
A protein called beta-amyloid has been linked to both sleep and Alzheimer's disease. During the day, the protein beta-amyloid builds up in the brain as a byproduct and plaque. During sleep, the brain aggregates and clears out the beta-amyloid plaque. This same plaque as been found to be linked to Alzheimer's disease. The gene APOE is responsible for many mechanisms in the brain such as maintenance of neurofibrils, microglia, and the blood brain barrier, which can all contribute to cognition. However, APOE is also responsible for aggregation and clearance of beta-amyloid plaque from the brain. Therefore, if an individual has an APOE allele which lessons the clearance of beta-amyloid, the individual may be at high risk for sporadic Alzheimer's disease later in life (especially if sleep-derivative factors contribute further).
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Types of Epistasis
There are multiple kinds of epistasis in that there are different ways in which a epistatic gene is involved in a system of genes which determine a particular trait. Examples of different types of epistasis include but are not limited to:
- Recessive epistasis
- Dominant epistasis
Recessive Epistasis
Recessive epistasis occurs when recessive alleles on one gene interrupt or mask the expression of another gene. For example, albinism in mice is recessive and will only occur if all the alleles for that gene are recessive. This gene if homozygous recessive cancels the pigment of all other pigment-related genes.
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Dominant Epistasis
Dominant epistasis occurs when there is complete dominance of one gene pair over another. That is, regardless of what kind of dominant or recessive alleles are present for one gene, if a dominant allele is present for another gene, then that gene's phenotype is overpowered and masked by this epistatic gene. For example, a gene determines if pigment for summer squash will be colored at all, and another gene has alleles which determine if the color will be yellow or green. So, it doesn't matter whether the fruit is coded to be yellow or green: If the epistatic gene has the dominant allele for white, then the fruit color will be white even if yellow and green pigments are present.
Causes of Epistasis
Epistasis can be caused by multiple factors, but it requires at least a trait or phenotype to be determined by multiple genes. It also requires a single gene to allow for ceasing or halting of the expression of a phenotype. Typically, epistasis occurs when an allele for a gene is essentially broken or the gene does not signal for or code particular proteins or enzymes it usually would have if a different version of the gene were present. Therefore, epistasis can be caused by multiple factors, including but not limited to:
- Mutagens or physical or chemical influences which can cause a gene to mutate and be broken, which can lead to an epistatic effect if it is involved in a polygenetic interaction to produce a particular phenotype.
- Recombination during the process of meiosis during which genes and alleles are exchanged between chromosomes with the same genetic information. This can result in certain alleles being paired together so that a particular gene might be all recessive alleles or all dominant alleles. This occurrence for the right genes can lead to dominant or recessive epistasis.
- Nondisjunction during meiosis in which the incorrect number of chromosomes is organized into sex cells. If a sex cell has additional genes, an epistatic effect occurs if the presence of the additional genes influences phenotypes.
Lesson Summary
Overall, epistasis is a genetic occurrence during which the presence of a specific gene or allele affects the phenotype or physical expression encoded by another gene or genes. There is more than one kind of epistasis such as dominant epistasis or recessive epistasis. Additionally, epistasis can be caused by or influenced by environmental factors as well as genetic factors. Some examples of epistasis include red hair in humans, albinism in humans and mice, and Alzheimer's disease.
Red hair in humans occurs due to one gene overriding others by preventing the pigment associated with red hair, pheomelanin, from turning into eumelanin. A faulty gene creates non-functional tyrosinase, which can prevent the production of melanin at all and therefore lead to albinism. A particular allele for the gene APOE is the epistatic gene that seems to affect Alzheimer's disease, which is influenced by both genetic and environmental factors.
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.
It's a bit like the second baker Michel stuffing all the food coloring bottles with cotton so no liquid could come out. Jacque makes a cake and the recipe says to add red dye to white icing in order to make pink roses. However, there is no red dye available, so the roses remain white.
Another Example: Alzheimer's Disease
Epistasis can be a factor in disease. For example, people with Alzheimer's disease tend to have a very severe form of the disease when they also have a certain apolipoprotein E4 (APOE) gene sequence. The APOE gene sequence somehow worsens the disease. This would be like if Jacques already had two incorrect ingredients in his recipe, and Michel made it worse by mixing and contaminating even more ingredients in the kitchen, resulting in an inedible cake.
Other Definitions of Epistasis
This lesson presents the classic, traditional definition of epistasis. However, as genes and their interactions are revealed to be more and more complex, the definition of epistasis has become contentious. Some would define epistasis as any time multiple genes affect one phenotype. In fact, this happens all the time. It turns out it's common to have multiple bakers working on one cake. So keep in mind that many single phenotypes are determined by a network of genes and gene products.
Lesson Summary
Epistasis is the phenomenon where one gene affects the phenotype of another gene. Red hair is due to a gene that is separate from genes that code for brown, blond, and black hair color. In the same way, albinism, which is a lack of pigment resulting in pale, white individuals, is due to a mutation in the TYR gene which codes for tyrosinase, which is necessary for the production of the pigment melanin, which prevents any pigment made in the body, despite what other skin color genes code for. An increased severity of Alzheimer's disease is thought to be due to the APOE gene. Red hair, albinism, and this disease severity are all examples of epistasis in humans.
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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.
It's a bit like the second baker Michel stuffing all the food coloring bottles with cotton so no liquid could come out. Jacque makes a cake and the recipe says to add red dye to white icing in order to make pink roses. However, there is no red dye available, so the roses remain white.
Another Example: Alzheimer's Disease
Epistasis can be a factor in disease. For example, people with Alzheimer's disease tend to have a very severe form of the disease when they also have a certain apolipoprotein E4 (APOE) gene sequence. The APOE gene sequence somehow worsens the disease. This would be like if Jacques already had two incorrect ingredients in his recipe, and Michel made it worse by mixing and contaminating even more ingredients in the kitchen, resulting in an inedible cake.
Other Definitions of Epistasis
This lesson presents the classic, traditional definition of epistasis. However, as genes and their interactions are revealed to be more and more complex, the definition of epistasis has become contentious. Some would define epistasis as any time multiple genes affect one phenotype. In fact, this happens all the time. It turns out it's common to have multiple bakers working on one cake. So keep in mind that many single phenotypes are determined by a network of genes and gene products.
Lesson Summary
Epistasis is the phenomenon where one gene affects the phenotype of another gene. Red hair is due to a gene that is separate from genes that code for brown, blond, and black hair color. In the same way, albinism, which is a lack of pigment resulting in pale, white individuals, is due to a mutation in the TYR gene which codes for tyrosinase, which is necessary for the production of the pigment melanin, which prevents any pigment made in the body, despite what other skin color genes code for. An increased severity of Alzheimer's disease is thought to be due to the APOE gene. Red hair, albinism, and this disease severity are all examples of epistasis in humans.
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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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Epistasis Overview & Examples | What is an Example of Epistasis in Biology?
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