Epistasis Examples in Humans

Lesson Transcript
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.

Epistasis occurs when one gene affects the outcome, or phenotype, of another gene. We'll look at three examples in this lesson: hair color, albinism, and Alzheimer's disease.

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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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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