Dominant vs Recessive Epistasis: Example & Analysis

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  • 0:00 What is Epistasis?
  • 1:35 Dominant Epistasis
  • 3:40 Recessive Epistasis
  • 5:37 Lesson Summary
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Lesson Transcript
Instructor: Erika Steele

Erika has taught college Biology, Microbiology, and Environmental Science. She has a PhD in Science Education.

This lesson will compare dominant and recessive epistasis and provide examples of each. It will also explain why dominant epistasis affects more individuals than recessive epistasis does.

What is Epistasis

Epistasis is a type of polygenic interaction where one gene controls the phenotype of another gene for a trait. Both genes have an influence on the physical appearance of the trait, but the one that shows epistasis masks the effect of the other. The important thing to remember about epistasis is that the epistatic gene has no effect on the genotype, or genetic composition, of the gene being controlled. It affects only the phenotype. If the epistatic gene was not present, the gene being controlled would be expressed normally.

An easy way to think of genes that show epistasis is that they are bully genes. The phenotype is going to be determined by the gene that shows epistasis because it masks the expression of the other gene. Albinism is an example of epistasis. If a person has the gene for albinism, their hair, skin, and/or eyes will little or no have pigment no matter what their genes say their color should be. It is because the gene for albinism, the bully gene, shoves them in a locker.

Before discussing dominant and recessive epistasis, let's briefly review these terms. 'Dominant' and 'recessive' describe how alleles, or variations of a gene, interact with each other. Dominant alleles are always expressed whether the person has one or two of the dominant allele. Recessive alleles require two copies to be expressed. Genes that show epistasis can be dominant or recessive.

Dominant Epistasis

Dominant epistasis happens when the dominant allele of one gene masks the expression of all alleles of another gene. If an organism inherits one or two copies of the dominant allele, they will have the trait. This diagram shows the inheritance pattern for a trait that shows dominant epistasis when both parents are heterozygous for the trait. Heterozygous means that two different alleles for a trait are inherited instead of two of the same allele.

Figure 3: In dominant epistasis, the majority of the individuals are affected. There is a 12:3:1 ratio.
Dominant epistasis ratio

Fruit and flower color in plants is a common example used to illustrate dominant epistasis. As shown in this figure, the squash comes in 3 colors. Yellow (AA, Aa) is dominant over green (aa). However, since squash color is polygenic, or determined by more than one gene, GeneB also determines squash color. Because GeneB shows epistasis, it is more important than GeneA in determining squash color. This is an example of a bully gene masking the expression of another gene.

You can determine whether a gene that shows epistasis is dominant or recessive by looking at the number of affected individuals in a population. As shown, the majority of individuals, 12, will be affected and turn out white if they have a BB or Bb genotype. The phenotype of GeneA will only be seen if two recessive alleles of the B gene are inherited. Three individuals will show the dominant (AA, Aa) yellow phenotype, and one individual will show the recessive (aa) green phenotype. Remember that yellow and green squash are always have a bb phenotype because GeneB shows dominant epistasis.

There are no simple examples of dominant epistasis in humans, but scientists believe this it is one of the mechanisms involved in complex diseases like Alzheimer's disease, autism, and diabetes.

Recessive Epistasis

Figure 4: Genes that show recessive epistasis can only mask a phenotype if two alleles are present.
recessive epistasis

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