Summary
One gene masks or modifies the expression of another gene at a different locus.
In depth
Epistasis is an interaction between genes where one masks the effect of another. In Labrador retrievers, the E gene (pigment deposition) is epistatic to the B gene (black versus brown); ee dogs are yellow whatever the B genotype. Ratios become 9:3:4 or 12:3:1.
It is typically revealed by modified dihybrid ratios.
In epistasis the ratios change from 9:3:3:1: recessive epistasis gives 9:3:4, as in Labrador coats, dominant epistasis gives 12:3:1, and complementary gene action gives 9:7, as in sweet pea flower colour, where two enzymes are needed in a pathway. It often reveals the order of steps in a biochemical pathway.
The word comes from standing upon, meaning that one gene hides another. It should not be confused with dominance, which is about alleles at one locus. In human disease, epistasis explains why modifier genes alter the severity of cystic fibrosis or sickle cell disease.
Example: the Bombay phenotype is a person who lacks the H antigen and thus appears as type O even if carrying A or B alleles.
Frequently asked questions
What is Epistasis?
One gene masks or modifies the expression of another gene at a different locus.
Which area of biology covers Epistasis, and how advanced is it?
Epistasis is one of the 54 topics in Genetics. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Dominant and Recessive, Test Cross, Monohybrid Cross.
Can you give an example of Epistasis?
The Bombay phenotype hides ABO type because of a gene needed to make H antigen.
What should I learn before Epistasis?
Work through the Beginner topics in Genetics first, such as Heredity, Gene, Allele, then return to Epistasis. Intermediate topics assume the Beginner vocabulary.
What topics are related to Epistasis?
The closest topics to Epistasis are Dominant and Recessive, Test Cross, Monohybrid Cross, Homozygous and Heterozygous. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Epistasis?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Epistasis. To practise the whole of Genetics, use the Genetics quiz.
How is Epistasis different from Dominant and Recessive?
Epistasis: One gene masks or modifies the expression of another gene at a different locus. Dominant and Recessive: A dominant allele shows its effect when present in one copy; a recessive allele only shows when two copies are present. Read both side by side and note what each one explains.
How should I study Epistasis step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Dominant and Recessive and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.