Tests of Epistasis


Defination of epistasis - A double mutant where one mutation masks the phenotype of another mutation.

Note that epistasis is not the same thing as dominance. With epistasis a mutation in one gene masks the expression of a different gene. With dominance, one allele of a gene masks the expression of another allele of the same gene.

For instance, a mutant gene that causes complete baldness would be epistatic to a mutant gene that determines hair color. Likewise, a mutation preventing an early step in a biochemical pathway will be epistatic to a mutation that prevents a step later in the same pathway (that is, an "upstream" mutant phenotype will obscure a "downstream" mutant phenotype). Two examples of how tests of epistasis can be used to analyze biochemical reactions are shown below.


QUESTION: Two mutations were isolated with different effects on proline utilization: the mutation put-1020 makes cells constitutively express the putP gene at high levels, while the mutation put-1222 prevents expression of the putP gene. To determine if these two mutations affect different "regulatory pathways" or if they both affect the same "regulatory pathway", the double mutant was constructed. The phenotype of the put-1020 put-1222 double mutant is constitutive expression of the putP gene. Suggest a simple explanation for these results. What results would you expect if the two mutations affected different pathways?

ANSWER: The put-1020 is epistatic to put-1222. The simplest explanation of these results is that both the put-1020 mutation and the put-1222 mutation affect the same regulatory pathway. If the two mutations affected independent regulatory pathways then the phenotype of the double mutant would probably be intermediate between the two extremes observed in the single mutants.


QUESTION: Yeast mutants designated ade are unable to synthesize adenine. The wild-type cells form white colonies. The ade4 mutants also form white colonies, but ade1 mutants accumulate an intermediate that makes the colonies red colored. If both of these genes affect the same biochemical pathway, the reactions could occur in two possible ways:

To determine the relative order of these two biochemical steps in the adenine biosynthetic pathway, an ade1 ade4 double mutant was constructed. If the ade4 gene product catalyzed an earlier step in the adenine biosynthetic pathway than the ade1 gene product, what color colonies would be formed? Which gene would be epistatic?

ANSWER: If the ade4 step was before the ade1 step then the colonies would not accumulate the red intermediate, and the colonies would have the white colored phenotype typical of ade4. This is the observed experimental result. Thus, the ade4 gene is epistatic to the ade1 gene.


For a detailed perspective on epistasis analysis, see Genetic measurement theory of epistatic effects


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Last modified July 15, 2002