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I have been trying to wrap my head around the concept of epistasis for a couple of days now, and I think I understand it, at least at a basic level, but I still don't understand some of the ways that the term is used.

What does it mean if gene A is epistatic to gene B? Does this simply mean that gene A masks the phenotype of gene B when expressed? or is there more to it?

Also, what does it mean when genes are in the same epistasis group? I am currently studying the transcriptional up-regulation of PSY3 in yeast as a result of UV treatment. PSY3 codes for a protein that forms the Shu complex along with 3 other proteins. One paper I am reading states that PSY3, is in the same epistasis group as the three other genes that code for these other proteins. What does this mean?

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What does it mean if gene A is epistatic to gene B? Does this simply mean that gene A masks the phenotype of gene B when expressed?

Yes. Note that this does not say anything about mechanism. If A converts 1 to 2, and B converts 2 to 3, then A is epistatic to B, but A is also upstream of B. In a signaling pathway, say B inhibits A and prevents it from signaling. A mutation in B allows A to signal. A mutation in A means there is no signaling. Mutations in both A and B means there is no signaling: A is epistatic to B, but A is downstream of B.

An epistasis group is a group of genes in the same pathway. Knocking out any combination of these genes will not result in a more severe phenotype than knocking out the most "sensitive" of these genes (the single mutant with the most severe phenotype).

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