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?