Problem:

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.

Required:

Question: What does it mean if gene A is epistatic to gene B?

Question: Does this simply mean that gene A masks the phenotype of gene B when expressed? or is there more to it?

Question: 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? Please describe your answer in detail.