Epistasis: synergy and antagonism

A system shows "epistasis" if the combined result of the function of its parts quantitatively differs from the additive result of the function of the same parts each one functioning independently from the others. If the combined result exceeds the additive one then the epistasis is characterized as "synergistic", whereas on the opposite case it is characterized as "antagonistic".

 

http://www.life.uiuc.edu/micro/316/topics/genetic-analysis/epistasis.html

http://www.calresco.org/lucas/fitness.htm

http://en.wikipedia.org/wiki/Epistasis