Epistasis is about interactions between mutations in an evolving systems.
- No epistasis corresponds to mutation effects “stacking” without any particular kind of interaction.
- Positive epistasis happens when the combined effect of the two mutations is more positive than the sum of their contributions.
- Negative epistasis is the same principle with negative effects.
Cancer is an example negative epistasis where the addition of a lot of mutations is needed to obtain a cancerous cell.
These interactions make the biological systems so complicated to study and understand.