For a QTL with k loci, there are unordered pairs of loci. We can define epistatic interactions for each of these pairs.
Epistatic interactions come in four types, thus there are possible interactions.
Additive by additive AA 2. Additive by dominance AD 3. Dominance by additive DA 4. If an interaction is not defined in the -interactions block, then it will be set to zero. To define the interactions, we have a block that looks similar to that defining QTL main effects.