A genetic framework was developed for the interpretation of statistical parameters estimated from a diallel experiment among a fixed set of lines. These included average direct genetic, average maternal genetic, general combining ability, reciprocal, and line and specific direct and maternal heterotic effects. The genetic model is based on direct and maternal additive and dominance genetic effects as would be expected in animal species. The model assumes that dominance is the underlying basis of heterosis. As an example, litter size at birth was analyzed from a 5 × 5 diallel cross with mice.