Additive fitness

Imagine that two loci encode two digestive enzymes. The enzyme produced by the A locus digests one kind of food, that by the B locus another kind of food. The genotypes might enable a certain number of joules of energy to be obtained by feeding in a particular environment:

genotype:
A1/A1 A1/A2 A2/A2 B1/B1 B1/B2 B2/B2

energy obtained: w₁₁ w₁₂ w₂₂ x₁₁ x₁₂ x₂₂

The fitness of an individual might be proportional to the total energy it obtains by feeding; its fitness for the two loci would then be found by adding those of its two genotypes together: