Multiplicative fitness

One of the simplest two locus models is multiplicative. In this model, an individual's fitness for its two genotypes is found by multiplying the fitnesses of each of its one locus genotypes.

Suppose, for example, that the A locus influences survival from age 1-6 months, such that

genotype:
A1/A1 A1/A2 A2/A2

survival chance1-6 months:
w₁₁ w₁₂ w₂₂

and the B locus influences survival from ages 6 - 12 months:

genotype:
B1/B1 B1/B2 B2/B2

survival chance 6-12 months:
x₁₁ x₁₂ x₂₂

The total chance of surviving from age 1 - 12 months would then be the product of the two genotypes that an individual possessed: an individual with the haplotypes A1/A1B1/B2 would have a chance of survival of w₁₁x₁₂

An individual with the genotype A1/A1 therefore has a chance of surviving from age 1 - 6 months of w₁₁ whether its genotype at the other locus is B1/B1, B1/B2, or B2/B2. The genotypes are independent: the effect of one genotype on survival is independent of the other locus.