We investigate sufficient conditions for the presence of coexistence states for different genotypes in a diploid diallelic population with dominance distributed on a heterogeneous habitat, considering also the interaction between genes at multiple loci. In mathematical terms, this corresponds to the study of the Neumann boundary value problemwhere the coupling-weights wi are sign-changing in the first variable, and the nonlinearities fi : [0, 1] → [0, +∞[ satisfy fi(0) = fi(1) = 0, fi(s) > 0 for all s ∈ ]0, 1[, and a superlinear growth condition at zero. Using a topological degree approach, we prove existence of 2 N positive fully nontrivial solutions when the real positive parameters λ1 and λ2 are sufficiently large.Mathematics Subject Classification (2010). 34B18, 47H11, 92D25.