The comb shape in chicken showing walnut, rose, pea and single forms is an example of
Chicken comb morphology demonstrates interaction between two unlinked loci R and P encoding transcription factors regulating comb development. Rose comb arises with R- pp, pea comb with rr P-, walnut comb novel phenotype requiring both dominants R- P- through synergistic interaction, single comb rr pp recessive baseline. Neither locus alone suffices for walnut; interaction creates new structure not predictable from single-gene dominance. Such dependence of one trait on multiple non-allelic genes defines gene interaction, distinct from incomplete dominance, codominance, or sex-linked inheritance, as autosomal loci cooperate developmentally.
Ref: Bateson and Punnett, 1905, Comb Genetics; Griffiths et al., Chapter 6: Gene Interaction