Figure 1. Switching vs. Refinement. (A) Switching between two functional ORs was considered to represent the serial expression of OR alleles in which the neuron conducts a second random initiation of gene activation (circle of arrows) after the shutdown of the OR chosen by a prior selection. The neuron is initially OR1⁺ but then switches to become OR2⁺, maintaining the one-receptor-per-cell rule. (B) Refinement is proposed to maintain monogenic selection after a failure of the initiation process to generate the choice of a single OR allele. The neuron is temporarily double positive, OR1⁺OR2⁺, prior to a competitive process (opposing arrows) through which only one OR emerges.