Fig. 7.

Model for the enhancer-dependent process of the singular OR gene expression. OSNs are fated to express a single OR gene from the repertoire of class I or class II genes to produce fixed populations of class I (blue) and class II OSNs (magenta). Subsequently, a functional OR enhancer is chosen according to the OSN fate to open up local chromatin structure to orchestrate the first step of OR gene choice. Class I OSNs activate one enhancer allele of the J element (green or red circle) or of a limited number of enhancer elements. By contrast, class II OSNs select one element from the class II enhancer repertoire spread over most chromosomes (white circle, H element is colored in yellow or blue). The selected enhancer element chooses one functional OR allele to express. In ΔJ heterozygous mutation, the intact J element allele (green) is activated to complement the ΔJ allele to fill in the class I OSN population. In class II enhancer (e.g., ΔH) heterozygous mutation, an intact element is selected randomly from the class II enhancer repertoire spread over chromosomes. In consequence, the expression of the H element linked OR genes are reduced to ~ 50%