Figure 4.

Model for insulator bypass. The yellow alleles are drawn only approximately to scale. Symbols are used as in Fig. 1. (A) Homologue pairing in y²/y^3c3 flies generates an unpaired loop in regions of heterogeneity between the alleles. This topology allows for bypass of the su(Hw) chromatin insulator by the y² body enhancer to act on its own promoter in cis. (B) The mutant phenotype of y²/Y males and hemizygous females (not shown) demonstrates that simple unpairing of yellow sequences is not sufficient to activate the y² promoter.