Correspondence Between Regression Models and Biological Models.

a. Six of the eight possible regression models represent hierarchical relationships between genes. If the upstream gene is a repressor we can identify gene order and the signal effect. If the upstream gene is an enhancer, we can identify only the signal effect. If the signal turns off an upstream enhancer, deleting either gene will have no effect. b. Non-hierarchical relationships can be distinguished if both genes are activated by the signal. Model 3 suggests buffering, while Model 4 suggests independent effects, i.e. no epistasis. If a potential regulator is turned off by the signal it has no effect on the target gene.