Equation 1. Definition of synergy ratios.

$$\text{synergy}\text{ratio}\frac{\text{observed}\text{phenotype}}{\text{expected}\text{phenotype}}=\frac{{\mathrm{\Phi }}^{\prime }\text{ab}}{1-(1-{\mathrm{\Phi }}^{\prime }\text{a})(1-{\mathrm{\Phi }}^{\prime }\text{b})}$$