Figure 5
Accounting for the occurrence of pleiotropy and epistasis. Mutation of any one of enzymes E2, E3, E4 would affect both fluxes Ja and Jb to separate trait components. Mutation of any one of enzymes E5a, E6a, etc would decrease flux Ja to a trait component but increase Jb to another trait component; the concentrations of trait components in pathway Ja would decrease, those in pathway Jb would increase. Epistasis would occur if a subsequent mutation occurred in any one of enzymes E5b, E6b etc. A branched metabolic pathway is thus a potential source of pleiotropy and epistasis; see the text for further discussion. This diagram, like that in Figure 4, emphasises the importance of adopting a systemic approach in understanding the potential effect, on a trait or traits, of a mutation in any one enzyme in enzyme-catalysed systems.