Figure 5
Accounting for the occurrence of pleiotropy and epistasis. Mutation of any one of enzymes E 2, E 3, E 4 would affect both fluxes J a and J b to separate trait components. Mutation of any one of enzymes E 5a, E 6a, etc would decrease flux J a to a trait component but increase J b to another trait component; the concentrations of trait components in pathway J a would decrease, those in pathway J b would increase. Epistasis would occur if a subsequent mutation occurred in any one of enzymes E 5b, E 6b 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.