Figure 1.

Dominance of mutations as predicted by the metabolic control theory. A loss-of-function mutation (c) reducing enzymatic activity to 50% (resp. 0%) when heterozygous (resp. homozygous) has a lethal but strongly recessive effect on metabolic flux (or fitness). A mutation decreasing less drastically the activity of the enzyme (b) has a milder and less recessive effect on metabolic flux (or fitness). However, by construction (i) the heterozygous effect of the lethal mutation is larger than that of the nonlethal (W_Ab > W_Ac) and (ii) overdominance cannot occur.