In pathway analysis, mutants blocked earlier require
Position of block predicts number of supplements capable of restoring growth due to sequential dependency of pathway. Early lesion abolishes synthesis of many downstream metabolites so exogenous provision of any compound beyond block including numerous intermediates and final product rescues prototrophy. Late lesion removes only final conversion so fewer compounds suffice for rescue. Consequently mutants blocked nearer start show broader supplementation requirement pattern while those near end need narrow rescue. Scoring breadth of positive growth therefore informs ordering of genes along biosynthetic assembly line logically.
Ref: Snustad & Simmons, Principles of Genetics, 7th ed., Chapter 14: Auxotroph Rescue Breadth Reflects Block Position