Phenotype of gene1 single mutation Phenotype of gene2 single mutation Phenotype of gene1 gene2 double mutation Model 1 GENE1 GENE2 → → X (Φa) Y (Φb) X (Φa) Model 2 GENE2 GENE1 → → X (Φa) Y (Φb) Y (Φb) Model 3 GENE1 → GENE2 → X (Φa) Y (Φb) Z (Φab) Model 4 GENE1, GENE2 → X (Φa) Y (Φb) X (Φa), Y (Φb), or Z (Φab) Models 1-4 derived from Anthony Michels, CV (2002) Genetic techniques for biological research: a case study approach. John Wiley and Sons. (http://www.amazon.com/ Genetic-Techniques-Biological-Research-Approach/dp/0471899194)

Table 1: Epistatic Models (Ems) for Substrate-Dependent Pathways.