Ordering gene functions in a regulatory pathway Imagine that we are studying the regulation of an enzyme and we find a recessive,trans- acting mutation in gene A,that gives uninducible enzyme expression.The simplest interpretation is that gene A is a positive activator of the enzyme: Model 1 enzyme Now,say that we find a recessive,trans-acting mutation in gene B that gives constitutive enzyme expression.The following model takes into account the behavior of mutations in A and B: Model 2 A enzyme The idea is that the gene for the enzyme is negatively regulated by gene B which in turn is negatively regulated by gene A.The net outcome is still a positive effect of gene A on enzyme expression.To distinguish the two models we will need more mutations. However,we can also modify Model 1 as shown below to fit the new data. Model 1 (revised) B enzyme The best way to distinguish the two possible models is to test the phenotype of a double mutant.In one case the A-B-double mutant is predicted to be uninducible and in the other case it is predicted to be constitutive.Ordering gene functions in a regulatory pathway Imagine that we are studying the regulation of an enzyme and we find a recessive, trans￾acting mutation in gene A, that gives uninducible enzyme expression. The simplest interpretation is that gene A is a positive activator of the enzyme: Model 1 Model 1 + enzyme A Now, say that we find a recessive, trans-acting mutation in gene B that gives constitutive enzyme expression. The following model takes into account the behavior of mutations in A and B: A Model 2 Model 2 – enzyme – B The idea is that the gene for the enzyme is negatively regulated by gene B which in turn is negatively regulated by gene A. The net outcome is still a positive effect of gene A on enzyme expression. To distinguish the two models we will need more mutations. However, we can also modify Model 1 as shown below to fit the new data. Model 1 Model 1 (revised) – + A enzyme B The best way to distinguish the two possible models is to test the phenotype of a double mutant. In one case the A–B–double mutant is predicted to be uninducible and in the other case it is predicted to be constitutive