This experiment represents a powerful form of genetic analysis known as an epistasis test

This experiment represents a powerful form of genetic analysis known as an epistasis test

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This experiment represents a powerful form of genetic analysis known as an epistasis test. In the example above, if  the double mutant were constitutive we would say that the mutation  B– is epistatic to  A – . Such a test allows us  to determine the order in which different functions in a regulatory pathway act. If the double mutant in the  example were constitutive, we would deduce that gene  BBBBB functions after gene  AAAAA in the regulatory  pathway. To perform an epistasis test, it is necessary that the different mutations under examination produce  opposite phenotypic consequences. When the double mutant is constructed, its phenotype will be that of the  function that acts later in the pathway.  Epistasis tests are of very general utility. If the requirement that two mutations have opposite phenotypes is met, almost any type of hierarchical relationship between elements in a regulatory
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Unformatted text preview: pathway can be worked out. For example, the LLLLL ccccc mutation is in a site, not a gene, but it is still possible to perform an epistasis between LLLLL ccccc and LLLLL sssss since these mutations satisfy the basic requirement for an epistasis test. One mutation is uninducible while the other is constitutive for LLLLL gene expression. When the actual double mutant, L LL L c cc c L LL L s ss s , is evaluated it is constitutive (this makes sense given what we know about the LLLLL operon since a defective operator site that prevents repressor binding should allow constitutive expression regardless of the form of the repressor protein). Formally, this result shows that a mutation in LLLLL is epistatic to a mutation in LLLLL . Even if we did not know the details of LLLLL operon regulation before hand, this epistasis test would allow us to deduce that the operator functions at a later step than the repressor....
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