If a mutation cannot complement mutations that fall in different complementation groups, but the mutation is recessive to the wild-type, then it may be a cis-dominant mutation that prevents expression of multiple genes. For example, if XYZ form an operon transcribed from XÆYÆZ and each of the mutations is recessive to the wild-type operon in trans ...
The easiest way to evaluate a complementation grid is to evaluate one column or row at a time. If the data is available, first check to see if all of the mutations are recessive to the wild-type. If any mutant is dominant, it should not be included in any of the complementation groups. As you go through each column or row, write down a list of all the alleles that cannot complement each other. When you are done, check to see if the results are internally consistant -- that is, ask yourself if there are any mutations that seem to be in more than one group or that complement some of the mutations in one group and not other mutation that seem to be in the same group. Circle any alleles with "weird" complementation behavior -- they should not be included in the groups because they are not simple recessive, loss-of-function mutations. Finally, double-check to see if you accounted for all of the mutant alleles.
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Last modified July 15, 2002