Terms Based on Phenotypic Consequences

Conditional Mutation. Phenotype of the organism changes only under a specific condition called the restrictive condition, but remains normal under other conditions called the permissive condition. 

Temperature-Sensitive Mutation. It is a special case of conditional mutation in which the restrictive condition is elevated or lowered temperature. Temperature-sensitive mutants are commonly used to analyze the function of essential genes, because inactivating mutations such as deletion mutations are not possible for these genes. 

Lethal Mutation. This refers to a mutation that causes cell death. 

Synthetic Lethality. A mutant organism is viable when separately carrying only one of two or more mutant genes. When these mutations occur together or when these individual mutant genes are combined together in the same organism, lethality results. 

Auxotroph. A mutation that causes a defect in the synthesis of an essential metabolite such that the growth of the organism is dependant on exogenous supply of this metabolite. 

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