Permissive temperature

The permissive temperature is the temperature at which a temperature sensitive mutant gene product takes on a normal, functional phenotype. When a temperature sensitive mutant is grown in a permissive condition, the mutated gene product behaves normally (meaning that the phenotype isn't observed), even if there is a mutant allele present. This results in the survival of the cell or organism, as if it were a wild type strain. In contrast, the nonpermissive temperature or restrictive temperature is the temperature at which the mutant phenotype is observed.