(iv) Resistant mutations
are identified by their ability to grow in the presence of an antibiotic (e.g. streptomycin, ampicillin, cycloheximide) or a pathogen, to which wild type is susceptible. These are very easy to score and therefore have been extensively studied, (v) Conditional mutations
are those which allow the mutant phenotype (including lethality) to be expressed only under certain condition (e.g. high temperature) called restrictive condition. Under other or normal condition described as permissive
condition, the mutant expresses normal phenotype. These mutants, if lethal or semi-lethal can be multiplied under permissive conditions and shifted to restrictive conditions for specific study. They have been extensively used for study of cell cycle (Physical Basis of Heredity 3. Genetics, Biochemistry and Dynamics of Cell Division
) or for a study of DNA replication (Chemistry of the Gene 2. Synthesis, Modification and Repair of DNA
Major emphasis in this section will be on morphological and lethal mutations, which are also described as macromutations
(identified in individuals) in contrast to micromutations
(identified only on the basis of a population, e.g. mutation for yield in a crop).