Types of Mutations
Various classifications of mutations are known, each based on a definite criterion. A classification based on the method of detection of mutations includes the following main types: (i)
Morphological mutations involve alterations in external form including colour, shape, size, etc. Examples include albino ascospores in
Neurospora, kernel colour in corn, curly wings in
Drosophila and dwarfism in pea. (ii)
Lethal mutations involve genotypic changes leading to death of an individual. These are perhaps the easiest to score for a study of mutation frequencies (see later for lethal mutations in
Drosophila; some albino mutations resulting from chlorophyll deficiency are also lethal), (iii)
Biochemical mutations are identified by a deficiency, so that the defect can be overcome by supplying the nutrient or any other chemical compound, for which the mutant is deficient. Such mutations have been studied mainly in prokaryotes like bacteria and fungi, but sometimes also in eukaryotes like
Drosophila and humans. They are discussed in detail in the next main topic,
(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).