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  Section: Genetics » Mutations » Morphological Level (Including Lethal Mutations)
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Types of Mutations

Mutations : 1.  Morphological Level (Including Lethal Mutations)
Brief History
Range of Mutations
Stages of Which Mutations Occur
Types of Mutations
Spontaneous Vs Induced Mutations
Mutation Rates and Frequencies
Induced Mutations 
Detection of Mutations in Drosophila 
Detection of Mutations in Plants
Use of Microbial Systems to Assess Potency of Mutagens
Practical Applications of Mutations
Effect of Genotypes on Induction of Mutations (Mutator Gene and Paramutations)
Adaptive Mutations and Genotrophs
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).

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