Mutation in a broad sense include all those heritable changes, which alter the phenotype of an individual Hugo de Vries used the term ‘mutation‘ to describe phenotypic changes which were heritable. He is therefore, credited to have differentiated between heritable and environmental variations. However, the term mutation is now used in a rather strict sense to cover only those changes which alter the chemical structure of the gene at the molecular level. These are commonly called gene mutations or point mutations. In practice, sometimes it is rather difficult to distinguish between gene mutations and structural changes in chromosome, because certain structural changes may have the same phenotypic effects as gene mutations and these structural changes may be too small to be detected cytologically. For instance, small deficiencies cannot be discovered by cytological observations. Although in Drosophila small deficiencies can also be detected in giant salivary gland chromosomes, in other organisms the only test for a deficiency is that it will not revert back to wild type character. However, gene mutations would be able to give reverse mutations.
The distinction between point mutations and chromosomal aberrations is thus rather superficial. If chromosomes are not studied under the microscope, in certain cases we may not be in a position to say with certainty whether a particular phenotypic character is due to point mutation or due to a structural change. Many mutations, described by de Vries in Oenothera lamarckiana, are now known to be due to certain numerical and structural changes in chromosomes. These are sometimes described as chromosomal mutations. For instance, ‘gigas’ mutant in O. lamarckiana was later found to be due to polyploidy.
Brief History
The earliest record of point mutations dates back to 1791, when Seth Wright noticed a lamb with unusually short legs in his flock of sheep. Wright thought that it would be worthwhile having a whole flock of these short legged sheep, which could not get over the low stone fence and damage the crop in the adjacent fields. In the successive generations, this trait was transferred and a line was developed where all sheep had short legs. This character resulted from a recessive mutation and the short legged individuals were homozygous recessit Once this mutation occurred in a particular cell, this will be carried in all the cells descending from this parent cell. This point mutation was discovered at a time when the science of genetics did not even have its birth. The short legged breed of sheep was known as Ancon breed.
The scientific study of mutations started in 1910, when T.H. Morgan started his work on fruitfly, Drosophila melanogaster, and reported white eyed male individuals among red eyed male individuals. Later it was found that the gene for this character is located on sex chromosome (X-chromosome) and expresses itself in a male individual (male individuals have one X-chromosome and one Y-chromosome; female has two X-chromosomes). When these rare white eyed males were crossed to their sister red eyed females, white eyed females could also be obtained in some cases proving that the females involved were heterozygous.
After the discovery of white eyed mutant, a thorough search for mutants was made by Morgan and his co-workers in Drosophila and about 500 different mutations were observed by geneticists all over the world. This search of mutations in Drosophila was
accompanied with mutation work in other organisms also e.g. maize, snapdragon, rodents, pea, fowl, man etc. However, in the last 40 years, increasing interest has been observed for mutations in microorganism, like Neurospora, bacteria (Escherichia coli) and bacteriophages, since these materials have been found to be very suitable for mutation work.
Range of Mutations
Although change due to single gene mutations may involve sudden and large changes (like that Ancon breed of sheep), they can also be so small that one may not be able to notice them. Therefore, earlier concept that, mutations could involve sudden and big changes only, was not correct. Mutations could also be observed which do not involve any change in morphology but only modify nutritional requirements as in case of Neurospara, similarly, mutations could be observed in pathogens, as a result of which a pathogen could lose or acquire virulence against a particular host.
Stages at which Mutation Occurs
Mutations can occur at any stage during the development. Depending upon this stage following situations will be met-
- If mutation occurs in a germinal cell, before differentiation of gametes, if would influence several gametes and will thus influence all the individuals derived from these affected gametes.
- If mutation occurs in a gamete or a zygote, a single individual will carry the mutation.
- If mutation occurs in a cell after the zygote has undergone one or more divisions, only a part of the body will show the mutant character. Such mutations will be called somatic mutations, in contrast to germinal mutations listed in (1) and (2) above.
Types of Mutation
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-
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Morphological mutations-
It 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.
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Lethal mutations-
It involve genotypic changes leading to death of an individual. These are perhaps the easiest to score for a study of mutation frequencies.
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Biochemical mutations-
They are identified, each by a nutritional 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.
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Resistant mutations-
They are identified by their ability to grow in the presence of an antibiotic (e.g. streptomycin, ampicillin, cycloheximide) or pathogen, to which wild type is susceptible. These are very easy to score ad therefore have been extensively studied.
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Conditional mutations-
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 or for a study of DNA replication.
Major emphasis in this chapter 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.
Spontaneous vs Induced Mutations
Mutations are rare events in nature and are then described as spontaneous mutations. Due to their rare occurrence, sometimes, it is difficult to identify and score them. This difficulty has been overcome by two methods-
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Selective systems-
Selective systems have been designed, which facilitate the selection of mutants against normal wild type, as in case of biochemical, resistance and conditional mutations, where under certain conditions only mutants will grow permitting selection of one mutant among even a million individuals.
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Induced mutations-
Induced mutations are used, when selective systems are not available and therefore, frequency of mutations need to be increased artificially, to allow convenient identification and scoring of mutations.
Important links
- Molecular Structure of Eukaryotic Chromatin| Levels of DNA packaging
- Structure of Metaphasic Chromosome
- Special Chromosomes- Lampbrush, Polytene, B-Chromosome
- Concept of Allelomorphism | Position Effect
- Gene Concept and Structure of Gene
- Interaction of Genes- Complimentary, Epistatic, Duplicate Gene etc.
- Inheritance of Multiple Factors
- Linkage- Types, Linkage Groups, Importance
- Concept of Crossing Over- Types, Mechanism, Significance
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