In this notes we will discuss about Mutation:- 1. Meaning of Mutation 2. Historical Account of Mutation 3. Characteristics 4. Types.
Notes # Meaning of Mutation:
The offsprings resemble their parents in one or several respects, yet there are differences between the two. These differences, whether large or small, are called variations. Some of the variations may be induced by environment, while others may be hereditary. Generally, the variations caused by environment are not permanent, hence non-heritable.
But the variations which appear due to changes in the hereditary mechanism are permanent and heritable. Sudden appearance of marked heritable variation in the nature of any organism in ordinary sense is known as mutation and the offsprings with unusual variability in characters are called mutants. A mutant individual or cell is one in which the changed phenotype is attributable to mutated gene or genes.
Many other definitions for mutation have been proposed from time to time by different biologists. Some are as under:
1. Sudden appearance of new hereditary character in the progenies of plants and animals was referred to as ‘sport’ or mutation by Darwin.
2. In the broad sense, it covers “any heritable change in the genotype” (Macromutation). In the narrower sense, “mutation is a change of gene” (micromutation) (Sinnot and others).
3. According to Bateson, “mutation is discontinuous variation”.
4. According to De Vries, “mutations are sudden and drastic heritable changes not traceable or ascribable to segregation or recombination”.
5. Stebbins describes mutation as discontinuous chromosomal change with genetic effect. He further states that the chromosomal change refers to chemical change in a small part of its chromomere, as well as to alteration of its physical structure.
6. Amatto and Otto (1956) have defined mutation as a change in the heredity constitution of a given species.
Notes # Historical Account of Mutation:
Little was known about mutation before 19th century. It was Darwin who first noticed several sudden changes in the organisms in nature. He called those changes as ‘sports’. Several years after Darwin, Bateson marked that some variations were not continuous.
They were called discontinuous variations. Hugo De Vries (Fig. 22.1), one of the three persons who rediscovered Mendel’s laws of inheritance, observed in 1901 sudden changes in Oenothera lamarkiana.
In Oenothera lamarkiana, gigas (large size), nanella (dwarf) and many other unusual changes, such as, changes in the colour and shape of flowers were marked by him. The new forms, he observed, differed in appearance from the normal forms and apparently they arose in small numbers in each generation.
On the basis of those observations he proposed a general theory of species formation by means of sudden discontinuous changes.
De Vries called such sudden changes as mutations. His observations were published in a book entitled. The Mutation Theory. Simultaneously several unique cases of mutation had been reported in nature, Ancon sheep (Fig. 22.2), one of the most interesting cases of mutation was first noticed in England.
The ancon sheep had a short leg (an unusual feature). That type appeared suddenly in the flock of sheep, existed for several years and then disappeared.
Again after eighty years, they reappeared in Norway and since then they are existing in some European countries. Besides these, many other remarkable cases of mutation have been noticed in both plants and animals. In 1904, Morgan reported white eyed Drosophila melanogaster in the population of red eye flies.
The sudden occurrence of Haemophilia disease in the royal family (Queen Victoria’s family) of England deserves special mention here. Queen Victoria came from a family in which Haemophilia A as not known, but suddenly the disease appeared in some of her sons and subsequently in several sons of her daughters.
Why did it appear? Perhaps, because something went wrong with the genetic machinery of one of the parents or apparently mutation to Haemophilia took place in germ cells of one of her parents so that she herself was already heterozygous for newly mutated character.
Notes # Characteristics of Mutation:
Originally the geneticists were of the opinion that mutations were spontaneous and random in effect, i.e., they were occasionally noticed but this statement seems to be contusive. Mutations may occur in an> cell at any stage of the life of an organism. Mutations occurring in reproductive cells (sperms and ova) are referred to as germinal mutations.
In sexually reproducing species only the mutations arising in germ cells are transmitted to the future generation. Mutations arising in body cells other than germ cells are called somatic mutations.
Somatic mutations, as for example, the fatal cancer of blood or the leukaemia, chronic myeloid, etc. arising in the body cells are not inherited as they affect only the mutant individuals and are not transmitted to future generations.
Asexually propagated species in which somatic mutations occur may become established as mutant strains. Many new varieties of fruit trees have originated as “bud mutations “. In vegetatively propagated mutant plants, the gamete will be either normal or mutant depending on which cell forms germinal tissue.
A mutation may be dominant or recessive, viable or lethal, sex-linked or autosomal. In diploid species, recessive mutation can produce bud mutation or mosaics only when the individual is heterozygous for the gene in question.
Mosaic in snapdragon flower, which is half purple and half lavender, sometimes develops on a branch which, in addition to one or more such mosaic flowers, also bears several wholly purple or wholly lavender colour flowers.
The plant itself was heterozygous for the recessive lavender genes. In some of the cells from which mosaic bud developed the normal allele has mutated to lavender. Dominant mutations may give rise to immediately observable bud, spores or mosaics.
Mutations are detected when some heritable changes occur in the characters of an organism. Since the characters are governed by genes, any change in them directly reflects the change in the genes (gene mutation). So the relationship between a gene and a character is apparent only due to mutation.
A gene mutation is also called point mutation. Sometimes, the mutations in genes do not result in any perceptible change in characters. Nevertheless, such changes are important from evolution point of view because they go on accumulating and have a cumulative effect on the phenotype.
Normally, the original activity of a gene is lost due to mutation and this is the reason why original or wild type genes are dominant and mutant genes are recessive.
In diploid organisms, for each character there are two genes located on two homologous chromosomes. As the mutation is a random process, the chance of simultaneous mutations in both the genes controlling a particular trait is very remote. If the mutation takes place in one of the two genes governing a particular trait, it results in heterozygosity because the other gene remains unaffected.
In such a case the mutation may not be immediately expressed because of the presence of the wild or unmutated genes. It may be expressed in later generations in a small number of individuals which are homozygous for mutant gene.
Recessive mutations perpetuate and are maintained in heterozygous state. In haploids, each character is usually governed by a single gene. So every mutation is expressed in them. Undesirable or unfit mutations are eliminated from the natural populations.
The mutated gene differs from original one in chemical composition or structure. The mutated gene like the original one must be capable of accurate self-replication in mutated form. Any change away from the standard form is called forward mutation. Sometimes mutant genes change towards the standard form. This is called reverse mutation, reversion or back mutation.
The truly mutated genes do not show reversion to the original gene. However, there are some rare exceptional cases in which reversion may be observed. Each individual gene mutates only very rarely, perhaps once in 1, 00,000 or once in a million cells; but as the number of genes in the majority of the organisms is very high, the overall mutation frequency per generation may be considerable.
Notes # Types of Mutation:
Geneticists have classified mutations in different ways according to their convenience.
Amatto (1950) considered three types of mutation:
1. Gene mutation (Mutation at gene level),
2. Chromosomal mutation (Mutation due to changes in the structure of chromosomes) and
3. Genomatic mutation (Mutation due to change in genome or basic chromosome number).
Recently Darlington and Mather have classified mutations into the following five types:
1. Gene mutation; (Mutation at gene level)
2. Structural mutation (change in chromosome structure);
3. Plastid mutation;
4. Numerical mutation; and
5. Cytoplasmic mutation.
Of the above mutational classes, numerical mutation and cytoplasmic and plastid mutations are described in separate chapters. Here only structural mutations and gene mutations will be discussed in detail.