In this article we will discuss about:- 1. Definition of Mutation 2. History of Mutation 3. Aniridia in Man 4. Detection 5. Types.
- Definition of Mutation
- History of Mutation
- Aniridia in Man as a Case of Mutation
- Detection of Mutation
- Types of Mutation
1. Definition of Mutation:
Heredity results accurate reproduction of genes. During cell-division the chromosomes divide to give rise to daughter chromosomes. The chromosomes are beset with genes and consequently chromosome duplication means in a way the duplication of genes.
Genes always arise from existing genes although the material for the synthesis comes from nutritional sources. The newly formed gene is an exact replica of the parent gene.
The process of gene reproduction though exact, sometimes presents an error in copying. The copy of the gene differs from the original and the modified gene reproduces its changed structure. This error in copying is Mutation.
Thus mutation is a change in gene, potentially capable of being transmitted in the changed form. Mutation occurs in genes (gene mutation) as well as in chromosomes (chromosomal mutation). Chromosomes usually reproduce faithfully and accurately. Sometimes a chromosome may alter by loss of some parts of genes, by reduplication, by translocation or by inversion.
2. History of Mutation:
Instances of sudden appearance of new hereditary types in plants and animals were in record by breeders. These new types were referred to as sports. The appearance of Ancon lamb (lamb with short and bowed legs), hornless cattle, double toed cats, albino rats are all examples of spontaneous mutations.
In Darwin’s time these sports were regarded as insignificant. They were considered as monstrosities rather than the origin of new viable types. Appearance of a number of sports in Oenothera lamarckiana led de Vries to put forward his mutation theory.
The striking mutants in Oenothera are forms like giants which had large size, nanella which were dwarfs and many other forms which differ in colour, size or shape of various parts. De Vries introduced the term ‘Mutation’ or ‘Saltation’ to ascribe for these sports.
3. Aniridia in Man as a Case of Mutation:
Genes are responsible for characters. In man the existence of gene is known which regulates the formation of normal iris of the eye. Sometimes this gene becomes so altered that the iris fails to develop. Failure of the iris to develop is known as aniridia. Mutation of this gene occurs in the germ cell of a man and is transferred to the zygote which finally develops into a new man.
The mutated gene is a dominant gene and as a result the new individual is born with no iris in the eye. The mutated gene persists in the germinal cells of this aniridic man who will eventually produce half of the germ cells with this mutant gene (Fig. 2.28).
4. Detection of Mutation:
Mutations produce both visible and lethal effects and it is possible to detect both these effects.
i. Detection of visible mutation:
Mutations may be produced artificially by applying different methods. If a visible nonlethal mutation is produced it will be detected readily provided the gene that mutates is a dominant one.
In one peculiar strain of Drosophila melanogaster known as ‘attached-X’ strain a very favourable condition is present and because of this condition a visible but recessive mutation can be detected very easily. In the ‘attached-X’ strain the females have two X chromosomes attached to one another at the ends nearest the centromere and a Y chromosome.
Because the two X chromosomes are attached they always tend to go to the same pole during meiosis. When such a female is crossed to a normal male then comes a theoretical possibility of the formation of four types of individuals.
(a) Attached X+Y—when the two attached X of the female unite with the Y chromosome of the male.
(b) X+Y male—When the Y chromosome of the female unites with the X chromosome of the male. This is a normal male.
(c) Attached X+X—When the two attached X of the female unite with the X of the male. Such individuals are with a total of three X chromosome? and are called super females which dies at a very early stage of development.
(d) Y+Y—When the Y chromosome of the female unites with the Y chromosome of the male. These are incomplete individuals and die at an early stage.
Thus a cross between attached X females and normal males produces ‘attached X’ females, normal males and some nonviable super females and incomplete individuals. The most noteworthy feature of such a cross is that the normal male offspring receives its X chromosome from the male and not from the female.
In the attached X method, normal males are treated with X’-rays and are then mated with attached-X females. As the sons in such a cross receive their X chromosome from the father, it becomes very easy to detect any mutation that may occur to the treated X chromosome-of the male.
ii. Detection of induced lethals:
In order to estimate the frequency of the induced lethal mutations that are produced in X chromosomes, male Drosophila flies are exposed to radiation of known kind and intensity. They are then-mated with non-radiated or normal females.
If a lethal is produced in the X chromosome of the male it will not be detected in the first generation of the females because they will be ‘masked’ by the ‘non-lethal’ of the X chromosome of the female parent. But if the resultant female offsprings are again mated to normal males the mutation will reappear in the next generation and will be manifested as a deficiency in the male population.
It is obvious that the treated X chromosome of the male passes into a female and subsequently again into the male. These grandsons have no X chromosome other than the one received from the grandfather and the presence of a lethal in that X chromosome results in the early death of all males.
A very simple and standard technique has been devised by Muller for the detection of new lethal in the X chromosome of Drosophila melanogaster. The technique is known as CIB method.
The method employs a special stock of female Drosophila fly. One X chromosome of such a female fly bears a recessive lethal gene (1) a dominant gene (B) for bar eye and a dominant gene (G) which is a crossover suppressor besides other genes.
Such females with one X chromosome as marker are mated to irradiated males. One fourth of the offsprings produced out of such a cross will receive a CIB X chromosome and an irradiated X chromosome and will be females.
One fourth of the offsprings, will be male with a GIB chromosome and will eventually die because of the presence of the lethal character (1). The remaining half of the progeny will not have the GIB chromosome and as such they will be normal in their eye structure. Such flies will be discarded. The CIB females with treated X chromosome will then be mated with normal males.
It is to be remembered that in a normal female crossing-over takes place between the two X chromosomes. But in CIB chromosome such a cross-over is not possible because of .the presence of the gene, C, which is a cross-over suppressor.
Thus the chance of transference of the induced lethal from the irradiated chromosome to the non-irradiated chromosome- becomes nil. The use of B gene is to identify the flies that have the cross-over suppressor (Fig. 2.29).
Thus when females with one CIB X- chromosome and one treated X chromosome are mated with normal males half of the male offsprings will fail to survive because of the presence of the lethal gene, 1. In case the irradiation induces any new lethal the other half of the males will die.
5. Types of Mutation:
i. Somatic mutations:
Mutations occurring in the genes which are somatic or vegetative in this location have been termed somatic mutation. Most of the mutations observed by de Vries in Oenothera lamarkiana were somatic mutations. Subsequently the presence of somatic mutation in the endospermic tissue of maize has been demonstrated by Emerson.
ii. Germinal mutation:
Mutation occurring in the germ cells (sperm or ovum) has been termed germinal mutation. Such mutations are again divided into gametic when occur in the gametes and zygotic when occur in the zygote.
iii. Spontaneous mutation:
The naturally occurring mutations are the spontaneous mutations. Natural agencies are responsible for the production of such mutations. The appearance of-a white-eyed male in a culture of wild type Drosophila of Morgan is a classical case of spontaneous mutation. Presence of such mutation speaks in favour of evolution.
iv. Induced mutation:
It is possible to induce mutation by the application of artificial means. Any agent that can produce mutation is called a mutagenic agent. Many of such agents are known. The important ones are X-rays, ultra-violet rays, radium, heat and temperature, mustard gas, coal tar, formaldehyde, caffine, etc.
v. Anomozygous mutation:
Mutations due to structural changes of a chromosome or a set of chromosomes have been termed anomozygous mutation. Structural changes in the chromosomes are caused by translocation, inversion, duplication and deficiency.
vi. Biochemical mutation:
Mutations that affect specific biochemical processes have been called biochemical mutation. Such mutation occurring in an individual inhibits its ability to synthesise essential materials like vitamin or amino-acid. The inhibition may be total or partial.
In partial inhibition there occurs a partial blockage in the steps of a synthetic process. Biochemical mutations of Neurospora is well studied. The alkaptonuria and phenylketonuria of man are examples of biochemical mutations.
vii. Lethal mutation:
Most mutations are lethal. Such mutations cause loss or alteration of a function during embryogenesis. Biochemical mutations are mostly lethal. But in some cases, organisms with biochemical mutations can thrive well if the metabolite which they cannot synthesise is supplied exogenously and in such cases biochemical mutations are not regarded as lethal.