They are changes involving chromosome morphology which result in changes in number and sequence of genes without altering policy or gene structure. Chromosome aberrations involve breaking of chromosome segments, their loss or union with same (intrachromosomal aberration) or different chromosomes (interchromosomal aberration). The important types are as follows:
(A) Deficiency:
It is the loss of chromosome segment. The lost segment may be terminal (deficiency or terminal deficiency) or intercalary (intercalary deficiency or deletion). The terms deficiency and deletions are also used interchangeably. Terminal deficiency involves a single break near the chromosome end. In intercalary/ interstitial deficiency or deletion, there are two breaks (double breaks), loss of separated segment and the mutation (Seth Wright, 1791) followed by Hornless (Polled) Cattle (1889).
Scientific study of mutations started with the noticing of white eye colour in Drosophila (Morgan, 1910). A mutation from wild to a new type is called ‘forward mutation’. The reversal of mutated gene back to its original/wild form is called reverse/back mutation.
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Mutations affecting vegetative cells are called somatic mutations. They are not inheritable, and disappear with the death of the individual. Somatic mutations occur in some of the cells from chimeras, patches or mosaics, e.g. variegation. In plants, somatic mutations can be preserved and passed on to future generations through vegetative propagation & tissue culture.
Germinal mutations occur in the sex cells and are inheritable. Depending upon the effect, mutations may be dominant or recessive. The recessive mutations show their effect after a few generations when they become homozygous. A single mutation affecting more than one character is called pleotropic mutation. Mutations have helped in development of new varieties of plants and animals, e.g. Sharbati Sonora from Sonora-64.
Spontaneous Mutations:
They are mutations which develop at random naturally, automatically or spontaneously in an organism due to internal reasons without any relation to any external/environmental factor.
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The frequency of spontaneous mutations is different for different organisms and their different genes, e.g., 1 in one million in Drosophila, 1 in 10 million cell generations in bacteria & 1 in 50,000 in human beings. The genes which mutate frequently are called MUTABLE GENES, e.g. R-gene of colour in maize (1 in 2000 gametes).
Stable genes do not mutate even once in several million gametes. Mutator genes increase frequency of spontaneous mutations, while antimutator genes prevent spontaneous mutations. Spontaneous mutations may be due to (i) Errors in replication (ii) Failure of proof reading machinery (iii) Presence of tautomeric form of nitrogen bases e.g. imino tautomer instead of amino group (e.g. cytosine, adenine) or enol group instead of keto-group (e.g. thymine, guanine). (iv) Slow spontaneous deamination of cytosine to uracil, (v) Back ground radiations.
Induced Mutations: They are mutations that are produced in response to specific external factors or chemicals. Muller (1927) was the first scientist to produce induced mutations in Drosophila with the help of X-rays (upto 150 times the spontaneous sex- lined lethal mutations). He got nobel prize in 1946.
Mutagens:
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There are physical and chemical factors which bring about new mutations and increase the frequency of spontaneous mutations.
Physical Mutagens:
They are physical factors which induce mutations. Physical mutagens are of three types-temperature, ultra-violet radiations and ionising radiations.
(i) Temperature:
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Increase in temperature increases the frequency of mutations. Q10 being 5 for Rice. Very low temperature is also mutagenic.
(ii) UV Radiations:
(Muller and Altenberg, 1932). They induce hydrolysis of cytosine and formation of thymine dimers. Thymine dimers cause bending and misreplication of DNA. Organisms overcome damage from UV radiations present in normal sunlight by photorepair (photoreactivation), excision repair and post replication repair.
(iii) Ionising Radiation:
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They include X-rays (man-made, Roentgen, 1895), cosmic ray (proton and helium nuclei present in space), a-particles (helium nuclei), p-particles (electrons), gamma rays (massless radiations). The latter three are emitted from radioactive elements and radio-isotopes. P-32 is radioisotope which gets incorporated in DNA but starts decaying and changed to sulphur.
DNA develops breaks in their regions. Ionising radiations produce electrically charged particles which ionise biomolecules and bring about several changes in DNA. There is deamination and dehydration of nitrogen bases, formation of peroxides and oxidation of deoxyribose.