Mutation due to Changes in Structure of Chromosome:
What is Mutation?
It is an abrupt and discontinuous process where a gene or chromosome undergoes a heritable change in its structure or its number.
Mutations were called sports by Darwin and discontinuous variations by Bateson.
The term mutation was coined by Hugo de Vries (1901).
De Vries observed a number of mutations in his experimental plant Evening Primrose (Oenothera Lamarckiana).
Mutation due to Changes in Structure of Chromosome (Chromosome Mutation or Chromosomal Aberration):
They are changes in the morphology of chromosomes that bring about change in the number and sequence of genes over the chromosomes without changing gene structure or ploidy of the chromosomes. Changes in chromosomal architecture can be found out either through a genetic test or observing the chromosomes after staining (example- G-banding) preferably in paired conditions. Structural changes include- deletion, addition, chromosomal rearrangement.
A part of a chromosome is deleted either from one of the pairs or both from homozygotic or heterozygotic chromosomes. During the pachytene of meiosis, deletion is detected as a loop.
Duplication or Addition:
Addition or duplication arises when part of a chromosome is added at the same position at a displaced position on the same arm or different arm of the same chromosome or different chromosome. Sometimes chromosome segment is attached in reverse order. As a result of duplication there arises bar eyes and hairy wings in drosophila.
It is a change in chromosome architecture in which part of the chromosome gets rotated or inverted in its position so that the sequence of the genes in the inverted region is reversed. It is believed that inversion occurs due to loop formation in a part of the chromosome, breakage at the points of intersection followed by reunion. Inversion is of two types paracentric and pericentric. When the inverted segment does not include centromere, it is called paracentric inversion, whereas pericentric inversion includes centromeres. Paracentric inversion will occur in one arm of the chromosome while pericentric inversion will take place at the interjunction of two arms. Inversions are quite common. At least, one inversion occurs in one newborn out of 100. They can be known with the help of chromosome banding (G-banding). Inversions reduce the frequency of recombinations by providing lethal or sterile cross-overs. They also lead to irregular disjunction and loss of chromosome segments. Occassionally they produce new variety and even species.
Translocation is the chromosomal rearrangement resulting in a change in the sequence and position of the genes. But there is no change in the total amount of DNA.
It is of two types, reciprocal translocation and transposition (simple translocation). Reciprocal translocation is the exchange of chromosomal segment between two pairs of non-homologous chromosomes. Besides, when a segment of a chromosome is broken by external or internal stress and attachment to any nearest non-homologous chromosome, it is called simple translocation or transposition. In Oenothera, Tradescantia and Rhoeo translocation in heterozygous conditions are naturally found. Translocation can also be induced artificially.