What are the Chromosomal mutations?
The other name of the chromosomal mutation is aberration. The structural changes in chromosomes which appear phenotypically are known as chromosomal mutations or aberrations. These alterations do not involve changes in the number of chromosomes but result from changes in the number or sequence of genes on chromosomes.
Some important structural changes in the chromosome which are very easy to understand are given below:
Deficiency or deletion
Deficiency signifies the loss or absence of a section of a chromosome and may involve one or more genes. Genie balance is usually disturbed due to deletions and this affects the phenotype. Deletions can be recognized by distortions of chromosomes during meiotic pairing of homologous chromosomes. Due to a terminal deletion, one of the paired chromosomes appears to be much longer than the other, whereas due to an intercalary deletion, the normal chromosome forms a loop near the lacking region of its homologue as only matching regions pair with each other.
The loss of terminal or intercalary segment of a chromosome is called deletion. In this type of chromosomal mutations, inheritance patterns of genes of deleted regions and cytological studies of pairing between normal and deleted chromosomes have helped to find out the relative positions of genes on chromosomes. Thus deletions have helped in constructing and verifying linkage maps of a variety of organisms like maize, Drosophila, bacteriophages, etc.
This is another type of structural change in chromosomal mutation. Here the presence of a part of a chromosome in excess of the normal complement is known as duplication. A broken section of a chromosome attaches itself to a normal homologous or nonhomologous chromosome or in the presence of a centromere it behaves like an independent chromosome and gets included in an otherwise normal nucleus. As a result, some genes are present in the cell in more than two doses. So, the presence of a part of a chromosome in excess of the normal complement is known as Duplication.
The duplication can be of the following types:
[a] Tandem Duplication : The duplicated segment remain adjacent to the normal corresponding, section of the chromosome. ABC DEFGH -» ABC DEFDEFGH.
[b] Reverse Tandem Duplication : The duplicated sequence is in the reverse order of the normal sequence. ABCDEFGH->ABC DEF FEDGH.
[c] Displaced Duplication : The duplicated segment is not adjacent to the nonl segment. It is either on the same side of the centromere (homobranchial) or on I two sides of the centromere (heterobranchial).
ABC DEFGH >ABCDEFGDEF(homobranchial) I——>ADEFBC DEFGH (heterobranchial)
[d]Transposed dulpication: The duplicated part of a chromosome gets attached to a non-homologous chromosome, either interstitially or terminally.
[e] Extra-Chromosomal Duplication : The duplicated segment in presence ( centromere behave as a complete chromosome).
In chromosomal mutations translocation involves the transfer of a part of a chromosome or a set of genes to a non-homologous chromosome. It results in a change in the sequence and position of genes but not their quantity. For example, if the original chromosomes were A B C D E F and G H IJ K L, the new ones may be A B C J K L and G H I D E F. Translocations are of the following three types: -
(i) Simple translocation: -This involves a single break in a chromosome; the broken piece gets attached to one end of a non-homologous chromosome.
(ii) Shift translocation : - Here, the broken part of a chromosome gets inserted in a non-homologous chromosome.
(iii) Reciprocal translocation: - In this case of chromosomal mutation, there is an exchange of chromosome part between two non-homologue. This is the most frequent type of translocation. Due to independent segregation of chromosomes translocations lead to the loss of genes in some of^ cells and a reduction in the number of viable gametes, or partial sterility.
Inversion is a type of chromosomal mutation involves a rotation of a part of a chromosome or a set of genes by 180° on its own axis. Breakage and reunion are essential for inversion. The net result is neither a gain nor a loss in the genetic material but simply a rearrangement of the gene sequence. For example, A B C D E F G gene order may be converted to A B D C E F G.
Inversions are of the following two types: -
(i) Paracentric inversion : - In this type, centromere is located outside the inversion loop. When a crossing over occurs within the loop, one product contains a centromere and the other does not. At anaphase this results in an abnormal chromosomal 'bridge' and a loss of an entire chromosomal section.
(ii) Pericentric inversion: - In such chromosomal mutation, centromere is located inside the inversion loop. When a cross- ovar between two chromatids occurs within the inversion loop, in the resulting chromatids there are some genes in double number while others are missing. Due to this imbalance, the cell is not practical.