Any type of change in structure of a normal chromosome, is referred to Structural Chromosomal aberration. These changes are also known as chromosomal mutations or structural chromosomal changes.
(a). It may take place in both somatic as well as in germ cells.
(b). It occurs during interphase or early prophase.
(c). It occurs due to breakage and reunion of the chromosome segments caused by radiations or chemicals.
(d). Structural changes causes changes in phenotype, fertility, viability and karyotype of an individual.
Type of Structural Changes
(1). Deletion or Deficiency:- It refers to loss of portion of segment from a chromosome. It is also well known as Deficiency. Deletion or deficiency has been observed in Drosophila, tomato and wheat etc.
Based on the locations of loss of genes, deletion are divided into two types.
- Terminal Deletion
- Interstitial Deletion
(A). Terminal Deletion:- Loss of a portion from terminal end of chromosome, known as Terminal deletion. It may be two types, (a). Heterozygous (deficiency occurs in only one chromosome of pair. (b). Homozygous (Deficiency occurs in both chromosomes of a pair).
(B). Interstitial Deletion:- Loss of intermediate portion of a chromosome, or between telomere or centromere, is known as Intercalary or interstitial deletion. The deleted portion comes out and remaining segments reunited.
Effects of Deletion:-
(a). Deletion causes reduction in pollen fertility.
(b). Small deletion (Heterozygous) is viable while large deletion (homozygous) is lethal for individual.
(c). Crossing over is also suppressed in presence of deletion due to loss of corresponding portion in homologous chromosomes.
(d). Deletion also affect phenotype due to deletion in short arm of chromosome which causes cri du chat (cry like cat) syndrome in Man.
Significance of Deletion:- Deletion are important for creating variability by chromosomal mutations and used as a cytological tool for gene mapping.
(2). Duplication:- It refers to the presence of a segment twice in the same chromosome. It is also known as repeat and reported by Bridges (1919) in Drosophila. It is reported in Maize, wheat, rice, Tradescantia and Nicotiana.
It is four type:-
- Tandem
- Reverse Tandem
- Displaced
- Reverse Displaced
- Tandem:- In this type of duplication, duplicated segment is present in the same sequence of genes by the original segment of a chromosome.
- Reverse Tandem:- In this type of duplication, duplicated segment is present in the reverse sequence of the genes by the original segment of a chromosome.
- Displaced:- In this type of duplication, duplicated segment is present away from the original segment but on the same arm of the chromosome.
- Reverse displaced:- In this type of duplication, duplicated segment is present away from the original segment but on the other arm of the chromosome.
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Effects of Duplication
(a). Duplication increases the number of genes in the chromosome.
(b). It causes reduction in the pollen fertility.
(c). Crossing over also suppressed due to duplication of the corresponding segment of the chromosome.
(d). Duplication is responsible for bar eye in Drosophila which produces oval shaped eye.
Significance of duplication:- Duplication do not reduce viability and are less harmful than deletions. Duplication leads to addition of some genes, which may have an important role in evolution.
(3). Translocation:- It refers to the one way or reciprocal transfer of the segment between non-homologous chromosomes. It is reported in Datura, Maize, Oenothera, Rye and wheat etc.
Translocation differs from the crossing over because it involves non-homologous chromosomes while crossing over involves non-sister chromatids of homologous chromosomes.
Translocations are three types:-
(a). Simple
Translocation:- In this type of translocation, a segment of a
chromosome is transferred and attached to the terminal end of a non-homologous
chromosome.
(b). Shifts:- In this type of translocation, a intermediate or intercalary segment from one chromosome transfer to the intercalary position of non-homologous chromosome, is known as shift.
(c). Reciprocal Translocation:- In this type of translocation, one break occurs in each chromosome and occurs mutual exchange of segments between non-homologous chromosomes, is known as Reciprocal Translocation. These translocations are very common and have great evolutionary significance.
Sometimes more than two non-homologous chromosomes are involve in translocation, which is known as Multiple Translocation.
Effects of Translocation:-
(a). Translocations lead to duplication and deletion of genes. Because, gametes with duplication or deficiency are inviable. So, it leads to pollen and ovule sterility.
(b). Crossing over is also suppressed in translocated chromosomes.
(c). Translocation change the size of chromosome and position of centromere also.
(d). Translocation also affect phenotype e.g. Progeny of individual heterozygote with a translocation involving chromosome number 21, can arise as Downy Syndrome in Human.
Significance of Translocation:- Translocations alter the chromosome size, chromosome number and karyotype and thus plays an important role in formation of species.
(4). Inversion:- It refers to the structural change in a chromosome in which a segment is oriented in a reverse sequence. Inversion firstly discovered by Sturtevant (1926) in Drosophila. Now, it is reported in Maize and Nicotiana etc.
Based on whether centromere is involved or not involved in inversions, It is classified into two types:-
(a). Paracentric Inversion:- When the centromere isn’t involved in inversion, is known Paracentric Inversion. In this type of inversion breaks occurs only in one arm of chromosome.
(b). Pericentric Inversion:- When the centromere is involved in the Inversion, is known Pericentric Inversion. In type of inversion, breaks occurs both in each of the two arms of a chromosome.
Effects of Inversion
(a). The crossing over in the inversion leads to formation of chromosomes with duplications and deficiency, which causes individual inviable and leads to 50% sterility.
(b). Gene order is changed in the inverted segment of a chromosome.
(c). Pericentric inversions changes the karyotype by shifting the position of centromere.
Significance of Inversion:- Inversion also play an important role in the evolution of new species by changing the karyotype of an individual.
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