Duplication
When any part of the genetic material __a single locus or a large piece of a chromosome __ is present more than once in a genome it is called Duplication.
If duplication is present only on one chromosome of the two homologue chromosomes, then at meiosis the chromosome bearing the duplicated segment forms a loop to meximise the point to point pairing.
Duplication may be of following types
1.Tandeem duplication
In this case the duplicated region is situated just by the side of the normal corresponding section of the chromosome and the sequences of genes are the same in normal and duplicated region.For example, if the sequence of genes in a chromosome is ABC. DEFGH (The full stop depicts the centromere) and if the chromosomal segment containing the genes DEF is duplicated, the sequence of genes in tandem duplication will be ABC. DEFDEFGH.
2.Reverse tandem duplication
Here, the sequence of genes in the duplicated region of a chromosome is just the reverse of a normal sequence. In the above mentioned example, therefore, the sequence of genes due to reverse tandem duplication will be ABC. DEFFEDGH.
3.Displaced duplication
In this case the duplicated region is not situated adjacent to the normal section. Depending on whether the duplicated portion is on the same side of the centromere as the original section or on the other side, the displaced duplication can be termed either homobranchial or heterobranchial.
Example. Homobranchial duplication = ABC. DEFG DEFH
Heterobranchial duplication = ADEFBC. DEFGH 4.
4:Transposed duplication
Here, the duplicated portion of chromosome becomes attached to a non-homologous chromosome. For example, if ABC.DEFGH and LMNOPQ. RST represent the gene sequences of two nonhomologous chromosomes, a transposed duplication will result into chromosomes with gene sequence ABC.GH and LMN DEF OPQ. RST. Such a transposed duplication may be either interstitial (e.g., LMNDEFOPQ. RST) or terminal (i.e., LMN OPQ. RST DEF ).
5. Extra-chromosomal duplication
In the presence of centromere the duplicated part of a chromosome act as independent chromosome.
GENETICAL EFECTS OF DUPLICATION
Due to duplication, there occur unequal crossing over which results in deletion and reduplication which produce distinct phenotypes as shown by the following examples :
Bar eye in Drosophila
The Bar Eye phenotype in Drosophilla is an example of duplication.Instead of normal oval eye shape, Bar_eyed flies have narrow,siit like eyes.This phenotype apperas to be inherited as sex_linked mutation. The classical studies of Bridges showed that the bar trait of Drosophila is associated with the duplication of a segment of the X-chromosome, called section 16A.Each added section 16A intensifies the bar phenotype (i.e., duplication behaves genetically as a dominant factor). However, the narrowing effect is greater if the duplicated segments are on the same chromosome (called position effect.
When any part of the genetic material __a single locus or a large piece of a chromosome __ is present more than once in a genome it is called Duplication.
If duplication is present only on one chromosome of the two homologue chromosomes, then at meiosis the chromosome bearing the duplicated segment forms a loop to meximise the point to point pairing.
Duplication may be of following types
1.Tandeem duplication
In this case the duplicated region is situated just by the side of the normal corresponding section of the chromosome and the sequences of genes are the same in normal and duplicated region.For example, if the sequence of genes in a chromosome is ABC. DEFGH (The full stop depicts the centromere) and if the chromosomal segment containing the genes DEF is duplicated, the sequence of genes in tandem duplication will be ABC. DEFDEFGH.
2.Reverse tandem duplication
Here, the sequence of genes in the duplicated region of a chromosome is just the reverse of a normal sequence. In the above mentioned example, therefore, the sequence of genes due to reverse tandem duplication will be ABC. DEFFEDGH.
3.Displaced duplication
In this case the duplicated region is not situated adjacent to the normal section. Depending on whether the duplicated portion is on the same side of the centromere as the original section or on the other side, the displaced duplication can be termed either homobranchial or heterobranchial.
Example. Homobranchial duplication = ABC. DEFG DEFH
Heterobranchial duplication = ADEFBC. DEFGH 4.
4:Transposed duplication
Here, the duplicated portion of chromosome becomes attached to a non-homologous chromosome. For example, if ABC.DEFGH and LMNOPQ. RST represent the gene sequences of two nonhomologous chromosomes, a transposed duplication will result into chromosomes with gene sequence ABC.GH and LMN DEF OPQ. RST. Such a transposed duplication may be either interstitial (e.g., LMNDEFOPQ. RST) or terminal (i.e., LMN OPQ. RST DEF ).
5. Extra-chromosomal duplication
In the presence of centromere the duplicated part of a chromosome act as independent chromosome.
GENETICAL EFECTS OF DUPLICATION
Due to duplication, there occur unequal crossing over which results in deletion and reduplication which produce distinct phenotypes as shown by the following examples :
Bar eye in Drosophila
The Bar Eye phenotype in Drosophilla is an example of duplication.Instead of normal oval eye shape, Bar_eyed flies have narrow,siit like eyes.This phenotype apperas to be inherited as sex_linked mutation. The classical studies of Bridges showed that the bar trait of Drosophila is associated with the duplication of a segment of the X-chromosome, called section 16A.Each added section 16A intensifies the bar phenotype (i.e., duplication behaves genetically as a dominant factor). However, the narrowing effect is greater if the duplicated segments are on the same chromosome (called position effect.
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