The chromosomes represent genetic material of an organism and are the most stable organic compound that maintains constancy both in number and structure. However chromosomes undergo unusual changes called as aberrations which can be numerical or structural. In numerical aberrations, increase or decrease in number of chromosomes are seen. Types of numerical aberrations are:
• Euploidy- complete set of chromosomes present in multiples
• Aneuploidy- partial change in chromosomes
When there is an increase in number of chromosomes compared to the chromosomal number of an organ, then the condition is called as hyperaneuploidy. It is represented as 2n+1, 2n+2 etc. Aneuploidy is also classified as Monosomy, Trisomy and Nullisomy.
Monosomy is hypoaneuploidy where one of alleles of the homologous pair is lost. Monosomy is found rarely in diploids and is commonly found in polyploidy. Depending on the chromosome number, that many types of monosomies can develop. When two different chromosomes are lost, it's denoted as 2n-1-1, when 3 different chromosomes of a different homologous pair are lost, it is represented as 2n-1-1-1. It is called as tri Monosomy. Trisomy is a type of hyperaneuploidy where the number of individual chromosomes is more than the number of chromosomes in an organism. Edward syndrome is caused because of a trisomy. Nullisomy is the condition where both the alleles of a gene of the same pair of homologous chromosomes are lost. It is represented as 2n-2. Usually nullisomies hardly survive.
Euploidy exists in three conditions; monoploidy, haploidy and polyploidy. Monoploidy refers to the normal condition where one set of chromosome is present. Haploidy is the presence of half the number of chromosomes in a somatic cell. Haploidy can be induced by X rays, temperature shock, colchisin and delayed pollination. Experimental methods of developing haploidy involve distant hybridization, production of androgenic plants. Haploids usually produce sterile plants. Polyploidy is the condition where the number of chromosomes present in multiple copies. Types of polyploidy include autopolyploidy, allopolyploidy and segmental alloploidy.
Structural chromosomal aberrations can be intra chromosomal or inter chromosomal. Intra chromosomal structural aberrations include deletion, duplication and inversion. Inter chromosomal aberrations include translocations. Deletions can be terminal or inter special and can be caused naturally and also by chemical mutagens and radiation. These can be identified by size of the chain, change in the position of centromere and formation of loops in pachytene stage. Deletion of a portion of a dominant allele may result in expression of a recessive character. This is called as pseudodominance.
Duplication results in structural chromosomal aberrations. Duplications occur in a lower frequency than deletions. Bar eye mutation in Drosophila results in duplication in X chromosome. Inversion is an intra-chromosomal aberration where segment of chromosomes are inverted on reversed by 180 degrees. Inversions can be paracentric, where centromere is not involved or pericentric where the centromere is involved in the inverted segments of chromosome. Translocations involve two non-homologous chromosomes and position of part of the chromosome is changed leading to change in arrangement of chromosomes. Types of arrangements in translocation include alternate, adjacent I and adjacent 2. In simple translocation, a single nick occurs and the terminal position of the chromosome gets translocated on another non-homologous chromosome. In shifted translocations, two nicks are created and interstitial chromosome segment gets translocated onto another non-homologous chromosome. In reciprocal translocation, two nicks occur on both non-homologous chromosomes and separated segments get interchanged. The translocated chromosomes show change in the size of the chromosome and in position of the centromere. During pairing of homologous chromosomes, the translocate part forms a loop. Translocation brings about new linkage groups or new variation can be linked with normal genes. Translocation in human beings can lead to leukemia.
• Euploidy- complete set of chromosomes present in multiples
• Aneuploidy- partial change in chromosomes
When there is an increase in number of chromosomes compared to the chromosomal number of an organ, then the condition is called as hyperaneuploidy. It is represented as 2n+1, 2n+2 etc. Aneuploidy is also classified as Monosomy, Trisomy and Nullisomy.
Monosomy is hypoaneuploidy where one of alleles of the homologous pair is lost. Monosomy is found rarely in diploids and is commonly found in polyploidy. Depending on the chromosome number, that many types of monosomies can develop. When two different chromosomes are lost, it's denoted as 2n-1-1, when 3 different chromosomes of a different homologous pair are lost, it is represented as 2n-1-1-1. It is called as tri Monosomy. Trisomy is a type of hyperaneuploidy where the number of individual chromosomes is more than the number of chromosomes in an organism. Edward syndrome is caused because of a trisomy. Nullisomy is the condition where both the alleles of a gene of the same pair of homologous chromosomes are lost. It is represented as 2n-2. Usually nullisomies hardly survive.
Euploidy exists in three conditions; monoploidy, haploidy and polyploidy. Monoploidy refers to the normal condition where one set of chromosome is present. Haploidy is the presence of half the number of chromosomes in a somatic cell. Haploidy can be induced by X rays, temperature shock, colchisin and delayed pollination. Experimental methods of developing haploidy involve distant hybridization, production of androgenic plants. Haploids usually produce sterile plants. Polyploidy is the condition where the number of chromosomes present in multiple copies. Types of polyploidy include autopolyploidy, allopolyploidy and segmental alloploidy.
Structural chromosomal aberrations can be intra chromosomal or inter chromosomal. Intra chromosomal structural aberrations include deletion, duplication and inversion. Inter chromosomal aberrations include translocations. Deletions can be terminal or inter special and can be caused naturally and also by chemical mutagens and radiation. These can be identified by size of the chain, change in the position of centromere and formation of loops in pachytene stage. Deletion of a portion of a dominant allele may result in expression of a recessive character. This is called as pseudodominance.
Duplication results in structural chromosomal aberrations. Duplications occur in a lower frequency than deletions. Bar eye mutation in Drosophila results in duplication in X chromosome. Inversion is an intra-chromosomal aberration where segment of chromosomes are inverted on reversed by 180 degrees. Inversions can be paracentric, where centromere is not involved or pericentric where the centromere is involved in the inverted segments of chromosome. Translocations involve two non-homologous chromosomes and position of part of the chromosome is changed leading to change in arrangement of chromosomes. Types of arrangements in translocation include alternate, adjacent I and adjacent 2. In simple translocation, a single nick occurs and the terminal position of the chromosome gets translocated on another non-homologous chromosome. In shifted translocations, two nicks are created and interstitial chromosome segment gets translocated onto another non-homologous chromosome. In reciprocal translocation, two nicks occur on both non-homologous chromosomes and separated segments get interchanged. The translocated chromosomes show change in the size of the chromosome and in position of the centromere. During pairing of homologous chromosomes, the translocate part forms a loop. Translocation brings about new linkage groups or new variation can be linked with normal genes. Translocation in human beings can lead to leukemia.
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