7 chromosomal aberrations

7 chromosomal aberrations

• 1.
  Mujahid Hussain (M.Phil.Botany) Department of Botany University of Sargodha, Sargodha, Punjab, Pakistan Chromosomal Aberrations 10/12/[email protected]
• 2.
  Chromosomal Aberrations “Variation inchromosomal structure or number is called Chromosomal Aberration” They can refer to changes in the number of sets of chromosomes (ploidy), changes in the number of individual chromosomes (somy), or changes in appearance of individual chromosomes through mutation-induced rearrangements. They can be associated with genetic diseases or with species differences. FISH and Banding techniques are used to detect chromosomal aberrations. 10/12/[email protected]
• 3.
  Structural Aberrations The chromosomalaberrations in which alternation of the structure of chromosome(sequence of genes or kind of genes in chromosome or no. of genes)occur—Structural Aberration. Types of Structural Aberrations:  Changes in the numbers of genes  Deletion/Deficiency (Terminal, Intercalary)  Duplication (Intrachromosomal, Interchromosomal)  Changes in the location of genes  Inversions (Paracentric, Pericentric)  Translocations (Intrachromosomal, Interchromosomal) 10/12/[email protected]
• 4.
  Translocation Deletion Insertion Inversion Isochromosome Ring chromosome Derivative chromosome ChromosomalStructure Abnormalities 10/12/[email protected]
• 5.
  Deletion “Loss of a(generally small) segment of chromosome”  Spontaneously or may be induced (radiation, UV, chemicals, viruses).  Detection is based on unpaired region of normal chromosome that produces a loop during pachytene stage.  Firstly observed by Bridges in 1917 in Drosophila. Terminal deletion: “Loss of either terminal segment of a chromosome”  In 1938, Muller postulated that loss of telomere makes chromosome unstable so it is not commonly observed.  E.g. maize. 10/12/[email protected]
• 6.
  Intercalary Deletion: “Loss ofsegment in between centromere and telomere”  It is commonly observed in drosophila. Effects of deletion:  Crossing over not occur.  Harmful effect on diploid organisms.  Morphological effects. A B C D E F G A B D E F G C 10/12/[email protected]
• 7.
  Duplication “Occurrence of asegment twice in the same chromosomes”  Given by Bridges in 1919.  Occurs due to unequal crossing over or crossing over in inversion or translocated heterozygotes.  It is detected by presence of loop in duplicated region during pachytene stage. Intrachromosomal Duplication:  Tandem – in this case sequence of genes in the duplicated segment is similar to the sequence of genes in the original segment of a chromosome. 10/12/[email protected]
• 8.
   Reverse tandem– the sequence of genes in the duplicated segment is reverse to the sequence of genes in the original segment of a chromosome. Interchromosomal Duplication:  Displaced : Duplicating segment is incorporated away from corresponding segment on the same chromosome.  Translocated: Duplicated chromosomal segment is incorporated on different chromosome. A B C D E F G A B C D E F F G 10/12/[email protected]
• 9.
  Effects of duplication: Origin of new genes mainly due to duplication results in evolution.  Activity of certain enzymes may be doubled. Ex: chromosome 6 of barley increased the activity of α- amylase. 10/12/[email protected]
• 10.
  Inversion “A chromosomal segmentis oriented in a reverse position (180o reversal )”  Studied by Sturtevant & Punnet in 1921 in Drosophila.  Detected by presence of inversion loop during pachytene  Absence of crossing over. Paracentric Inversion: “Inverted segment does not include centromere and confined to one arm” Pericentric Inversion: “Included the centromere” 10/12/[email protected]
• 11.
  Effects of Inversion: Partial male sterility.  Formation of Recessive mutation.  Move active genes to sites generally inactive; loose gene function and vice versa . A B C D E F G H I J K A B C H G F E D I J K 180O 10/12/[email protected]
• 12.
  Translocation “In translocation, changein position of the segment occurs in such a way that they become integrated into same or homologous or non homologous chromosome”  Detected by formation of cross shaped configuration at pachytene between two non homologous chromosomes.  Occurs spontaneously or may be induced by mutagens. Types:  Based on involvement of chromosomes: Intrachromosomal Translocation:  Intraradial: Shift occurs in same arm.  Extraradial: Shift occurs in different arm. 10/12/[email protected]
• 13.
  Interchromosomal Translocation:  Fraternal:Shift occurs to homologous chromosome.  External: Shift occurs to non homologous chromosome. Reciprocal Translocation: “Exchange between segments of non homologous chromosomes or regions of same chromosome”. Non reciprocal Translocation: “Movement of a chromosome segment to non homologous chromosome or region of same chr. without reciprocal change”. 10/12/[email protected]
• 14.
   Based onno. of breaks involved: Simple Translocation: “It involves one break. Terminal segment of chromosome integrated at the one end of non homologous chromosome” Shift Translocation: “It requires three breaks. Intercalary segment of a chromosome is integrated within a non homologous chromosomes” Ring Chromosome: “Break occurs in each arm & the 2 sticky ends join while distal fragments are lost” Robertsonian Translocation: “Breakage of 2 acrocentric chr. near centromeres & fusion of 10/12/[email protected]
• 15.
  long arms. Shortarms are lost” Effects of Translocation:  Alters the chromosome morphology.  Damage to DNA may result in formation of recessive lethals.  Lead to impaired fertility. Uses of Structural Aberration:  Study of chromosome pairing and its behaviour during cell division  For locating genes on particular chromosome  Used in plant breeding by increasing the dosage of certain desirable genes for increasing the activity  Important role in evolution  Desirable characters can be detected using inversion 10/12/[email protected]
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• 17.
  Numerical Aberrations “Change inthe number of chromosomes is called as numerical aberration or numerical abnormality”. Numerical Aberration has two types:  Aneuploidy (Hyperploidy, Hypoploidy)  Euploidy (Monoploidy, Diploidy, Polyploidy) 10/12/[email protected]
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• 19.
  Aneuploidy “Change in numberof individual chromosomes, but not in complete set” e.g., 2n ± 1 Mainly arises due to non disjunction. – tolerated in plants – usually lethal in animals – most well known examples in human genetic diseases. Types of Aneuploidy:  Hperploidy “having chromosomes more than disomic condition (2n)”  Trisomy(2n+1) : “Addition of one chromosome to one pair in diploid set” 10/12/[email protected]
• 20.
  It has twotypes :  Simple trisomics – increase in chromosome number in one pair only (2n+1)  Double trisomics – addition of one chromosomes in two different pairs (2n+1+1)  Tetrasomy(2n+2): “Addition of two chromosomes to one pair or two different pairs”  Simple tetrasomics – addition of two chromosomes to one pair(2n+2)  Double tetrasomics – two chromosomes are added each to two different pairs(2n+2+2) 10/12/[email protected]
• 21.
   Hypoploidy: “Having chromosomesless than disomic condition(2n)” – Mostly occurs in polyploids e.g. wheat, tobacco etc.  Monosomy (2n-1): “lacking one chromosome from a diploid set” – Used to map genes in plants.  Double Monosomics – lack of one chromosomes each from two different pairs (2n-1-1).  Nullisomy (2n-2): “lacking one pair of chromosomes from a diploid set” 10/12/[email protected]
• 22.
  Type No. ofchromosomes Example Normal diploid 2n AABBCC Monosomic 2n-1 AABBC Nullisomic 2n-2 AABB Polysomic Extra chromosomes a) Trisomic 2n+1 AABBCCC b) Double trisomic 2n+1+1 AABBBCCC c) Tetra somic 2n+2 AABBCCCC d) Pentasomic 2n+3 AABBCCCCC 10/12/[email protected]
• 23.
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• 24.
  Euploidy “A condition inwhich one or more full sets of chromosomes are present in an organism” Types:  Monoploidy(n): “Single basic set of chromosomes” e.g. in wheat: 2n₌6x₌42 where x₌7, neurospora etc.  Polyploidy: “More than two multiples of haploid chromosomes sets” Triploidy (3n) and Tetraploidy (4n).  Autopolyploidy, Allopolyploidy 10/12/[email protected]
• 25.
   Autopolyploids: “Polyploids whichoriginates by multiplication of the chromosomes of a single species” a. Autotriploids(3n) Formation of autopolyploids occurs when: • Diploid gamete is fertilized by a haploid sperm • An ovum is fertilized by two sperms • Fertilization occurs between diploids(2n) and tetraploids(4n) – e.g. seedless banana, apple, sugarbeet, watermelon etc. b. Autotetraploids(4n) A diploid cell is treated with heat, cold or colchicine to tranform it into autotetraploid. – e.g. rye, grapes, alfalfa etc. 10/12/[email protected]
• 26.
   Allopolyploids: “Polyploids whichoriginates by combining complete chromosomal sets from two or more species”  Mostly Autotetraploids  Formation occurs when two different spp. are combined. Resulting individual is sterile but is treated with colchicine to form a tetraploid–a new species. a. Natural Alloploids – wheat, cotton, tobacco, mustard, oats, brassica etc. As wheat (Triticum aestivum: 2n ₌42) is actually formed from three different species: Triticum aegilopoides (2n₌14), Aegilops speltoides (2n₌14), Aegilops squarrosa (2n₌14). 10/12/[email protected]
• 27.
   Artificial Alloploids– Domesticated coffee, cotton, raphanobrassica etc. As raphanobrassica (2n ₌36) is formed from Raphanus sativus (2n ₌18) and Brassica oleraceae (2n₌18). Impotance of Polyploidy:  plays an imp. role in evolution as Alloploidy gives rise to new species.  Autopolyploids are more adaptive to changing environment as compared to their respective diploids. 10/12/[email protected]
• 28.
   Plants commonlyexhibit polyploidy  30-35% of ferns and flowering plants are polyploids  Many of the fruits, grains and ornamental plants are polyploids  Polyploid strains often display desirable agricultural characteristics  wheat  cotton  strawberries  bananas  large blossom flowers 10/12/[email protected]