chromosomal aberration

structural and numerical aberration, types, related diseases

1. Sub: Principles of cytogenetics (GP-502) Topic : chromosome, structural aberration and their types ,numerical aberration and their types, related diseases. To, Dr.A.R.Aher Asstt.Professor from, R.C.S.M college of agriculture Sunil.L kolhapur K-19/061

2. What are chromosomes? Tiny thread-like structure found in the nucleus of most living cells, carrying genetic information in the form of genes Made up of protein ( histone )and DNA

3. “variation in chromosomal structure or number is called as Chromosomal Aberration” Classification of chromosomal aberration- Structural chromosomal Aberration- Change in the structure of chromosome Numerical Chromosomal Aberration –Change in the number of chromosome Chromosomal Aberrations

4. Factors responsible for chromosomal aberration Due to error in cell division(non disjunction of chromosome and chromatids ) Maternal age environment

5. Structural Aberration Change in the structure of chromosome All structural aberrations are produced by chromosomal breakage . Chromosome breakage is caused by 1.High frequency radiation – X-ray ,gamma-rays alpha and Beta rays ,neutrons, 2.Chemical agents -Ethyl methane sulphonate ,base analogue ,insecticide ,herbicide and fungicide 3.Viruses –Measles virus

6. Deletion/deficiency “ 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. 1.Terminal deletion: “Loss of 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.

7. 2.Intercalary Deletion: “Loss of segment 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.

8. Deletion occurs in chromosome 5 (humans)

9. Duplication “ Occurrence of a segment 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. Types of 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. Reverse tandem – the sequence of genes in the duplicated segment is reverse to the sequence of genes in the original segment of a chromosome. Displaced : Duplicating segment is incorporated away from corresponding segment on the same chromosome. Translocated : Duplicated chromosomal segment is incorporated on different chromosome. 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.

11. Inversion “ A chromosomal segment is oriented in a reverse position ( 180 ֯ reversal )” Studied by Sturtevant & Punnet in 1921 in Drosophila. Detected by presence of inversion loop during pachytene Absence of crossing over. Types of inversion : Paracentric Inversion : “Inverted segment does not include centromere and confined to one arm” Pericentric Inversion : “Included the centromere ” Effects of Inversion: Partial male sterility. Formation of Recessive mutation.

12. Translocation “In translocation, change in position of the segment occurs in such a way that they become integrated into same or homologous or non homologous chromosome” Occurs spontaneously or may be induced by mutagens.

13. Types: 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” Reciprocal translocation : it is produced when two non homologous chromosome exchange their segments reciprocally.

14. 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

15. Numerical aberration “Change in the number of chromosomes is called as numerical aberration or numerical abnormality’’ Introduction : – In somatic cells of diploid organism ,two copies of the same genome are present ( i.e ,2n=2x) , while their gametes contain a single genome that (n=x) A deviation from the diploid (2n=2x) state represents a numerical chromosomal aberration , this is often referred as heteroploidy . Individual possessing variant chromosome number are known as heteroploids

16. Aneuploidy “Change in number of 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.

17. Types : Hyperploidy “having chromosomes more than disomic condition (2n)” 1 . Trisomy (2n+1 ) : “Addition of one chromosome to one pair in diploid set” It has two types : 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) 2 . 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)

18. B. Hypoploidy : “Having chromosomes less than disomic condition(2n)” – Mostly occurs in polyploids e.g. wheat, tobacco etc. 1 . 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). 2 . Nullisomy ( 2n-2 ): “lacking one pair of chromosomes from a diploid set”

19. Application in crop improvement Aneuploidy are useful in crop improvement in various ways. Some of the use of aneuploidy in plant breeding are briefly presented below: 1. Locating Genes : Aneuploidy are useful tools for locating genes on a specific chromosome Monosomics and nullisomics are used for this purpose. Monosomic analysis has been used in wheat, cotton, tobacco,oat and other crops for locating genes on specific Chromosome. 2 . Interspecific Gene transfer: Monosomics are also used in transferring chromosomes with desirable genes from one species to another. 3.Aneuploids are used for developing alien addition and alien substitution lines in various crops 4. Primary trisomis useful in identification of chromosomes involved in translocations

20. Down syndrome Patau’s syndrome Edward syndrome

21. Euploidy “A condition in which one or more full sets of chromosomes are present in an organism” Types: Monoploid (x): “Single basic set of chromosomes” e.g. in wheat: 2n=6x=42 where x=7 Haploid(n): gametic chromosome number of an individual e.g. in rice : 2n=24 where n=12

22. Polyploidy Presence of more than two genome in an individual is known polyploidy Types : Autopolyploids : Polyploids which originates by multiplication of the chromosomes of a single species Allopolyploids : Polyploids which originates by combining complete chromosomal sets from two or more species

23. Genetic effects of polyploids Polyploidy has marked effects on the morphology of plants. The distinct features of polyploids are increases in general Vigour and Size of various plant parts Such features are referred as Gigantism . Autoploidy have the following important features 1. Stems are thicker and stouter 2. Leaves are fleshy, thick, larger and darker green in colour . 3, Roots are stronger and longer 4. Flowers. pollens and seeds are larger than diploids 5.Maturity duration is longer and growth rate is slower than diploids. 6.Water content is higher than diploids, etc.

24. Autopolyploid 1 . Autotriploids (3n ) Formation of autotriploids 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. 2 . Autotetraploids (4n ) A diploid cell is treated with heat, cold or colchicine to tranform it into autotetraploid . – e.g. rye, grapes, alfalfa etc.

25. Application in crop improvement Autopolyploidy has found a limited application in crop improvement . Triploid watermelon are produced by crossing tetraploid (4x , female) and diploid (2x,Male) lines. These watermelons produce only rudimentary seeds . which are not objectionable when chewed. Triploid Sugarbeet produce larger roots and more sugar per unit area than diploid , Some (3x) sugarbeet varieties are grown in Europe Autotriploid clone of tea , TV 29 , has been released for commercial cultivation India TV29 gives higher leaf yield than diploid clones. Autotetraploid commercial crop varieties of rye e. g., Double Steel and Tetra Petkus

26. Allopolyploids “ Polyploids which originates by combining complete chromosomal sets from two or more species” Mostly are 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. Natural Allopolyploids – wheat, cotton, tobacco, mustard, oats, brassica etc. Artificial Allopolyploids – triticle , raphanobrassica etc.

27. Application in crop improvement Triticale is the only synthesized allopolyploid that has succeeded as a crop species . Varieties of Triticale have been released for commercial cultivation in several countries e.g Australia, Canada ,Italy ,Portugal etc. In India , TL 419 has been released and several promising lines are being evaluated.

28. Thank you..