CHROMOSOMAL ABERRATIONS APPLIED ASPECTS-GENETICS

This presentation will give details about chromosomes and chromosomal aberration and mosaichism .

1. Topic: Structural and numerical variations in chromosomes and their implication DR.M.VASANTHHAKOHILA POST GRADUATE DEPARTMENT OF ANATOMY-GKMC DATE 13.06.24

2. TABLE OF CONTENT Structure of chromosomes 1. Structural Changes (Variations in Chromosome structure): Deletions Duplications Translocations Inversions 2. Numerical Changes (Variations in Chromosome Number): a. Aneuploidy: Hypoploidy Hyperploidy b. Euploidy: Monoploidy Polyploidy Autopolyploid and allopolyploid Implication of chromosomal aberrations in plant breeding and crop improvement

3. What are chromosomes? Chromosomes are the structures that hold genes. These are made up of a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic  chromosomes  include packaging proteins called histones bind to and condense the DNA molecule to maintain its integrity. Many chromosomes have two segments, called arms, separated by a pinched region known as the centromere. The shorter arm is called the p arm . The longer arm is called the q arm .

4. 1. Structural Change (Variations in Chromosome structure) Any change which alter the basic chromosome structure is known as structural change. Chromosomes are the vehicle of hereditary material or genes. Any alteration, addition or deletion of chromosomal part leads to alteration of number, position or sequence of genes in the chromosome . These changes are categorized into four classes- Deletions Duplications Translocations Inversions

5. Deletions A portion of the chromosome is missing or deleted. The chromosome becomes shorter due to loss of one or more genes . Duplications A portion of the chromosome is duplicated, resulting in extra genetic material.

6. Translocations A portion of one chromosome is transferred to another chromosome. There are three main types of translocation. Simple translocation- In simple translocation, segment from one chromosome will break and attached to another chromosome. Reciprocal translocation- In a reciprocal translocation, segments from two different chromosomes have been exchanged.

7. Robertsonian translocation- In a Robertsonian translocation, an entire chromosome arm has attached to another at the centromere. Loss of chromosomal segment.

8. Inversions A portion of the chromosome has broken off, turned upside down, and reattached. As a result, the genetic material is inverted. An inversion is produced when there are two breaks in a chromosome and the intercalary segment reunites in reverse order i.e., the segment rotate by 180°. Inversion is of two types- Pericentric inversion - If the inverted segment includes the centromere, the inversion is called pericentric inversion; Paracentric inversion- If it does not include centromere the inversion is called as paracentric inversion.

9. 2. Numerical Changes (Variations in Chromosome Number) The organisms are usually diploid (2n), i.e., they possess two sets of chromosomes . A deviation from the diploid state represents a numerical chromosomal aberration which often referred as heteroploidy. Individual possessing the variant chromosome numbers are known as heteroploids Heteroploidy can be mainly of two types (1) Aneuploidy (2) Euploidy

10. (1) Aneuploidy It involves addition or deletion of one or few chromosomes to the usual diploid set of chromosomes. Aneuploid changes in chromosome number do not involve the whole genome.

11. Hypoploidy ( i ) Monosomy They arise by the loss of one chromosome from the diploid set i.e., 2n-l.

12. (ii) Nullisomy These arise by the loss of a particular pair of chromosomes i.e., 2n-2.

13. Hyperploidy ( i ) Trisomy These arise by addition of an extra chromosome to the normal diploid set with the genetic formula, 2n + 1.

14. (ii) Tetrasomy These arise by the addition of an extra pair of chromosome to the diploid set with a chromosomal formula 2n + 2. By this a particular chromosome is represented in four doses instead of normal two.

15. (2) Euploidy Normally organism possesses two sets of chromosomes i.e., they are diploid (2n). At times there is addition or loss of complete one set (n) or more than one set of chromosomes is observed. It is called as euploidy . Euploidy is of following types: Monoploidy : Presence of a single copy of a single genome is known as monoploidy, denoted by x. Polyploidy : Organisms having more than two normal sets of chromosomes (2n) are called polyploids. Organisms with three sets of chromosomes (2n + n) = 3n, are triploids. T hose with four sets of chromosomes (2n + 2n) = 4n, are tetraploids and those with five sets (2n + 3n) = 5n, are pentaploids and so on.

16. There are two major kinds of polyploids according to the origin of chromosomes- Autopolyploid and allopolyploid Autopolyploids are those polyploids which have same basic set of chromosomes multiplied. For instance, if a diploid species has two similar sets of chromosomes (AA), an autotriploid will have three similar sets (AAA) and an autotetraploid will have four such sets (AAAA ). Allopolyploids are those polyploids which contains two or more distinct (different) genomes. If we double the chromosome number in a F 1  hybrid (AB) which is derived from two distinctly different species, the resulting polyploidy will be allotetraploid (AABB). common wheat is an allohexaploid having genome configuration AABBDD (three distinct species).

17. Implication of chromosomal aberrations in plant breeding and crop improvement Aneuploids have been used to determine the phenotypic effects of loss or gain of different chromosomes . Aneuploids are also used to produce alien addition and alien substitution lines which are useful in gene transfer from one species to another . Aneuploid analysis permits the identification of location of a gene onto a specific chromosome Aneuploids are useful in the identification of chromosomes involved in translocation. Chromosome doubling of haploids produces homozygous disomic plants (2n) (double haploids) in just two years as compared to 6-7 generations required for their production through selfing .

18. Triploid watermelons 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 diploids . Autotetraploid varieties of some forage crops like Clove variety Tora, and Berseem variety Pusa Giant (PAPAYA) have been released for commercial cultivation . Many ornamentals are autotetraploids , they generally have larger flower and longer flower duration than do doploids .

19. NUMERICAL ABERRATIONS Trisomy syndromes - autosomes Patau’s syndrome (13) Edward’s syndrome (18) Down’s syndrome(21 ) Monosomy syndromes – allosomes Turner’s syndrome (XO)

20. Patau’s syndrome Karyotype shows trisomy 13 Brain malformation Microphthalmia Harelip or cleft palate Polydactyly Congenital heart disease

21. Edwards syndrome Karyotype shows trisomy 18 Elongated head Broad & flat nose Low-set ears Micrognathia Contraction of fingers Rocker bottom foot Child dies with in few weeks

22. Down’s syndrome Karyotype shows trisomy 21;translocation D/G; translocation 21/22; 21/21 Incidence is 1 in 700 births Round face with epicanthic fold(mongolism) Mental retardation Hypotonia Single palmar crease(simian crease) Long protruding tongue Flat oxiput

23. Down’s syndrome contd.... Excess nuchal skin Short stature Short stocky hands and feet Congenital heart disease Music lovers Affected adults develop Alzheimer’s disease

24. Down’s syndrome contd.... Seen in babies born to elderly primi Due to non disjunction in gametogenesis Mother carrying translocation between two 21 chromosomes will have repeated translocated down babies

25. Turner’s syndrome

26. Turner’s syndrome Karyotype 45, X0(classical); XX/X(mosaic); 46XX,qi with iso -chromosome of long arm of 1X Barr body absent Incidence is 1 in 5000 live born female infants Cause is non-disjunction in gametogenesis In 75% cases X chromosome is maternal in origin Two active X chromosomes required for development of ovaries and their function

27. Turner’s syndrome

28. Turner’s syndrome contd... Female in phenotype Short in stature Webbing of neck Bilateral cubitus valgus “Shield” chest Low set ears Streaked gonads Primary amenorrhoea Lymphoedema , co- arctation of aorta

29. PRADER WILLI SYNDROME A genetic disorder that causes obesity, intellectual disability and shortness in height. Prader-Willi syndrome is a genetic disorder usually caused by deletion of a part of chromosome 15 passed down by the father. The most common symptoms of Prader-Willi syndrome are behavioral problems, intellectual disability and short stature. Hormonal symptoms include delayed puberty and constant hunger leading to obesity. There is no cure for Prader-Willi syndrome but many patients will benefit from a supervised diet. Some symptoms can be treated with hormone therapy.

30. Genetic counseling is defined as a process that gives information about the risk of developing or transmitting a genetic condition to the next generation and management or treatment options of the genetic ailment . GENETIC COUNSELLING DEFINITION

31. Some of the genetic conditions (sometimes referred to as ‘hereditary disorders’) people talk to a genetic counsellor about are : Cystic fibrosis Fragile X syndrome Haemochromatosis Haemophilia Huntington’s disease Muscular dystrophy Neural tube defects