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Chromosomal aberrations
1. Chromosomal aberrations can be numerical, involving a change in chromosome number, or structural, involving a change in chromosome structure. 2. Common numerical aberrations include trisomies like Down syndrome (trisomy 21) and monosomies. Structural aberrations include translocations, deletions, inversions, and ring chromosomes. 3. Nondisjunction during cell division can cause numerical aberrations by failing to separate homologous chromosomes or sister chromatids. Structural aberrations arise from chromosome breakage and rearrangement.
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Chromosomal aberrations
- 1. Chromosomal Aberrations Mohsin Raza& Farrukh Ahmad
- 2. Types Numerical - changein no. Structural – change in structure
- 4. Monosomy loss of singlechromosome Monosomy of autosomes is lethal Turner syndrome XO i.e. loss of sex chr. Cause : 1. non disjunction – one gamete receives 2 copies of homologous chr. & other will have no copy 2. loss of chr. As it move towards pole of cell during anaphase .
- 5. Trisomy Gain of homologouschr. Trisomy 21 (down’s ) , trisomy 18 , klinefelter’s syndrome (47 XXY)
- 6. Nondisjunction Failure of bivalentsto separate during meiosis 1 , the gametes get both homologues of one chr pair Sometimes in meiosis 2 when sister chromatids fail to separate - gets 2 copies of one of the homologues It can also occur during early mitotic div. of zygote - there will be presence of 2 or more cell lines
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- 8. Causes Aging effect onprimary oocyte Radiation Delayed fertilization after ovulation Effects – monosomy or trisomy
- 9. Polyploidy Multiples of haploidno. Triploidy or tetraploidy Foetus does not survive Cause -1. retention of polar body 2. Formation of diploid sperm 3. Dispermy – fertilization by 2 sperms
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- 11. Structural Abnormalities Rearrangement becauseof chr. Breakage & subsequent reunion in a different configuration Balanced – chr. Complement is complete Unbalanced – when there is incorrect amount of genetic material
- 12. Structural Abnormalities 1. Translocations– reciprocal or robertsonian 2. Deletions 3. Insertions 4. Inversions – paracentric or pericentric 5. Ring chr. 6. Isochromosomes
- 13. Translocation Transfer of geneticmaterial from one chr to another Reciprocal – 2 chr break & exchange fragments no. remains 46 unique to a particular family chr imbalance during gamete formation
- 14. Robertsonian Translocation Breakage of2 acrocentric chr near centromeres & fusion of long arms Short arms lost – no importance they contain genes for rRNA Chr no. 45 , but no loss of genetic material hence balanced
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- 16. Deletions ❑ Loss ofpart of chr. • Deletions of larger portions are usually incompatible with life • 10-15% are due to balanced translocations in one parent 85-90% are true deletions
- 17. Deletions Terminal – Cri– du chat syndrome ( 5p-) Interstitial – 2 breaks & middle part is lost eg. prader villi syndrome Microdeletions
- 18. Insertions Rare nonreciprocal typeof translocation A segment of chr gets inserted into another chr
- 19. Inversion 2 break rearrangement Segmentis reversed 3. Pericentric – when centromere is involved 4. Paracentric – only one arm is involved
- 20. Ring Chromosomes • Breakoccurs in each arm & the 2 sticky ends join Distal fragments are lost 1/5 cases of turner’s syndrome
- 21. Philadelphia Chromosome Involves translocationbetween long arm of chr 22 & 9 There is shorter arm of chr 22 called philadelphia chr Found in chronic myelogenous leukemia Indicates better out come
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- 23. Down’s Syndrome First identifiedby Dr Langdon Down in 1866 Trisomy 21 1in 700 live births Associated with increased maternal age
- 24. Clinical Findings Newborn –hypotonia , increased sleepiness ,excess nuchal skin Mental retardation Small stature Craniofacial findings – brachycephaly ( flat occiput ) , epicanthic folds , upward slanting eyes , protruding tongue, low set ears , flat nose , low nasal bridge , high arched palate
- 25. Cont….. ❑Short broad hands ❑Clinodactyly( incurving ) little finger ❑ASD,VSD , PDA ❑Anal duodenal atresia ❑Happy & affectionate
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- 27. Edward’s Syndrome Trisomy 18 Describedby Edward in 1960 Rare to find live born Do not live beyond few months Features – mental retardation , failure to thrive , hypotonia , prominent occiput , low set ears , receding jaw , short sternum , clenched fists, rocker bottom feet.
- 28. Patau’s Syndrome Trisomy 13 Diein a month Growth & mental retardation Sloping forehead , hypertelorism , microphthalmia , cleft lip , cleft palate , polydactyly , polycystic kidneys , bicornuate uterus
- 30. Turner’s Syndrome X monosomy, 45 X described by Turner in 1938 Phenotype is female Short stature Webbing of neck Cubitus valgus Low posterior hair line Broad chest with widely spaced nipples High arched palate
- 31. Coarctation of aorta VSD Horseshoekidney Renal hypoplasia Streak gonads Secondary sexual characters do not develop Turner’s Syndrome
- 32. Klinefelter’s Syndrome Harry klinefelterin 1942 47XXY Male phenotype with sex chromatin positive Patients are tall thin , eunuchoid Poorly developed secondary sexual characters Testis are small , scrotum & penis show hypoplasia Pubic ,chin & axillary hair absent
- 33. Klinefelter’s Syndrome ❑Normal intelligence ❑Testicularbiopsy shows hyalinisation of seminiferous tubules ❑Absent spermatogenesis ❑Low serum testosterone , high FSH & LH
- 34. 5p- (cri-du-chat) Syndrome ❑deletion(the length of which may vary) on the short arm of chromosome 5 ❑small at birth, may have respiratory problems , the larynx doesn't develop correctly, which causes the signature catlike cry. ❑microcephaly), an unusually round face, a small chin, widely set eyes, folds of skin over their eyes, and a small bridge of the nose.
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