Revision summary
Structural abnormalities rearrange chromosome parts after breakage or unequal crossing-over. Types are deletion, duplication, inversion, translocation, ring, and isochromosome. Cri-du-chat is 5p deletion; 22q11 is a common microdeletion. Philadelphia t(9;22) is a somatic translocation in CML. Robertsonian 14;21 can cause translocation Down syndrome. Balanced parental carriers explain some recurrent unbalanced offspring.
Model answer
Introduction
Structural chromosomal abnormalities are changes in the shape and arrangement of chromosomes, not in the simple count of 46. They arise when breakage and repair go wrong in meiosis or mitosis, and the clinical picture depends on how much DNA is lost, gained, or moved.
Body
Causes at the chromosome
- A break in the DNA-protein chromosome, from errors of recombination, radiation, some chemicals, or fragile sites, is the starting event.
- Repair may join the wrong ends, producing deletions, duplications, inversions, translocations, rings, or isochromosomes.
- Unequal crossing-over between repeated sequences is a common meiotic cause of deletion and duplication.
- Robertsonian fusion joins two acrocentric long arms, which is a structural change that also alters count.
- Parental balanced rearrangements can become unbalanced gametes, which is why family karyotypes matter in counselling.
Main types with examples
- Deletion: loss of a segment. Cri-du-chat is a 5p deletion with a characteristic cry and developmental delay. 22q11 deletion (DiGeorge / velocardiofacial) is a common microdeletion with heart and immune findings.
- Duplication: extra segment, sometimes the partner of a deletion from unequal crossing-over. Charcot–Marie–Tooth type 1A is a teaching duplication of a myelin gene region on chromosome 17.
- Inversion: a reversed segment. Pericentric inversions can be balanced in a parent and still produce unbalanced recombinant children.
- Translocation: exchange between chromosomes. The Philadelphia chromosome t(9;22) fuses BCR–ABL in chronic myeloid leukaemia, a somatic structural change. Reciprocal translocations in the germline can be balanced in a parent and unbalanced in a child.
- Robertsonian translocation between 14 and 21 is a cause of translocation Down syndrome, distinct from free trisomy 21, which is numerical.
- Ring chromosome forms when two ends of one chromosome fuse after terminal breaks.
- Isochromosome is a mirror chromosome with two identical arms, seen in some Turner variants involving X.
Anthropological and clinical note
- These are cytogenetic facts used in medical anthropology of disability, stigma, and prenatal testing, not racial traits.
- Mosaicism can blunt or patch the phenotype when the rearrangement is post-zygotic.
Flow diagram
flowchart TD B[Chromosome break] --> DEL[Deletion] B --> DUP[Duplication] B --> INV[Inversion] B --> TRA[Translocation] TRA --> P[Philadelphia CML] DEL --> C[Cri-du-chat 5p]
Conclusion
Structural abnormalities come from breakage, unequal recombination, and mis-repair, yielding deletions, duplications, inversions, translocations, rings, and isochromosomes. Cri-du-chat, 22q11, Philadelphia t(9;22), and 14;21 Robertsonian Down syndrome are the standard examples.
Quick related
Students also ask
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"Chromosomal aberrations can play havoc with the human body and mind." Explain with suitable examples
Next question on this syllabus topic (2021 · Q4(c)). View answer →
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Is Down syndrome always a structural abnormality?
No. Most Down syndrome is free trisomy 21, a numerical error. A minority is a Robertsonian translocation, which is structural.
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Can a parent look healthy and still cause these?
Yes. A balanced translocation or inversion carrier may be healthy and still produce unbalanced gametes.
PYQ trend
When UPSC asked this
Related PYQs from other years, newest first. Open a question to read it.
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2024 · Q3(a) · Anthropology GS 1 · 20 marks
What is meant by karyotype? How does its analysis help in diagnosis of the chromosomal aberrations in man? -
2021 · Q4(c) · Anthropology GS 1 · 15 marks
"Chromosomal aberrations can play havoc with the human body and mind." Explain with suitable examples -
2016 · Q7(b) · Anthropology GS 1 · 15 marks
Explain the significance of screening and counseling for genetic disorders. -
2015 · Q6(c) · Anthropology GS 1 · 15 marks
Discuss chromosomal aberrations in man illustrating with examples
More from this topic
Q3(a) · UPSC Mains 2024 · Anthropology GS 1 · 20 marks
What is meant by karyotype? How does its analysis help in diagnosis of the chromosomal aberrations in man?
Numerical and structural aberrations (disorders).
Karyotype means chromosome complement; karyogram is its ordered image. Metaphase arrest, hypotonic spreading and G-banding permit homologous pairing and notation. It diagnoses trisomy 21, Turner, Klinefelter and other numerical aberrations. It also finds large deletions, inversions and balanced or unbalanced translocations. Multiple cells must be examined for mosaicism. A normal karyotype does not exclude microdeletions or single-gene disease; use FISH, microarray or sequencing appropriately.
Q4(c) · UPSC Mains 2021 · Anthropology GS 1 · 15 marks
"Chromosomal aberrations can play havoc with the human body and mind." Explain with suitable examples
Numerical and structural aberrations (disorders).
Aberrations are numerical or structural changes of chromosomes. Down syndrome is trisomy 21 with body and cognitive effects. Turner is 45,X; Klinefelter is 47,XXY; both alter gonads and sometimes learning. Deletions and unbalanced translocations can be as severe as extra chromosomes. Outcomes vary; counselling and care matter as much as the karyotype name.
Q7(b) · UPSC Mains 2016 · Anthropology GS 1 · 15 marks
Explain the significance of screening and counseling for genetic disorders.
Numerical and structural aberrations (disorders).
Screening searches a defined group for genetic risk. Newborn, cascade, and prenatal programmes are the main types. Counselling explains risk without directing the choice. Significance is prevention, early treatment, and informed reproduction. Stigma and eugenics are the failures to avoid.
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