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Home/Anthropology/Mains PYQ/2018/Paper I/Human Genetics & Human Biology
UPSC MainsAnthropology Optional Paper I 2018 20 Marks Model Answer Available

Structural Anomalies of Autosomes — 2018 Paper I

Question · 2018 · Paper I · 20 Marks

Describe the mechanisms for structural anomalies of autosomes with diagrams.

Model Answer

VAID ICS

Approach

  • Demand of Question: Explain how structural abnormalities of autosomes arise through chromosome breakage, abnormal recombination and faulty repair.
  • Structuring the Response: Classify anomalies into deletion, duplication, inversion, translocation, ring chromosome and isochromosome, with simple diagrams and human examples.
  • Key Dimensions to Cover: Chromosome breakage, unequal crossing-over, deletion, duplication, inversion, reciprocal/Robertsonian translocation, ring chromosome and clinical consequences.

Model Answer

Introduction: Structural chromosomal anomalies arise when an autosome undergoes breakage followed by loss, duplication or abnormal rearrangement of chromosomal segments. They may be balanced, with no net loss or gain of genetic material, or unbalanced, producing altered gene dosage and phenotypic abnormalities.

Normal chromosome:

A — B — C — D — E — F — G

  1. Deletion

Deletion results from loss of a chromosome segment after one or more breaks.

Normal:
A — B — C — D — E — F

Deletion:
A — B — D — E — F

It may be:

  • terminal — loss from chromosome end;
  • interstitial — loss of an internal segment.

Mechanism:

Chromosomal breakage → Segment lost → Reduced gene dosage

Example: Cri-du-chat syndrome results from deletion of the short arm of chromosome 5, del(5p).

Microdeletions may also arise through non-allelic homologous recombination.

  1. Duplication

Duplication occurs when a chromosomal segment is present more than once.

Normal:
A — B — C — D — E

Duplication:
A — B — C — C — D — E

A major mechanism is unequal crossing-over between misaligned homologous chromosomes during meiosis.

Misalignment → Unequal crossing-over → One deletion + One duplication

Duplication increases gene dosage and can disturb normal development.

  1. Inversion

An inversion occurs when a chromosome segment breaks at two places, rotates 180°, and reinserts.

Normal:
A — B — C — D — E — F

Inversion:
A — B — E — D — C — F

Two types occur:

Paracentric inversion: Centromere is not included.

Pericentric inversion: Centromere lies within the inverted segment.

Inversions are generally balanced, so carriers may appear normal. However, crossing-over inside an inversion loop during meiosis may produce abnormal gametes.

Thus:

Inversion carrier → Abnormal meiotic pairing → Unbalanced gametes

  1. Reciprocal translocation

A reciprocal translocation results when segments from two non-homologous chromosomes are exchanged.

Before:

Chr. 1: A — B — C | D — E
Chr. 2: P — Q — R | S — T

After:

Chr. 1: A — B — C | S — T
Chr. 2: P — Q — R | D — E

Balanced carriers may be phenotypically normal but can produce unbalanced gametes, leading to:

  • infertility;
  • miscarriages;
  • congenital abnormalities.
  1. Robertsonian translocation

Robertsonian translocation involves fusion of the long arms of two acrocentric chromosomes, particularly chromosomes:

13, 14, 15, 21 and 22

Diagrammatically:

Chr. 14 long arm + Chr. 21 long arm → Fused chromosome

The short arms are usually lost.

A balanced carrier may possess 45 chromosomes but remain phenotypically normal.

An important example is familial Down syndrome caused by a Robertsonian translocation involving chromosome 21, commonly t(14;21).

  1. Ring chromosome

Ring chromosomes arise when both terminal portions of a chromosome break and the remaining ends fuse.

Normal:
A — B — C — D — E — F

Terminal breaks → End loss → ○ Ring

Thus:

Two terminal breaks → Loss of distal segments → End-to-end fusion → Ring chromosome

Ring chromosomes may be mitotically unstable and produce variable clinical manifestations.

  1. Isochromosome

An isochromosome forms through abnormal division of the centromere, producing a chromosome with two identical arms.

Normal centromeric division:

p-arm | Centromere | q-arm

Abnormal transverse division may produce:

q-arm | Centromere | q-arm

Consequently:

Loss of one arm + Duplication of the other arm

This creates both monosomy and trisomy for different chromosome-arm regions.

Basic mechanisms producing structural anomalies

Structural autosomal abnormalities arise mainly through:

  1. chromosome breakage and defective repair;
  2. unequal crossing-over;
  3. non-allelic homologous recombination;
  4. abnormal centromeric division;
  5. errors in meiotic recombination.

Thus:

DNA/chromosome breakage → Faulty recombination or repair → Structural rearrangement

Balanced versus unbalanced anomalies

Balanced rearrangement Unbalanced rearrangement
No major net gain/loss of DNA Genetic material gained or lost
Individual may be normal Phenotypic abnormalities common
Reproductive risk may remain Developmental defects likely
Example: balanced translocation Example: deletion/duplication

Conclusion

Structural anomalies of autosomes result from breakage, abnormal recombination and incorrect repair of chromosomes. Deletions and duplications alter gene dosage directly, while inversions and balanced translocations may remain clinically silent in carriers but generate abnormal gametes. Their effects therefore depend upon the amount, position and functional importance of genetic material involved.

2026 Distinguishing features of nuclear and mitochondrial DNA. 2026 How would you find out that a particular disease/disorder is inherited and, if so, in which manner? Elaborate with suitable examples. 2026 What do you understand by fertility and mortality? How are these influenced by biological and socio-ecological factors? 2026 In what manner is our understanding of serogenetics and cytogenetics useful in reproductive biology? What are the major ethical issues involved in reproductive technology?

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