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Home/Anthropology/Mains PYQ/2023/Paper I/Human Genetics & Human Biology
UPSC MainsAnthropology Optional Paper I 2023 15 Marks Model Answer Available

Structural Abnormalities of Chromosomes — 2023 Paper I

Question · 2023 · Paper I · 15 Marks

Describe the causes of structural abnormalities of chromosomes with suitable examples.

Model Answer

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Introduction Structural chromosomal abnormalities occur when the physical structure of a chromosome is altered, leading to a loss, gain, or rearrangement of genetic material. Unlike numerical abnormalities (aneuploidy), which involve whole extra or missing chromosomes, structural aberrations involve chromosome breakage and faulty repair mechanisms.

Major Causes of Structural Abnormalities The underlying agents that cause chromosomal breaks are known as Clastogens. They can be categorized into four primary causes:

  1. Errors in Meiosis (Endogenous/Spontaneous Causes):
    • Unequal Crossing-Over: Misalignment of homologous chromosomes during prophase I of meiosis can lead to reciprocal deletions and duplications.
    • Faulty DNA Repair: Double-strand breaks occur naturally; if the Non-Homologous End Joining (NHEJ) repair pathway fails, it leads to translocations or inversions.
  2. Physical Agents (Radiations):
    • Exposure to ionizing radiation (X-rays, gamma rays, UV rays) directly breaks the phosphodiester bonds in the DNA backbone.
    • Example: High rates of structural chromosomal translocations and deletions were observed in survivors of the Hiroshima/Nagasaki bombings and the Chernobyl disaster.
  3. Chemical Agents (Mutagens):
    • Certain chemicals induce chromosomal breaks or interfere with spindle fiber formation.
    • Examples: Alkylating agents (mustard gas), benzene, agricultural pesticides, and historical teratogens like Thalidomide.
  4. Biological Agents (Viruses):
    • Certain viruses integrate their DNA into the host chromosome, causing structural disruption and breakage.
    • Example: Human Papillomavirus (HPV) and Epstein-Barr Virus are known to cause structural chromosomal aberrations leading to oncogenesis (cancer).

Resulting Types of Structural Abnormalities (with Examples) These causes lead to specific structural changes, which can be balanced (no loss of genetic info) or unbalanced:

  • Deletion (Unbalanced): Loss of a segment. Example: Cri-du-chat syndrome (deletion of the short arm of chromosome 5; 5p-).
  • Duplication (Unbalanced): A segment is repeated, causing gene dosage imbalance. Example: Charcot-Marie-Tooth disease (duplication on chromosome 17).
  • Translocation (Balanced/Unbalanced): Exchange of segments between non-homologous chromosomes.
    • Example: Philadelphia Chromosome (reciprocal translocation between chromosomes 9 and 22), causing Chronic Myeloid Leukemia.
    • Example: Robertsonian Translocation (fusion of two acrocentric chromosomes), a major cause of familial Down Syndrome.
  • Inversion (Balanced): A segment breaks, flips 180 degrees, and reattaches. (Paracentric or Pericentric).

Conclusion Structural chromosomal abnormalities play a critical role in medical genetics and biological anthropology. Understanding their clastogenic causes—ranging from environmental mutagens to spontaneous meiotic errors—is vital for genetic counseling, managing hereditary diseases, and studying evolutionary cytogenetics.

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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UPSC Anthropology Optional 2023 — Paper I

28 questions

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1(c) Race and Ethnicity

10 Marks150 words
Syllabus: Paper I > 9.5 Race and racism > Race and ethnicity
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5(e) William Ogburn and Cultural lag

10 Marks150 words
Syllabus: Paper I > 6. Anthropological Theories > Cultural lag (Ogburn)
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