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Home/Anthropology/Mains PYQ/2020/Paper I/Human Genetics & Human Biology
UPSC MainsAnthropology Optional Paper I 2020 15 Marks Model Answer Available

Numerical Aberrations in Sex Chromosomes — 2020 Paper I

Question · 2020 · Paper I · 15 Marks

How may numerical aberrations in sex chromosomes lead to genetic disorders ?

Model Answer

VAID ICS

Approach

  • Demand of Question: Explain how abnormal numbers of X or Y chromosomes arise and show how they produce characteristic genetic disorders.
  • Structuring the Response: Explain nondisjunction and dosage imbalance, then discuss major sex-chromosome aneuploidies with their karyotypes and manifestations.

Model Answer

Introduction

Numerical aberrations of sex chromosomes result when an individual possesses an abnormal number of X or Y chromosomes. They usually arise through meiotic or mitotic nondisjunction, producing aneuploid conditions such as monosomy or trisomy.

Mechanism

Normally:

Female = 46,XX
Male = 46,XY

During meiosis, failure of homologous chromosomes or sister chromatids to separate can produce gametes containing an abnormal number of sex chromosomes.

Nondisjunction → Abnormal gamete → Fertilization → Sex-chromosome aneuploidy

  1. Turner syndrome — 45,X

Turner syndrome results from monosomy of the X chromosome.

Major features include:

  • phenotypic female;
  • short stature;
  • gonadal dysgenesis or streak ovaries;
  • delayed or absent secondary sexual development;
  • infertility;
  • webbed neck in some cases;
  • possible cardiovascular abnormalities.

The absence of a second sex chromosome alters normal gene dosage, particularly involving genes that escape X-inactivation.

  1. Klinefelter syndrome — 47,XXY

Affected individuals are generally phenotypic males with an additional X chromosome.

Common features include:

  • tall stature;
  • small testes;
  • reduced testosterone;
  • sparse facial/body hair;
  • gynecomastia in some individuals;
  • impaired spermatogenesis and infertility;
  • variable learning difficulties.
  1. Triple-X syndrome — 47,XXX

Individuals are phenotypically female and may show:

  • above-average stature;
  • mild developmental or learning difficulties in some cases;
  • usually normal sexual development;
  • fertility often retained.

Many affected individuals remain undiagnosed because manifestations may be mild.

  1. XYY syndrome — 47,XYY

Affected males possess an additional Y chromosome.

Features may include:

  • tall stature;
  • generally normal sexual development;
  • usually normal fertility;
  • variable language or learning difficulties.

Earlier claims linking XYY constitution with inherent criminal aggression are not supported by modern genetic evidence.

Mosaic conditions

If nondisjunction occurs during mitosis after fertilization, two or more chromosomal cell lines may develop, such as:

45,X / 46,XX

The clinical phenotype may consequently be milder or more variable.

Why sex-chromosome aneuploidies are viable

Sex-chromosome abnormalities are generally more compatible with survival than most autosomal aneuploidies because of:

  1. X-inactivation, which compensates for much of the additional X-chromosome material;
  2. relatively small gene content of the Y chromosome.

However, genes escaping X-inactivation create dosage imbalances and contribute to clinical abnormalities.

Summary

Disorder Karyotype Phenotypic Sex Major Effect
Turner syndrome 45,X Female Short stature, gonadal dysgenesis
Klinefelter syndrome 47,XXY Male Hypogonadism, infertility
Triple-X 47,XXX Female Usually mild manifestations
XYY syndrome 47,XYY Male Tall stature, variable learning issues

Conclusion

Numerical sex-chromosome aberrations arise mainly through nondisjunction, producing altered gene dosage and developmental disturbances. Their effects range from relatively mild phenotypes to significant abnormalities of growth, sexual development and fertility.

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? 2026 Why the classifications of Indian populations, based on somatometric and somatoscopic characteristics failed to develop a common understanding of the population diversity? 2026 Canalization and catch-up growth. 2026 Genetic polymorphism and natural selection.

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