Class 12 Biology Inheritance Variation CHROMOSOMAL DISORDERS


  • It is caused due to absence or excess or abnormal arrangement of one or more chromosomes.
  • Aneuploidy – failure of segregation of chromatids during cell division cycle results in the gain or loss of chromosome.
  • Polyploidy – Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism.
  • Down’s syndrome – gain of extra copy of chromosome 21 (trisomy 21)
  • It was first described by Langdon Down (1866).
  • Affected individual is short with small round head, furrowed tongue and partially open mouth.
  • Broad palm with characteristic palm crease.
  • Physical, psychomotor and mental development is retarded.


Pic shows baby suffering from Down’s syndrome.

  • Turner’s syndrome – loss of an X-chromosome in human females i.e. 45 with XO
  • Such females are sterile as ovaries are rudimentary
  • Klinefelter’s syndrome – presence of an additional copy of X-chromosome resulting into karyotype, 47, XXY.
  • Sterile individuals

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