Principles of Inheritance and Variation

Sex determination - in humans

  • Females possess 2 isomorphic sex chromosomes, XX.
  • Males contain 2 heteromorphic sex chromosomes, XY.
  • The ‘Y’ chromosome is shorter and is a heterochromatin (tightly packed and darkly stained).
  • The Y – chromosome carries only holandric genes eg., SRY that produces TDF. (Testis Determining factor) which acts on Sox 9 to initiate testis differentiation and development of sertoli cells.
  • Drosophila also possess sex chromosomes XX and XY.
  • Other type of sex chromosomes with its location in organism is given below
  • XX(Female) - XO(male) type in round worms, bugs, grass hoppers, cockroaches
  • ZO(Female) - ZZ(male) type in Butterflies and moths.
  • ZW(Female) - ZZ(male) type in Birds and in some reptiles.

Part-1: Watch this video for the topic from 0:47 to 10:36

Part-2: Watch this video for the topic from 0:12 to 6:20

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