Nondisjunction

• What is nondisjunction?


  chromosomal mutation that takes place during meiosis or mitosis -> members of a pair of homologous chromosomes or a pair of sister chromatids fail to separate properly from each other
• When does nondisjunction of homologous chromosomes occur?

  During meiosis I
• When does nondisjunction of sister chromatids occur?

  Meiosis II and anaphase of mitosis
• During meiosis I,

  • during metaphase I, bivalents align at the equator
  • one bivalent fails to separate, nondisjunction occurred
  • results in one pole with an extra chromosome and the other pole with one less chromosome
  • 2 of the daughter cells will have (n+1) chromosomes while the other 2 will have (n-1) chromosomes
• During meiosis II,

  • during metaphase II, chromosomes align at the equator in single row
  • sister chromatids of one chromosome fail to separate, nondisjunction occurred
  • results in one pole with an extra chromosome and the other pole with one less chromosome
  • 2 of the daughter cells will have n number of chromosomes, one of the other 2 will have (n+1) chromosomes and the other will have (n-1) chromosomes
• During anaphase of mitosis,

  • during metaphase, chromosomes align at the equator in single row
  • sister chromatids of one chromosome fails to separate, nondisjunction occurred
  • results in one pole with an extra chromosome and the other pole with one less chromosome
  • one of the daughter cells will have (2n-1) chromosomes and the other will have (2n+1) chromosomes