3.3  Meiosis

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Essential Idea:

Alleles segregate during meiosis allowing new

combinations to be formed by the fusion of gametes


  • One diploid nucleus divides by meiosis to produce four haploid nuclei
  • Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number
  • DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids
  • The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation
  • Orientation of pairs of homologous chromosomes prior to separation is random
  • The halving of the chromosome number allows a sexual life cycle with fusion of gametes
  • Crossing over and random orientation promotes genetic variation
  • Fusion of gametes from different parents promotes genetic variation


  • Non-disjunction can cause Down syndrome and other chromosomal abnormalities
  • Studies showing age of parents influences chances of non-disjunction
  • Description of methods used to obtain cells for karyotype analysis – e.g. chorionic villus sampling and amniocentesis and the associated risks


  • Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells