8.1 origins of variation

• factors which increase genetic variation

  • mutation
  • independant assortment and crossing over in meiosis
  • random fertilisation in sexual reproduction
• mutation

  change in DNA base sequence
• why do mutations not always lead to different amino acid sequences?

  • genetic code is degenerate
  • mutation may code for same amino acid
• insertion mutations


  • bases inserted into DNA base sequence
  • causing a frame shift
  • different amino acid sequence forms
• deletion mutation


  • bases deleted in DNA base sequence
  • causing frame shift
  • different amino acid sequence produced
• independant assortment

  • different combinations form of parental chromosomes
  • occurs in prophase 1
• crossing over

  • occurs in prophase 1
  • sections of homologous chromosomes are swapped
  • non sister chromatids entangle and form chiasma
  • DNA strand break and fragments join
  • form new combination of allele
• random fertilisation

  • gamete is haploid so contains half number of chromosomes
  • meiosis allows for variation
  • every gamete contains different DNA