• Exchange of one base for another results in same amino acid
    • often an alteration in 3rd position of codon
      • tRNA wobble
  • No change in protein function
  • Exchange of one base for another results in changed amino acid 
    • transversion → exchanges a purine to a pyrimidine or a pyrimidine to a purine
      • e.g., C → A or G → T
    • transition → exchanges a purine for another purine or a pyrimidine to another pyrimidine
      • e.g., A → G or C → T
  • Variable change in protein function
    • if the new amino acid is similar to old (leu → ile) the protein will most likely function the same
    • if the new amino acid is different (glu → val) the protein folding/stability will likely be affected
  • e.g., sickle-cell anemia
    • glu → val mutation in β-globin gene
  • Exchange of one base for another results in a stop codon
  • Loss of function mutation as peptide is truncated
  • Deletion or addition of 1 or 2 bases resulting in misreading of all nucleotides downstream
  • Loss of function mutation as peptide is completely different
Large Segment Deletion
  • Unequal crossover at meiosis results in loss of large segment of DNA
  • Loss of function mutation
  • e.g., α-thalassemia 
    • deletion of α-globin gene 
Change at splice site
  • Alteration in base sequence at mRNA splicing site results in altered splicing
    • can remove parts of exon
    • can leave parts of intron
  • Variable effect on protein function as number of spliced amino acids varies
  • e.g., β-thalassemia
Triplet repeat expansion
  • Expansion of short nucleotide sequence results in longer polypeptide
    • can be in coding or noncoding region
  • Addition of amino acids affects protein structure/folding and affects function
  • Disease display anticipation
    • earlier disease onset in successive generations
  • e.g., myotonic dystrophy, Huntington's disease, and Fragile X
  • In-frame mutations 

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