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  • agc
    Member
    • May 2010
    • 26

    #1

    Gapped Alignment

    Several short-read aligners claim that they can perform gapped alignment - I'm new to short-read aligners, and I don't understand what 'gapped alignment' means. How is it different from standard alignment?

    Thanks.
  • Pepe
    Member
    • Mar 2009
    • 30

    #2
    If the organism that you are sequencing has a deletion in comparison with the reference, the gapped aligner will do something like:

    Code:
    read: ATGAT-ATGATGA
    ref:  ATGATGATGATGA
    Aligners that do not allow for gaps will not be able to map this read.

    Comment

    • agc
      Member
      • May 2010
      • 26

      #3
      What if it is more than one base that is missing, IE:

      Code:
      read: ATGAT----ATGA
      ref:  ATGATGATGATGA
      Will that be aligned as well? What is the maximum of missing bases allowed for it to still be aligned?

      Thanks.

      Comment

      • nilshomer
        Nils Homer
        • Nov 2008
        • 1283

        #4
        Originally posted by agc View Post
        What if it is more than one base that is missing, IE:

        Code:
        read: ATGAT----ATGA
        ref:  ATGATGATGATGA
        Will that be aligned as well? What is the maximum of missing bases allowed for it to still be aligned?

        Thanks.
        It will, as long as there is not some other more likely alignment. I would review the Smith-Waterman and Needleman-Wunch sequence comparison algorithms. These algorithms compute actual probability of alignments (well log-odds-ratios) so the following two alignments do not have the same odds:

        Code:
        read: ATGAG-A
        ref:  ATGAGGA
        
        read: ATGAGA
        ref:  ATGAGG
        You can see how if you prefer deletions (from the read) over base edits, the former can be preferred (have a better odds ratio). Anyhow, a good intro to bioinformatics book (or course) will clear this up for you.

        Comment

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