Genetic maps of Chromosomes

Cross-over frequency estimates separation distance between genes.
The unit of [separation] distance is map unit.

  • 1 map unit (centimorgan, cM) = 1% recombination frequency
  • map of all genes known on a chromosome is
    . . - linkage map
    . . - chromosome map
    . . - genetic map

    For example, on one chromosome of the fruit fly, Drosophila, are
    . the following genes:
  • yellow body color (y) - wildtype (Y)
  • ruby eyes (rb) - wildtype (Rb)
  • short wing cross vein (cv) - wildtype (Cv)

    Drosophila
    recombination
    %
    y rb cv
    y 0.0
    rb 7.5 0.0
    cv 13.3 6.2 0.0
    Drosophila
    genetic map
    cM
    y - rb rb - cv
    7.5 6.2
    13.3
    ..... ..... ..... .....

    Note: 7.5 + 6.2 = 13.7
    _not: 7.5 + 6.2 = 13.3

  • but {7.5, 6.2, 13.3} are estimates not absolute numbers
  • "error" is +4 in (y-rb and rb-cv) or -4 in (y-cv)
    . . - or dividing error is +2 in (y-rb and rb-cv) and -2 in (y-cv)
  • "correction" is
    . . - (-1) in (y-rb), (-1) in (rb-cv), and (+2) in (y-cv):
    . . . - (y-rb) = 7.5 -1 = 7.4
    . . . - (rb-cv) = 6.2 - 1 = 6.1
    . . . - (y-cv) = 13.7 - 2 = 13.5
    which adds up, 7.4 + 6.1 = 13.5

    y - rb rb - cv
    7.4 6.1
    13.5
    ..... ..... ..... .....


    We had made considerable progress in genetic mapping

  • fruit fly (Drosophila)
  • mustard (Arabidopsis)
  • Corn (Zea mays)
  • Human (Homo sapiens) disease traits
  • and others


    Then we developed more 'accurate' estimates of loci (gene locations) -
    DNA sequencing to estimate map distances in nucleotide bases

    Nucleotide base is

  • ribose sugar
  • phosphorylated on 5'-C
  • with nucleotide base [T, C, A, or G] bonded to 1'-C of riboses

    DNA molecule is polymer of nucleotides
  • phosphate
    . . . - 5'-C bonded to
  • ribose, 1'-C bonded to base
    . . . - 3'-C bonded to
  • phosphate
    . . . - 5'-C bonded to
  • ribose, 1'-C bonded to base
    . . . - 3'-C bonded to
  • phosphate
    . . . - 5'-C bonded to
  • ribose, 1'-C bonded to base
    . . . - 3'-C bonded to
  • phosphate
    . . . - 5'-C bonded to
  • ribose, 1'-C bonded to base

    DNA sequence is order of nucleotides from 5'-end to 3'-end
    and "map unit" becomes 1 nucleotide base,
    which is not equal to 1 percent recombination


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    © 2004 Prof. LaFrance, Ancilla College