Autosomal Linkage by 703L73nU

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									Autosomal Linkage
    A basic assumption…
• So far we have assumed that an individual
  who is AaBb produces gametes of four
  types in equal proportions:



    AB       Ab       aB       ab
        A big ‘what if?’…
• Each chromosome hold numerous genes
• What if the two genes (A and B) are held on
  the same chromosome?
• Now we realise that…
                                B
AaBb could           A              b
be this:                    a
              At meiosis
• Pairs of homologous chromosomes separate
  and one from each pair goes into each
  gamete                               B
          B                 A
                                    Gametes
  A           b
      a                                 b
                                a
            At meiosis
• Pairs of homologous chromosomes separate
  and one from each pair goes into each
  gamete
• And so our assumption breaks down and
  now AaBb produces only the following
  gametes:
               AB       ab
        Crossing over and
          recombinants
• During meiosis, SYNAPSIS occurs and
  segments of DNA are exchanged between
  homologous chromosomes:

    a     a     A      A    Homologous
                               pairs of
                             chromatids

    b     b     B     B
          Crossing over and
            recombinants
 • During meiosis, SYNAPSIS occurs and
   segments of DNA are exchanged between
   homologous chromosomes:

                               Synapsis

Chiasma
      Crossing over and
        recombinants
• During meiosis, SYNAPSIS occurs and
  segments of DNA are exchanged between
  homologous chromosomes:

                            Crossing over
      Crossing over and
        recombinants
• During meiosis, SYNAPSIS occurs and
  segments of DNA are exchanged between
  homologous chromosomes:

                            Recombinant
                            chromosomes
       Crossing over and
         recombinants
• During meiosis, SYNAPSIS occurs and
  segments of DNA are exchanged between
  homologous chromosomes:
   a     A     a      A     Recombinant
                            chromosomes

  b      b     B     B
 These genes are said to be
         LINKED
• Now AaBb which is                 B
                           A            b
                               a


Produces large number of the gametes AB and ab
  and small numbers of the (recombinant) gametes
  Ab and aB
                A question…
• In frogs, the webbed foot condition is dominant over
  the non-webbed condition and the rough condition is
  dominant over the smooth condition. The two genes,
  W and R, are linked.
• A pure breeding webbed, rough frog is mated with a
  non-webbed smooth frog.
• What would you expect in:
   – The F1;
   – The F2?
• What would affect the ratios of phenotypes in the F2?

								
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