Linkage and Chromosome Mapping in Eukayotes

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Chapter 7

• Linkage and Chromosome Mapping in Eukayotes

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Unit Factors

• 1903 realized there were more unit factors
  than chromosomes
• Unit factors also segregated together so must
  be on the same chromosome linked and
  demonstrate linkage
• no independent assortment
• should separate as a unit but not always the case
  because of crossing over
• can make maps of location of genes on the
  chromosome

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Independent Assortment

• Independent assortment genes on 2 chromosomes
• 4 genetically different gametes

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Linked without Crossing-Over

• Genes segregate on same chromosome and have
  different meiotic consequences
• Linkage without crossing over
• 2 types of gametes receive genes from one or
  other homolog parental or non-crossover gametes
• usually only theoretical as crossing over is
  common

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Linked with Crossing-Over

• Linkage with crossing over
• 2 new allele combinations recombinant or
  cross-over gametes
• based on how close the genes are, further apart
  see increase in recombinant gametes and decrease
  in parental gametes
• see 1111 ratio in gametes if 100
  crossing-over occurs

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Linkage Ratios

• See unique F2 generation ratio when mate
  heterozygous at the same loci
• Brown eye (bw) and heavy wings (hv) linked,
  recessive mutant genes
• New terminology linked genes are placed above
  or below a single or double horizontal line
• Linkage groups 1 for each chromosome

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Crossing-Over

• Determine distance between genes during mapping
• Complete linkage too close together for
  cross-over to occur
• Distance between loci can be used to determine
  number of recombinant gametes formed

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Morgan Mapped X Chromosome

• Morgan looked at genes on the Drosophila X
  chromosome
• Saw puzzling results in some F2 generations
• most followed what was expected but some were
  unique in their phenotype
• saw in several gene pairs

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2 Questions

• What is the source of gene separation?
• Why did frequency vary depending on genes looked?
• Morgan saw chiasmata formation in synapsed
  chromosomes and thought that may be the point of
  genetic exchange
• crossing over between 2 X chromosomes may lead to
  unique phenotypes recombination gametes
• parental gametes have no crossing-over
• genes in linear order along the chromosome
• closer 2 genes are the less likely to see
  crossing over

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Mapping Genes

• Morgans student realized that the frequency of
  unique phenotypes may predict the location of the
  genes in relation to other genes on the
  chromosomes
• Predicted gene order to be yellow, white and
  miniature with yellow and white close together
  and miniature further away
• more yellow/mini and white/mini than
  yellow/white, white between yellow and mini

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Estimate of Distances

• Help make chromosome map of 3 genes on X
  chromosome
• 1 recombination between 2 genes 1 map unit
• distance between yellow and white is 0.5 mu
• doesnt quite add up perfectly
• Crossing over not restricted to the X chromosome
  but in autosomes as well
• In Drosophila only female has crossing over,
  other organisms have both sexes conducting
  crossing over

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Single Cross Overs

• Limited number of cross-overs in each tetrad
• Too close together, unlikely to occur
• Non-sister chromatid cross over but not between
  loci goes undetected
• If cross over is between genes, can detect the
  cross over because see unique pheotype
• one chromatid for each chromosome is left
  unaffected

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Potential Gametes

• 50 potential gametes are recombined
• Non-cross over chromatid will yield expected
  phenotype
• If get 100 cross over will see equal numbers of
  gametes and it will appear to act as if 2 genes
  on 2 different chromosomes and independent
  assortment