Gene Linkage

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Gene Linkage
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II. Linkage and Crossing Over

• Linkage happens when genes dont assort
  independently.
• Genes on the same chromosome are linked.
• Genes linked on the same chromosome segregate
  together.
• Crossing Over involves reciprocal exchange of
  chromosome segments between homologous increases
  genetic variation (recombination).

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Crossing Over

• Linkage without crossing over creates only
  parental (noncrossover) gametes.
• Linkage with crossing over creates parental
  gametes and recombinant (crossover) gametes.
• Interlocus distance is proportional to the degree
  of crossing over between.
• Little or no crossing over in close genes.
• Frequent, even multiple crossovers between
  distant genes.
• Chromosome map, determined from recombination
  rates, indicates relative locations of genes on
  a chromosome.

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No Linkage Independent Assortment
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Linkage without Recombination
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Linkage with Recombination
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chiasma
Chiasmata Between SynapsedHomologs During
Meiosis Prophase I
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If two genes are on different chromosomes
Half look like they got a set of the parents
chromosomes
And half look like they got a mix of both parents
chromosomes
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Fig. 4.6
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Crosses of Two X-linked genes
Expect only parental types if no crossing over
occurs - Confirm this for yourself with a
Punnett square.
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