Linkage,%20Sex%20linkage,%20Pedigrees

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Linkage, Sex linkage, Pedigrees
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Linked genes

• We know two traits should assort independently.
  Why?
• So, offspring phenotype ratios should be
• 9331
• But theyre not
• Why?

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Linked genes

• The genes reside on the SAME chromosome
• What produces recombinants (purple, round red,
  long)?
• Crossing over!

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Linkage

• If genes reside on the SAME chromosome, then how
  are recombinants generated?
• What happens in Meiosis I?
• Crossing over recombines (shuffles) alleles

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Sex Linkage Recombination

• T. H. Morgan and his students discovered both
  phenomena in fruit flies
• Eye color sex linkage
• Body color, wing type, other eye color
  autosomal linkage

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Recombination

• G gray body (dom)
• g black body (rec)
• L long wings (dom)
• l short wings (rec)
• GgLl x ggll ?
• GgLl ALL Gray long
• Few Ggll ggLl

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Recombination

• G gray body (dom)
• g black body (rec)
• L long wings (dom)
• l short wings (rec)
• All on same chromosome
• GgLl x ggll ?
• Few Ggll ggLl
• Recombination frequency 17

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Recombination freq Genetic map

• Recombination frequency 17
• If possibility of crossing over is at all
  points along chromosome, then the farther apart
  two genes are, the greater the chance of
  recombination between them

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Sex linkage

• Morgan students made true breeding red-eyed
  female and true-breeding white-eyed males

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Sex linkage

• Made true breeding red-eyed female and
  true-breeding white-eyed males
• All offspring red-eyed
• Now lets cross those F1 offspring

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Sex linkage

• Cross those F1 offspring
• ONLY males have white eyes, and approximately ½
  of all males are affected
• Why are females never affected?

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Sex linkage

• Red female with white male
• We know male can only contribute a white allele
• If mother is heterozygous?

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Sex linkage

• Red female with white male
• We know male can only contribute a white allele
• If mother is heterozygous?
• 11 white-eyed red-eyed, irrespective of sex

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Common sex-linked disorders

• Red-green color blindness
• Lack either red sensitive or green-sensitive cone
  cells in retina
• Hemophilia
• Lack one or more blood-clotting proteins
• Duchenne muscular dystrophy
• Progressive muscle wasting

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Conclusions

• Appearance of few recombinants signals some
  degree of linkage
• For any X-linked gene, males with a recessive
  allele will show the recessive phenotype.
• When looking at pedigrees of disease phenotypes,
  an excess of affected males suggests that disease
  gene lies on X-chromosome