Sex linkage & mapping - Chromosome Theory of...

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Chromosome Theory of Inheritance Sutton and Boveri (1902) Mendel’s factors (genes) behave like chromosomes during meiosis Genes are in pairs; so are chromosomes Alleles of genes segregate equally into gametes; so do homologous chromosomes Different genes assort independently; so do different chromosomes Hypothesis: Genes are located on chromosomes Proof of the Chromosome Theory of Inheritance came from the study of sex-linked genes in Drosophila
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Sex linked inheritance Read Chapter 4 pp.105 – 113 Problems Chapter 4, 23-29, 31-40
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The Fly Room Thomas H. Morgan’s laboratory, Columbia University, 1910
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Drosophila melanogaster Short life cycle - 12 days Many progeny Easy to grow in laboratories
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Calvin Bridges, an undergraduate dishwasher, discovered a white-eyed fly (normal flies have red eyes) in one of the fly bottles he was about to wash
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X X
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Chromosome numbers Human 2n = 46 22 + XY Mouse 2n = 40 19 + XY Drosophila 2n = 8 3 + XY zebraish 2n = 50 25 Chromosomes: sex chromosomes autosomes
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X P F1 X F2 X w+ /X w+ X w /Y (males are hemizygous) X w+ /X w X w+ /X w+ X w+ /Y X w /Y red is dominant. all daughters had red, but 1/2 of the sons had white this is what you would expect if eye color gene is on the X chromosome. White
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X P F1 X F2 Reciprocal Cross X w /X w X w+ /Y white X w+ /X w X w /Y white X w+ X w X w Y X w+ X w X w X w X w Y X w+ Y
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Mutant phenotype (white eye) was inherited in the same manner as the X chromosome, with rare exceptions. X P X w X w X w+ Y Expected F1s: Red-eyed females and White-eyed males. However, a white-eyed female or a red-eyed male appeared 1/2000 times.
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Calvin Bridges proposed: aberrant chromosome segregation ( non- disjunction ) caused rare exceptions.
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Fig 6.5 A map of genes on the X chromosome of Drosophila
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Sex determination Sex chromosome: XY male humans XX female mouse etc. ZZ male butterflies ZW females birds ploidy: haploid male honeybees diploid female # X/ # sets of autosomes: Drosophila
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Sex determination Mammalian: SRY gene (sex-determining region Y) encodes testis-determining factor
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Sex determination Drosophila : ratio of X chromosome and autosomes XX AA female X:A = 1 XY AA male X:A = 0.5 XX AAA intersex X:A = 0.67 0.5 < X: A male 0.5 < X: A < 1 intersex X: A > 1 female
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Recognizing X-linked inheritance X-linked recessive traits -reciprocal crosses don’t give same results -more males than females have phenotype -offspring of affected male are not affected X-linked dominant traits -all daughters of affected males are affected -sons of affected males are not affected
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Examples of X-linked genes in humans. Red-green color blindness (recessive). Some males in the room may not be able to see this number.
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If a normal man marries a woman with normal vision whose father was red-green cb, what will the chance of their first child being a colorblind son? How about a colorblind daughter?
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