2011 Lect 14 Gene linkage - Genetic Geneticlinkage 1 Mendel...

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Genetic linkage 1
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Mendel’s 2nd Law Mendel s 2nd Law Principle of independent assortment : The genetic units for different characters move into gametes independently This is for genes on non homologous chromosomes only What happens when genes are on the same chromosome? 2
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How do we know genes are on h chromosomes? X linkage and the case of the white eyed fruitfly 3
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Fruitflies and genetics Th H M ’ l b k d f itfli Thomas H. Morgan’s lab worked on fruitflies 4
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Are genes on chromosomes? One day Thomas Morgan found a male with a new mutation: white eyes instead of red Hypothesis: The gene for this trait is located on the X chromosome And white is recessive 5
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Other possibilities: 1. Gene is located on the Y chromosome : XY W 2. The trait is suppressed in females (eg hormone dependent) h h t i ll l i l rr shows up phenotypically only in males 6
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Approach: Cross 1: white eyed male with red Cross 1: white eyed male with red eyed female Parentals : Females: X R X R males: X r Y Prediction? 7
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Approach: Cross 1: white eyed male with red Cross 1: white eyed male with red eyed female This phenotype would also fit a suppression hypothesis 8
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Cross 2: F1 X F1 Parentals : X R X r females X R Y males Prediction for F2: 9
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Cross 2: F1 x F1 X R Y X R X r 10
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Cross 2: F1 x F1 ½ female and red eyed X R Y X R X R X R X R Y ¼ male and red eyed ¼ male and X r X R X r X r Y white eyed 11
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Conclusion? F2 data fit the X linkage But to invalidate the F2 data fit the X linkage hypothesis well suppression hypothesis Especially the phenotypic ratios: You need to see white eyed females: 3:1 red to white X r X r But linked to sex X X 12
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A cross to find female white eyed flies Cross 3: Backcross F1 females with X r Y white eyed males: X R X r 13
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A cross to find female white eyed flies Cross 3: B k F1 f l i h Backcross F1 females with white eyed males: X r Y Offspring: ¼ female and red X R X R X r X R Y ¼ female and white ¼ male and red ¼ male and white X r X r X r
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