97_09_LB_PopGen2

97_09_LB_PopGen2 - Bio 97 Population Genetics II Last Time...

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1 Population Genetics II ©1999-2005 Lee Bardwell Bio 97 Last Time 1. The Hardy-Weinberg Equation 2. Hardy-Weinberg Equilibrium 3. How to use HW 4. Forces that can cause deviation from HW ratios ©2009 Lee Bardwell Population Genetics Allele freqs Genotype freqs Gene pool Hardy Weinberg Law: ©2001 Lee Bardwell A a AA Aa aa (p+q) 2 = p 2 +2pq +q 2 = 1 HW equilibrium: allele freqs remain constant from generation to generation, under certain conditions Try using both allele counting methods to determine the allele freqs in a pop of 10 people , consisting of 1 homozyg w.t , 6 hets and 3 homozyg recessive At home exercise - Method 1: just count (there are 20 alleles total) Method 2: p = freq(AA) + 1/2 freq(Aa), q = freq(aa) + 1/2 freq(Aa),
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2 Try using both allele counting methods to determine the allele freqs in a pop of 100 people , consisting of 10 homozyg w.t (AA) , 60 hets (Aa) and 30 homozyg recessive (aa) Let’s assume this pop is 10X larger so we don’t have to worry about drift* Is this pop at HW equil? *in reality would have to be much larger Forces that can cause deviation from HW ratios 1. Gene flow (e.g., migration) 2. Genetic drift 3. Mutation 4. Natural Selection 5. Non-random mating (e.g., inbreeding) ©2005 Lee Bardwell You would think that NS would eliminate “bad” (lower fitness) alleles from the gene pool. But then why do we still have disease-causing alleles in our gene pool? Natural Selection ©2009 Lee Bardwell Forces that keep “bad” alleles in gene pools Delayed age of onset (e.g. Huntington’s) Slow selection against recessive alleles in diploids Mutation-selection equilibrium Heterozygote superiority ©2000 Lee Bardwell Sometimes, bad alleles become good Natural Selection
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3 Forces that keep “bad” alleles in gene pools Delayed age of onset (e.g. Huntington’s) Slow selection against recessive alleles in diploids Mutation-selection equilibrium Heterozygote superiority ©2009 Lee Bardwell Natural Selection A very weak selective advantage can still lead to a dominant allele becoming fixed (= essentially the only allele in
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