320-10-25&26topost

320-10-25&26topost - Lectures 25& 26 Genetic...

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Unformatted text preview: Lectures 25 & 26 Genetic diversity in populations Text chapter 21; Question bank E . Principles of population genetics- why is there genetic polymorphism in populations?- (how) do allele frequencies change over time? Human prehistory - written in our genes- reading the molecular clock- origins of human genetic diversity- are we still evolving? Genetics of quantitative and complex traits- Is “heritability” real? useful?- Genetic knowledge = genetic determinism? Applying Mendelian genetics to populations… 1. Calculating allele genotype frequencies; why allele frequencies don’t change over time. (Hardy-Weinberg-Castle equilibrium) 1. When & why allele frequencies do change- mutation, selection, genetic drift Population genetics Questions: How much polymorphism in natural populations? Why do lethal /deleterious mutations persist? Insights: Evolution = change in allele frequencies over time “Molecular clock” - tracking prehistory in DNA Text chapter 21; Question bank E . A a A a 0.5 0.5 1. Genotype frequencies from allele frequencies AA aA Aa aa 0.25 0.25 0.25 0.25 A a 0.5 A a 0.5 A a A a A a a a A A 0.25 0.5 0.25 For 2 individuals But for a population…. 1 1 A a A a p q p q Genotype frequencies from allele frequencies A A A A a a a a a a A A A A a a a a a a ”gene pool” frequency a = q = 1 - p AA aA Aa aa p 2 pq pq q 2 AA aA Aa aa 2pq Genotype frequencies in next generation: p 2 q 2 frequency A = p where 0 < p < 1 a a AA AA a a a a A a A a Allele frequencies from genotype frequencies AA Aa aa = freq AA + 1/2 freq Aa 2pq Substituting calculated genotype frequencies: p 2 q 2 frequency A = # AA + 1/2 # Aa # individuals in population frequency a = freq aa + 1/2 freq Aa frequency A = p 2 + 1/2 ( 2pq ) = p 2 + pq Since q = 1-p, = p 2 + p(1-p) = p 2 + p - p 2 = p => Allele frequencies do not change from one generation to the next. (Hardy/Weinberg/Castle equilibrium) Relation between allele and genotype frequencies 1 p Genotype frequency frequency 1 q frequency a A a a AA A a 2pq- Most when p = q = 0.5 A a- Few when q small a a Two island populations- 10,000 40,000 a a AA Combined population a A p = 0.2 q = 0.8 A a after 1 generation AA a a 0.04 0.64 0.32- Genotype frequencies reach HWC equilibrium in 1 generation & then stay constant same after n generations Example question Q. In a population at Hardy-Weinberg equilibrium, the frequency of people affected by a recessive Mendelian condition is 1/1600. What is the proportion of heterozygotes in the population?...
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This note was uploaded on 09/12/2010 for the course AMS 316 taught by Professor Xing during the Spring '09 term at SUNY Stony Brook.

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320-10-25&amp;26topost - Lectures 25& 26 Genetic...

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