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Population Genetics

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  1. Population Genetics Definitions of Important Terms Population: group of individuals of one species, living in a prescribed geographical area Subpopulation: localized, distinct breeding group Gene pool: collection of all gene forms (alleles) in a population Allele frequency: % of one allele in the gene pool Ex. % A or % a Phenotype frequency: % of one type of individual in the population. Ex. % A- or % aa 1

  2. Population Genetics Definitions of Important Terms Evolution: change in allele frequencies within a population Selection: phenotype with advantage increases Natural Selection - Charles Darwin Eugenics - forced selection in humans Fixation (extinction): loss of an allele from the population Ex. 10% B, 90% b 0% B, 100% b alleles 2

  3. Population at Equilibrium Ideal Population: random mating, no changes in allele frequencies, phenotype frequencies are predictable Allele frequencies A = 20% a = 80% Genotype frequencies 4% AA 32% Aa 36% wild-type 64% aa 64% mutant 3

  4. Hardy-Weinberg Equilibrium When individuals mate at random and allele frequencies are unchanged, genotypic and phenotypic ratios rapidly approach an equilibrium. At equilibrium, frequencies should follow a binomial distribution. (p + q) 2=p2 + 2pq + q2= 1 Frequencies of Alleles p + q = 1 p = frequency of wild-type allele ex. p = 0.2 q = frequency of mutant allele ex. q = 0.8 4

  5. Hardy-Weinberg Equilibrium p2 + 2pq + q2 = 1 p + q = 1 ex. p = 0.2 q = 0.8 Frequencies of Genotypes p2 = frequency of homozygous wild type ex. (0.2) 2 2pq = frequency of heterozygous ex. 2 (0.2) (0.8) q2 = frequency of homozygous mutant ex. (0.8) 2 5

  6. Determining Allele Frequencies Calculating allele frequencies based on genotypes 4 AA 32 Aa 64 aa 100 Total Allele frequency = 2 (# homozygotes) + 1 ( # heterozygotes) 2 (total # individuals) Frequency of A = 2(4) + (32) = 0.2 2 (100) Frequency of a = 2(64) + (32) = 0.8 2 (100) 6

  7. Determining Allele Frequencies - Autosomal Recessive Using frequency of homozygous mutants (q2 ) to determine frequency of the mutant allele (q) Phenotype # Observed Genotypes apterous 50 apap wild type 250 ap+ap+ or ap+ap 300 Total Know frequency of q2 = 50 / 300 = 0.167 Calculate q = q2 = 0.167 = 0.408 Calculate p, p + q = 1, p = 1- q = 1 - 0.408 = 0.592 7

  8. Hardy-Weinberg Equilibrium - Autosomal Recessive Once allele frequencies known, determine genotype frequencies If p = 0.592 and q = 0.408 ap+ap+ ap+apapap p2 + 2pq + q2 = 1 (0.592) 2 2 (0.592)(0.408) (0.408) 2 0.350 0.483 0.167 8

  9. Determining Allele Frequencies - X-linked Incompletely dominant trait - bar eyes Hemizygous males: allele frequency = phenotype frequency 9

  10. Determining Allele Frequencies - X-linked Females: allele frequency = phenotype frequency 10

  11. Determining if a Population is at Equilibrium Observed frequencies: Human MN blood types 11

  12. Determining if a Population is at Equilibrium Observed frequencies: Human MN blood types Predictions based on calculated allele frequencies Do observations fit expectations? Chi square analysis. 12

  13. Determining if a Population is at Equilibrium Chi square analysis requires numbers (no percentages) Must convert to expected numbers (300 total) 13

  14. Determining if a Population is at Equilibrium Degrees of freedom = ( k - r ) k = # genotypes, r = # alleles = 3 - 2 = 1 Probability = < 0.01 Significant difference indicates population is evolving 14

  15. Assumptions of Hardy-Weinberg • Mutation rate must be constant A a not A a Rare mutations, little effect 15

  16. Spontaneous Mutation Frequencies Excerpt from Table 24.6 16

  17. Assumptions of Hardy-Weinberg • Migration can not occur Apterous flies escape more easily - ap ap and ap decline Founder Population Small group migrates to new location - begins new population Amish - distinct allele frequencies and phenotypes Gene Flow - decreases differences between populations 17

  18. Assumptions of Hardy-Weinberg 3) Population must be infinitely large Random changes occur over generations - genetic drift Small population can lose allele by chance - allele extinction Bottlenecks decrease diversity 18

  19. Genetic Drift Computer-generated examples of genetic drift Figure 24.12 19

  20. Assumptions of Hardy-Weinberg • Selection must not occur If q2 is less viable, frequency of q allele declines. q2 q2 (1-S) after one generation S = selection S = 1, lethal S = 0, no selection F = W = fitness = 1 - S 20

  21. Calculating Selection (S) q2 q2 (1-S) after one generation Initial frequency aa = 0.64 After selection = 0.2 21

  22. Types of Selection Stabilizing Selection - Heterozygote superiority Ex. HbS HbS HbA HbS HbA HbA Allele frequencies approach 0.5 From: www.sparknotes.com/biology/evolution/naturalselection/section1.html 22

  23. Types of Selection Directional Selection - against one extreme Ex. aa selected against Frequency of a allele declines From: www.sparknotes.com/biology/evolution/naturalselection/section1.html 23

  24. Example of Directional Selection Peppered Moths - Industrial melanism Figure 21.19 24

  25. Types of Selection Disruptive Selection - against heterozygotes Can lead to distinct populations Isolation - Speciation From: www.sparknotes.com/biology/evolution/naturalselection/section1.html 25

  26. Assumptions of Hardy-Weinberg • Mating must be random Chance of two genotypes mating must depend only on number of individuals of that genotype within population. If mating is not random, no change in allele frequencies, but rapid change in genotype frequencies Ex. More aa x aa - increase in aa More AA x aa - increase in Aa 26

  27. Types of Non-Random Mating Positive Assortive Mating Similar phenotypes attract Increases frequency of homozygotes Negative Assortive Mating Opposites attract Increases frequency of heterozygotes Inbreeding Increases frequency of homozygotes Increases expression of rare recessives 27

  28. Multiple Alleles in Populations - Polymorphic Loci Calculations become much more complex 28