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Transcript of 1 Population Genetics Definitions of Important Terms Population: group of individuals of one...
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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 poolEx. % A or % a
Phenotype frequency: % of one type of individual in the population. Ex. % A- or % aa
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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 populationEx. 10% B, 90% b 0% B, 100% b alleles
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Population at Equilibrium
Ideal Population: random mating, no changes in allele frequencies,phenotype frequencies are predictable
Allele frequenciesA = 20%a = 80%
Genotype frequencies
4% AA 32% Aa 36% wild-type
64% aa 64% mutant
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Hardy-Weinberg Equilibrium
When individuals mate at random and allele frequencies are unchanged, genotypic and phenotypic ratios rapidly approachan 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
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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
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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) = 0.22 (100)
Frequency of a = 2(64) + (32) = 0.82 (100)
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Determining Allele Frequencies - Autosomal Recessive
Using frequency of homozygous mutants (q2 ) to determinefrequency of the mutant allele (q)
Phenotype # Observed Genotypes apterous 50 ap ap 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
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Hardy-Weinberg Equilibrium - Autosomal Recessive
Once allele frequencies known, determine genotype frequencies
If p = 0.592 and q = 0.408
ap+ ap+ ap+ ap ap ap p2 + 2pq + q2 =
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(0.592) 2 2 (0.592)(0.408) (0.408) 2
0.350 0.483 0.167
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Determining Allele Frequencies - X-linked
Incompletely dominant trait - bar eyes
Hemizygous males: allele frequency = phenotype frequency
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Determining Allele Frequencies - X-linked
Females: allele frequency = phenotype frequency
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Determining if a Population is at Equilibrium
Observed frequencies: Human MN blood types
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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.
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Determining if a Population is at Equilibrium
Chi square analysis requires numbers (no percentages) Must convert to expected numbers (300 total)
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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
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Assumptions of Hardy-Weinberg
1) Mutation rate must be constant
A a not A a
Rare mutations, little effect
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Spontaneous Mutation Frequencies
Excerpt from Table 24.6
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Assumptions of Hardy-Weinberg
2) 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
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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
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Genetic Drift
Computer-generated examples of genetic drift
Figure 24.12
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Assumptions of Hardy-Weinberg
4) 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
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Calculating Selection (S)
q2 q2 (1-S) after one generation
Initial frequency aa = 0.64 After selection = 0.2
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Types of Selection
Stabilizing Selection - Heterozygote superiority
Ex. HbS HbS HbA HbS HbA HbA
From: www.sparknotes.com/biology/evolution/naturalselection/section1.html
Allele frequencies approach 0.5
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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
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Example of Directional Selection
Peppered Moths - Industrial melanism
Figure 21.19
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Types of Selection
Disruptive Selection - against heterozygotes
From: www.sparknotes.com/biology/evolution/naturalselection/section1.html
Can lead to distinct populations
Isolation -Speciation
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Assumptions of Hardy-Weinberg
5) Mating must be random
Chance of two genotypes mating must depend only onnumber 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
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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
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Multiple Alleles in Populations - Polymorphic Loci
Calculations become much more complex