Population Genetics

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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 DriftComputer-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 MatingSimilar phenotypes attractIncreases frequency of homozygotes

Negative Assortive MatingOpposites attractIncreases frequency of heterozygotes

InbreedingIncreases frequency of homozygotesIncreases expression of rare recessives

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Multiple Alleles in Populations - Polymorphic Loci

Calculations become much more complex