population genetics 2015 03-20 (agb 32012)
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POPULATION GENETICS
1. Introduction
2. Terminologies
3. Hardy - Weinberg Law
4. Calculations
5. Factors affecting HWL
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Population is a group of individuals of the same species occupying a givenarea that can freely interbreed and produce fertile offspring in nature
POPULATION
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The gene pool consists of all alleles at all gene loci inall individuals of the population
Gene pool
Allele frequency The percentage of an organism in a population thatcarrying a particular allele
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Total number of allele in populationAllele frequency =
Number of specific type of allele
DOMINANT ALLELE?
Recessive allele?
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Total number of allele in population
Number of specific type of allele
Allele frequencies in a gene pool determine the geneticchange for a population
Composition of a gene pool may change over time due tocertain factors
If no change in allele frequencies from one generationto the next, the gene pool is considered static
Genetic Equilibrium
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G. H. Hardy W. Weinberg
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Frequency of alleles and genotypes in apopulation will remain constant fromgeneration to generation if the population isstable and in genetic equilibrium
Five conditions1. A large breeding population2. Random mating3. No mutation4. No immigration or emigration5. No natural selection
Hardy-Weinberg equilibrium (1908)
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Organisms are diploid
Only sexual reproduction occurs
Generations are non overlapping
Mating is random
Population size is infinitely large
Allele frequencies are equal in the sexes
There is no migration, mutation or selection
Homozygous Dominant
Homozygous Recessive
Heterozygous
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Frequency of dominant allele (B) = p
Frequency of recessive allele (b) = q
B b
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BB Bb bb
(p + q)2 = p2 + 2pq + q2
p2- Frequency of homozygous dominant
q2- Frequency of homozygous recessive
2pq - Frequency of heterozygous
BB
Bb
bb
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A plot of Hardy-Weinberg equilibrium genotype frequencies as a function of allele frequencies
Calculation
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100 cats84 black and 16 white
q2 = 16/100 = 0.16q = √0.16 = 0.4
Frequency of each genotype?
p + q = 1P + 0.4 = 1
p = 1 – 0.4= 0.6
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p2 - Frequency of homozygous dominant = 0.6 × 0.6 = 0.36
q2- Frequency of homozygous recessive = 0.4 × 0.4 = 0.16
2pq - Frequency of heterozygous = 2 × 0.4 × 0.6 =0.48
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If it is three alleles ?p q r
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Factors affecting HWL
Population size
Type of mating
Mutation
Migration
Natural selection
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Original population
Survivors
New population
Bottle Neck Effect
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