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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