Population Genetics

Yuwono, M.D., M.Sc., Ph.D.Departement of Microbiology & Molecular Medicine

Fakultas Kedokteran Universitas SriwijayaE-mail: yuwono71@yahoo.com

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Mendelain Populations and The Gene Pool

• Inheritance and maintenance of alleles and genes within a population of randomly breeding individuals

• Study of how often or frequent genes and/or alleles appear in the population

• Genotypic frequencies – how often do certain allelic combinations appear

• Allelic frequencies - how often does an individual allele appear

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Genotypic frequencies: frequency a particular genotype appears (combination of alleles)for months at rightout of 497 months collected

BB appears 452 timesBb appears 43 timesbb appears 2 times

FrequenciesBB 452 ÷ 497 = 0.909Bb 43 ÷ 497 = 0.087bb 2 ÷ 497 = 0.004Total 1.000

BB

Bb

Bb

bb

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• What about alleles that do show simple dominant - recessive relationship?

• How does genotypic frequency really demonstrate flux or change in frequencies of the dominant allele?

• What if there are multiple alleles?• Allelic frequencies

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Allelic frequency

Allelic frequency = Number of copies of a given allele divided by sum of counts of all alleles

BB appears 452 timesBb appears 43 timesbb appears 2 times

497 months 994 alleles

FrequenciesB (904 + 43) ÷ 994 = 0.953b (43 + 4) ÷ 994 = 0.047Total 1.000

BB

Bb

Bb

bb

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• Can also calculate it from the genotypic frequenciesBB was 0.909Bb was 0.087bb was 0.004• Therefore frequency of: B = Frequency of BB + ½ frequency of Bb f(B) = 0.909 + ½ 0.087 = 0.909 +0.0435 = 0.953 f(b) = 0.004 + ½ 0.087 = 0.004 + 0.0435 = 0.047

What about multiple alleles?6

Genotype NumberA1A1 4A1A2 41A2A2 84A1A3 25A2A3 88A3A3 32Total 274

f(A1) = Total number of A1 in population divided by total

number of alleles7

Genotype Number Number of A1

A1A1 4 2 X 4A1A2 41 41A2A2 84A1A3 25 25A2A3 88A3A3 32Total 274

f(A1) = ((2 X 4) + 41 + 25) ÷ (2 X 274) = (8 +41 + 25) ÷ 548

= 74 ÷ 548= 0.135

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• Allelic frequencies at X linked locus same principle• However remember for humans that males only have one X

So thatF (one allele = 2 X the homzygous genotype) + the number of

heterozygotes + the males with the phenotype all divided by the number of alleles in the population (2 X females) plus males.

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Hardy – Weinberg “Law”

• Frequencies of alleles and genotypes within a population will remain in a particular balance or equilibrium that is described by the equation

• Consider a monohybrid cross, Aa X Aa• Frequency of A in population will be defined as p• Frequency of a in population will be defined as q

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Gametes A (p) a (q)A (p) AA(pp) Aa(pq)

a (q) Aa(pq) aa(qq)

Frequency of AA offspring is then p2

Frequency of aa offspring is then q2

Frequency of Aa offspring then 2pqFrequency of an allele being present is = 1

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p2 + 2pq + q2 = 1

Where p = frequency of “dominant” allele and q = frequency of “recessive” allele

For the moth example (0.953)2 + (2 X (0.953 X 0.047)) + (0.047)2

= 0.908 + (2 x 0.045) + 0.002= 0.908 + 0.09 + 0.002 = 1 Is this good enough?

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Can be extended to more than two alleles

Two alleles(p + q)2 = 1

Three alleles(p + q + r)2 = 1

And X – linked alleles Can be used to determine frequencies of one

allele if the presence of one allele is known13

Conditions or Assumptions for The Hardy – Weinberg Law to be True

• Infinitely large population (?)• Randomly mating population (with respect to trait)• No mutation (with respect to locus or trait)• No migration (with respect to locus or trait)• No natural selection (with respect to locus or trait)• Frequencies of alleles do not change over time

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Population variation

How is it quantitated?

• Proportion of polymorphic loci

• Heterozygosity

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Population Variation in Space and Time for Alleles Blue MusselCline–Systematic Variation in Allele Frequency Across Geography

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Temporal variation

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Population variationVariation at many loci

How is it detected?PCRSequencingProtein electrophoresisVNTRsSNTRs

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How is Population Variation of Loci Obtained

• Random events• Mutation

Gain and loss of genes from the gene pool:• Founder effect• Bottleneck effect• Random genetic drift• Selection• Migration

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Mutations may be Lost or Fixed within a Population

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Selection against recessive lethal

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Fitness

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Terms

Mendelian populationGene poolGenotypic frequenciesHardy-Weinberg lawGenetic driftRandom matingClineRandom genetic drift

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Terimakasih

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