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Population Genetics

Population Genetics. 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

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Population Genetics

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  1. Population Genetics

  2. 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

  3. Genotypic frequenciesfrequency a particular genotype appears (combination of alleles)for moths at rightout of 497 moths collectedBB appears 452 timesBb appears 43 timesbb appears 2 timesFrequenciesBB 452 ÷ 492 = 0.909Bb 43 ÷ 492 = 0.087bb 2 ÷ 492 = 0.004Total 1.000 BB Bb Bb bb

  4. 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

  5. Allelic frequencyAllelic frequency = Number of copies of a given allele divided by sum of counts of all allelesBB appears 452 timesBb appears 43 timesbb appears 2 times492 moths 994 allelesFrequenciesB (904 + 43) ÷ 994 = 0.953b (43 + 4) ÷ 994 = 0.047Total 1.000 BB Bb Bb bb

  6. Can also calculate it from the genotypic frequencies • BB was 0.909 • Bb was 0.087 • bb was 0.004 • Therefore frequency of B = Frequency of BB + ½ frequency of Bb • f(B) = .909 + ½ 0.087 = .909 + .0435 = .9525 • F(b) = 0.004 + ½ 0.087 = 0.004 + 0.0435 = 0.047 • What about multiple alleles?

  7. Genotype Number • A1A1 4 • A1A2 41 • A2A2 84 • A1A3 25 • A2A3 88 • A3A3 32 • Total 274 • f(A1) = Total number of A1 in population divided by total number of alleles

  8. Genotype Number • A1A1 4 • A1A2 41 • A2A2 84 • A1A3 25 • A2A3 88 • A3A3 32 • Total 274 • f(A1) = Total number of A1 in population divided by total number of alleles

  9. Genotype Number Number of A1 • A1A1 4 2 X 4 • A1A2 41 41 • A2A2 84 • A1A3 25 25 • A2A3 88 • A3A3 32 • Total 274 • f(A1) = ((2 X 4) + 41 + 25) ÷ (2 X 274) • = (8 +41 + 25) ÷ 548 • = 74 ÷ 548 • = 0.135

  10. Allelic frequencies at X linked locussame principle • However remember for humans that males only have one X • So that • F(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.

  11. 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

  12. 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 2pq • Frequency of an allele being present is = 1

  13. p2 + 2pq + q2 = 1 • Where p = frequency of “dominant” allele • and q = frequency of “recessive” allele • For the moth example • (0.9525)2 + (2 X (0.953 X 0.047)) + (0.047)2 • 0.907 + (2 x 0.045) + .002 • .907 + .09 + .002 = .999 • Is this good enough?

  14. 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 det4ermine frequencies of one allele if the presence of one allele is known

  15. 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

  16. Population variation • How is it quantitated? • Proportion of polymorphic loci • Heterozygosity

  17. Population variation in space and time for allelesBlue musselCline –systematic variation in allele frequency across geography

  18. Temporal variation

  19. Population variation • Variation at many loci • How is it detected? • PCR • Sequencing • Protein electrophoresis • VNTRs • SNTRs • Synonymous vs. non-synonymous variations or chnages

  20. 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

  21. Mutations may be lost or fixed within a population

  22. Selection and speciation • Selection coefficient • Heterozygote superiority

  23. Selection against recessive lethal

  24. Fitness

  25. Problems • Text Study Guide • 22.1 - 22.5 pg 502 – 505 • 1-15

  26. Terms • Mendelian population • Gene pool • Genotypic frequencies • Hardy-Weinberg law • Genetic drift • Random mating • Cline • Random genetic drift

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