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Dean Adrian G QUIDEZ . Population genetics

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1. 1. Population Genetics By:Quidez Dean Adrian
2. 2. Mendelain populations and the gene pool Inheritance and maintenance of alleles and genes within a population of randomly breeding individualsStudy of how often or frequent genes and/or alleles appear in the populationGenotypic frequencies – how often do certain allelic combinations appearAllelic frequencies - how often does an individual allele appear
3. 3. Genotypic frequencies BBfrequency a particulargenotype appears(combination of alleles)for moths at right Bbout of 497 moths collectedBB appears 452 timesBb appears 43 timesbb appears 2 times BbFrequenciesBB 452 ÷ 492 = 0.909Bb 43 ÷ 492 = 0.087bb 2 ÷ 492 = 0.004Total 1.000 bb
4. 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. 5. Allelic frequency BBAllelic frequency = Numberof copies of a given alleledivided by sum of counts ofall alleles BbBB appears 452 timesBb appears 43 timesbb appears 2 times492 moths Bb994 allelesFrequenciesB (904 + 43) ÷ 994 = 0.953b (43 + 4) ÷ 994 = 0.047 bbTotal 1.000
6. 6. Can also calculate it from the genotypic frequenciesBB was 0.909Bb was 0.087bb was 0.004Therefore frequency of B = Frequency of BB + ½ frequency of Bbf(B) = .909 + ½ 0.087 = .909 + .0435 = .9525F(b) = 0.004 + ½ 0.087 = 0.004 + 0.0435 = 0.047What about multiple alleles?
7. 7. Genotype NumberA1A1 4A1A2 41A2A2 84A1A3 25A2A3 88A3A3 32Total 274f(A1) = Total number of A1 in population divided by total number of alleles
8. 8. Genotype NumberA1A1 4A1A2 41A2A2 84A1A3 25A2A3 88A3A3 32Total 274f(A1) = Total number of A1 in population divided by total number of alleles
9. 9. Genotype Number Number of A1A1A1 4 2X4A1A2 41 41A2A2 84A1A3 25 25A2A3 88A3A3 32Total 274f(A1) = ((2 X 4) + 41 + 25) ÷ (2 X 274) = (8 +41 + 25) ÷ 548 = 74 ÷ 548 = 0.135
10. 10. Allelic frequencies at X linked locus same principleHowever remember for humans that males only have one XSo 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.
11. 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 equationConsider a monohybrid cross, Aa X AaFrequency of A in population will be defined as pFrequency of a in population will be defined as q
12. 12. Gametes A (p) a (q)A (p) AA(pp) Aa(pq)a (q) Aa(pq) aa(qq)Frequency of AA offspring is then p2Frequency of aa offspring is then q2Frequency of Aa offspring then 2pqFrequency of an allele being present is = 1
13. 13. p2 + 2pq + q2 = 1Where p = frequency of “dominant” allele and q = frequency of “recessive” alleleFor the moth example(0.9525)2 + (2 X (0.953 X 0.047)) + (0.047)20.907 + (2 x 0.045) + .002.907 + .09 + .002 = .999Is this good enough?
14. 14. Can be extended to more than two allelesTwo alleles(p + q)2 = 1Three alleles(p + q + r)2 = 1And X – linked allelesCan be used to det4ermine frequencies of one allele if the presence of one allele is known
15. 15. Conditions or assumptions for the Hardy – Weinberg law to be trueInfinitely 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. 16. Population variationHow is it quantitated? Proportion of polymorphic loci Heterozygosity
17. 17. Population variationVariation at many loci How is it detected? PCR Sequencing Protein electrophoresis VNTRs SNTRsSynonymous vs. non-synonymous variations or chnages
18. 18. How is population variation of loci obtainedRandom events MutationGain and loss of genes from the gene pool Founder effect Bottleneck effect Random genetic drift Selection Migration
19. 19. Mutations may be lost or fixed within a population
20. 20. Selection and speciation Selection coefficient Heterozygote superiority
21. 21. Selection against recessive lethal
22. 22. Fitness
23. 23. TermsMendelian populationGene poolGenotypic frequenciesHardy-Weinberg lawGenetic driftRandom matingClineRandom genetic drift