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Lecture 13


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Gregory Wagner

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Lec13 BIOL207 2014-02-03 CHAPTER 6 A. Population Genetics a. Population defined: a large group of individuals of the same species who are capable of mating with each other (means live near each other, etc…) b. Main question: what is the frequency of a given allele in a population and how does it change? Can use this information to e.g. i. Calculate disease risks (medicine/public health, agriculture) ii. Follow migration patterns (current, historical anthropology; ecology and conservation) iii. Monitor populations (conservation biology, public health, agriculture & foresty) iv. Follow evolution (selective pressure changes allele frequency) c. Changes in allele frequency (deviations from expected frequencies); these indicate something is happening with the population (e.g. migration, disease, selection) B. Calculating allele frequency a. How do we calculate the observed genotypic frequency? i. Can be easily inferred with co-dominant (or semi-dominant) alleles. Because of this, molecular markers are heavily used in population genetics b. p = frequency of one allele (most common allele; usually dominant) c. q = frequency of the other allele (rarer allele; usually recessive) d. p + q = 1, e.g. 0.5 + 0.5 = 1; or 0.9 + 0.1 = 1, etc. e. calculating p, q i. by observation of genotypes (may be inferred from phenotype) e.g. AA 1125 individuals Aa 750 individuals aa 125 individuals TOTAL 2000 individuals p= 2(AA) + 1(Aa) / 2(total individuals) q= 2(aa) + 1(Aa) / 2(total individuals) p= 2(1125) + 1(750) / 2(2000) p = 0.75 q=2(125) + 1 (750) / 2(2000) q=0.25 f. Note !!! Knowing allele frequencies alone does not tell you anything about the population structure, or fitness advantage/disadvantage of various alleles e.g. each of these populations each has p=0.8, q=0.2 #1 #2 #3 AA 640 800 700 Aa 320 0 200 aa 40 200 100 For p=0.8, q=0.2 and 1000 individuals, HWE is AA =640 , Aa= 320, aa=40, so only #1 is in HWE g. Given p, q the only way to predict genotypic frequencies is to use HW formula, but if predictions do not match observations, then we suspect a change is happening in the population i.e. population is not in HW equilibrium C. Hardy-Weinberg Equilibrium a. HWE: p,q and genotypic frequency are stable from generation to generation b. HW Equilibrium requires certain conditions i. random mating – 1. most mating (e.g. humans) is non-random i.e. assortative ii. no new alleles (no gene flow) 1. no migration 2. no mutation iii. no natural selection 1. neither allele can confer a fitness advantage iv. large population size 1. no genetic drift: loss (or fixation) of one allele due to random sampling (fixation means frequency goes to 1; loss means frequency goes to 0) a.
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