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Lecture 13

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University of Alberta
Biology (Biological Sciences)
Lesley Harrington

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); you may want to study human migration patterns/ movement patterns, observing the mountain pine beetle and its migrations; allele frequency may change with migration  allele frequency can change with a variety of factors; if the population is static, the allele frequency will change  if not migration, it might be natural selection/ changes in the environment, mating preferences iii. Distribution of O negative blood is most prominent in South America and NativeAmerica; theA-allele is very uncommon in South American and in FN Northern Canadian communities  there are different distributions in the population; the B-allele is originated in asia and is most common inAsia, but is fairly rare in indigenous populations of North and SouthAmerica; as you move from France  China, increase in the frequency of the B-allele; well accepted by the Mongol invasions, brought the B-allele when they invaded Europe. iv. Monitor populations (conservation biology, public health, agriculture & foresty); selective pressure and selection will affect frequency; v. When observing polar bear populations, we want to conserve vi. 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; we need to be able to accurately count the occurrence of an allele in a population; co-dominant alleles are useful because you can distinguish the heterozygotes from the homozygotes. ii. Blood type studies are important because its codominant! 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. If both alleles are equally abundant in the population, they will both be 0.5; 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; 75% percent of the alleles are bigA q=2(125) + 1 (750) / 2(2000) q=0.25; 25% of the alleles are smallA We double the number of alleles so we multiply our total individuals by 2 (# of alleles = pop # x 2) THIS IS THE FREQUENCY OFTHEALLELES IN THE POPULATION! 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 isAA=640 , Aa= 320, aa=40, so only #1 is in HWE A(0.5) p a (0.5) q 2 A(0.5) p AA(0.25) P Aa (0.25) P2 a (0.5) q Aa (0.25) PQ aa (0.25) Q A(0.5) x a(0.5) x 2  should be theAa (0.25 x 2 = 0.5) We can use HWP to calculate the genotypic frequencies based on our allelic frequencies; 1 = p+q 1= (p+q) 2 1 = p + 2pq + q 2 A(0.9) p a (0.1) q 2 A(0.5) p AA(0.81) P Aa (0.09) PQ 2 a (0.5) q Aa (0.09) PQ aa (0.01) Q 1 = p2 + 2pq + q2… 1 = 0.81 + 2x0.09 + 0.01 If the population follows this model, then it is in the HWE; if we have lessAAand more aa, then you’ll see that the HWP principles aren’t met and this population is migrating or not part of stuff. You can calculate the frequency of the genotypes you will see in a population, this genotype is true forANY FREQUENCY! 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 genotyp
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