Microsoft Word - Population Genetics 2B F11.doc

Microsoft Word - Population Genetics 2B F11.doc - BIS 2B...

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BIS 2B Fall 2011 STUDY GUIDE _____________________________________________________________________________________ This study guide has two major parts. PART I is a primer on the use of the Hardy-Weinberg model to calculate genotype frequencies from allelic frequencies, and allelic frequencies from genotype frequencies. It also highlights some of the pitfalls of assuming that a population is in H-W equilibrium, when it is not. Note especially that the mere fact that allelic or genotypic frequencies sum to 1 does NOT mean a population is in H-W equilibrium, and that you can readily convert from allelic to genotypic frequencies (and vice versa) using the H-W model. Part I also contains a series of questions that test your mastery of population genetics. PART II contains a series of general questions on population genetics and natural selection. Some of the questions are quite tough, so don’t be easily daunted. _____________________________________________________________________________________ PART I: A PRIMER ON HARDY-WEINBERG EQUILIBRIUM & BASIC POPULATION GENETICS I. SOME BASIC ESSENTIALS The five conditions required for a Hardy-Weinberg equilibrium are: 1. No mutation 2. Random Mating 3. No gene flow ( i.e . no immigration or emigration) 4. Large population size (genetic drift does not affect allelic frequencies) 5. No selection ( ., no advantage to individuals carrying particular alleles or genotypes at a locus) ! If these conditions are satisfied, then there should be no change in genotype frequencies or allele frequencies across generations. This means that if we have information about allele or genotype frequencies for one generation, we can determine the expected frequencies for future generations under Hardy-Weinberg equilibrium. A first step is to compare expected genotypic frequencies (assuming equilibrium) with observed genotypic frequencies: if they match we assume that the 5 conditions are satisfied and we can go on. If they don't match, then we CANNOT use the H-W formula to calculate genotype frequencies from observed allelic frequencies. (We may be able to determine which condition is violated.) Keep in mind that if you know the frequencies of all of the genotypes, then you can always calculate allelic frequencies just by counting the number of copies of each allele in each genotype. But, if genotype frequencies are NOT in H-W equilibrium, you CANNOT calculate the allelic frequencies by taking the square root of the frequencies of one or the other homozygote class. This is a classic “trick” that will be played on you, so beware!
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BIS 2B Pop Gen Page 2 There are several different ways to use information about genotype and allele frequencies, depending on what you know at the beginning and what you want to find out. II. HOW TO ESTIMATE GENOTYPE FREQUENCIES IF YOU KNOW ALLELE FREQUENCIES:
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Microsoft Word - Population Genetics 2B F11.doc - BIS 2B...

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