Unformatted text preview: The The Hardy‐Weinberg Principle Today ’s questions: Will blonds go extinct? I. Deriving the Hardy‐Weinberg principle II. Applying the Hardy‐Weinberg principle Hypothesis: Recessive alleles decline in frequency over time, because they are recessive. Prediction (reported in a 2002 BBC news story): The allele for blonde hair is recessive and will gradually be reduced to a frequency of 0. Corollary 1: The extinction of the allele will happen in the next 200 years. Corollary 2: The last blonde will be from Finland, because this is where the blonde allele is currently at highest frequency. The essential issue is: What happens to the frequency of alleles in a population—especially if they are completely dominant and completely recessive—just due to segregation during meiosis and transmission to offspring? …no selection, nothing else going on … just recessiveness or dominance or co‐dominance or incomplete dominance, and random mating with respect to the gene and alleles in question To answer this question, we need to shift our thinking … 1. Evolution = 2. Instead of predicting the frequencies of genotypes from a particular mating, we need to predict the frequencies of genotypes from many thousands of matings in a population. I. Deriving the Hardy‐Weinberg Principle Consider the simplest possible situation: one gene, with 2 alleles that have a simple dominant‐recessive relationship. Call the alleles A1 and A2 … A1 is dominant to A2 The genotypes that are possible are A1A1 A1 A2 A2 A2 Conclusions, under Hardy‐Weinberg conditions: If the allele frequencies in a population are given by p1 and p2, then the genotype frequencies are given by p12, 2p1p2 and p22. When all individuals in the population breed and produce the same number of offspring, the allele frequencies in the population will not change, generation after generation. (They remain at p1 and p2 forever.) Will blondes go extinct? What happens to the frequency of alleles in a population—especially if they are completely dominant and completely recessive—just due to segregation during meiosis and transmission to offspring? What’s so big about that? II. Applying the Hardy‐Weinberg Principle The Hardy‐Weinberg principle It assumes that, with respect to the gene in question, there is: • No • No • No • No • Problem 1: Survey 100 humans; determine genotypes at the MN blood type locus. MM MN NN 18 50 32 1. What is the observed frequency of the genotypes? 2. What is the observed frequency of the M and N alleles? fr(M) = fr(N) = 3. Given the observed allele frequencies, what are the expected genotype frequencies under the null hypothesis of no evolution and random mating with respect to MN genotype? 4. Compare the observed and expected values: Do the data support the null hypothesis? Problem 2: Survey a tapeworm population and determine genotype frequencies at an autosomal locus. AA Aa aa 0.30 0.33 0.37 1. What is the observed frequency of the genotypes? ...
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- Spring '09
- Biology, 200 years, Hardy‐Weinberg Principle