chapter23 - BIOL 1020 CHAPTER 23 LECTURE NOTES Chapter 23...

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BIOL 1020 – CHAPTER 23 LECTURE NOTES Chapter 23: Population Genetics (Microevolution) I. Microevolution is a change in allele frequencies or genotype frequencies in a population over time A. population – a localized group of individuals capable of interbreeding and producing fertile offspring, and that are more or less isolated from other such groups B. gene pool – all alleles present in a population at a given time C. phenotype frequency – proportion of a population with a given phenotype D. genotype frequency – proportion of a population with a given genotype E. allele frequency – proportion of a specific allele in a population 1. diploid individuals have two alleles for each gene 2. if you know genotype frequencies, it is easy to calculate allele frequencies 3. example: population (1000) = genotypes AA (490) + Aa (420) + aa (90) allele number (2000) = A (490x2 + 420) + a (420 + 90x2) = A (1400) + a (600) freq[A] = 1400/2000 = 0.70 freq[a] = 600/2000 = 0.30 note that the sum of all allele frequencies is 1.0 II. Genetic equilibrium in populations: the Hardy-Weinberg theorem A. the Hardy-Weinberg theorem describes the frequencies of genotypes in a population based on the frequency of occurrence of alleles in the population that is in a state of genetic equilibrium (that is, not evolving) 1. the usual case for calculations: if allele “A” is dominant to “a”, and they are the only two alleles possible at the A- locus, then p = freq[A] = the frequency of occurrence of the A-allele in the population q = freq[a] = the frequency of occurrence of the a-allele in the population 2. Then p + q = 1 (following the sum rule for probability) 3. Allele associations follow the product rule for probability, so you multiply to predict the genotype frequencies: ( p + q ) x ( p + q ) = p 2 + 2 pq + q 2 p 2 = frequency of homozygous dominant genotypes 2 pq = frequency of heterozygous genotypes q 2 = frequency of homozygous recessive genotypes note that ( p + q ) x ( p + q ) = 1 x 1 = 1, so p 2 + 2 pq + q 2 = 1 B. Hardy-Weinberg equilibrium 1. if the Hardy-Weinberg theorem can be used to accurately predict genotype frequencies from allele frequencies for a population then the population is in Hardy-Weinberg equilibrium or genetic equilibrium 2. in such cases you can use data from one generation to predict the allele, genotype, and phenotype frequencies for the next generation 3. such populations are not evolving, but are static instead C.
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