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

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