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The Genetic Analysis of Populations and How They Evolve Chapter 24 p. 659 Population Genetics Population: a local group belonging to a single species within which mating can occur Gene pool: The set of genetic information carried by all interbreeding members of a population What is the focus? In the study of population genetics, the focus is on the groups rather than individuals Also on the measurement of allele and genotype frequency rather than on the distribution of genotypes from a single mating Allele frequency: represents the freq of alleles in contrast to genotype freq. Populations are dynamic They grow/expand or diminish based on birth and death rates Migration Merging with other populations Over time, this can lead to changes in the genetic structure of the population How to measure Allele Frequency: Refer to page 661 for this problem In a population of 100, 36 are type M(M), 48 are type MN and 16 are type N(N) 36 M is actually 72 ( additional M) 16 N is actually 32 (additional N) 48MN will include an additional 48M alleles Therefore there are 72+48 or 120M alleles out of 200 alleles (100 in population with 2 alleles) 120/200= 60% of M allele in population 48+32= total number of N alleles 80/200= 40% of population Outline of Chapter 24 The Hardy-Weinberg law A model for understanding allele, genotype, and phenotype frequencies for single gene traits in a genetically stable population Calculations beyond Hardy-Weinberg Measuring how selection and mutations change allele frequencies over time 3 Tenets of the Hardy-Weinberg Law 1. allele frequencies predict genotype frequencies 2. at equilibrium, allele and genotype frequencies do not change from generation to generation 3. equilibrium is reached in one generation of random mating The Hardy-Weinberg law clarifies the relations between genotype and allele frequency within a generation and from one generation to the next Five assumptions Infinitely large population Individuals mate at random No new mutations appear in gene pool No migration into or out of population No genotype-dependent differences in ability to survive and reproduce If all assumptions hold, population is in Hardy- Weinberg equilibrium Hardy-Weinburg Equilibrium p2 +2pq+q2=1 In the random combination of gametes in the population, the prob that sperm and egg both contain the R allele (RR) is pxp=p2 Then chance that gametes will carry unlike alleles (Rr) (pxq) + (pxq)=2pq And the chance that a homozygous recessive (rr) individual will result is qxq=q2 Step 2 Fig. 20.3 Use gamete allele frequency to calculate genotype frequencies in the zygotes of next generation Two steps in translating the genotype frequencies from one generation to the next Step 1 Fig. 20.2 Calculate allele frequency of gametes – same as adults p2 is the probability that both gametes in a fertilization event will carry the R allele, it is also the measure of the freq of RR homozygotes in the next generation 2pq describes the freq of Rr q2 is a measure of the freq of homozygous recessive (rr) zygotes p2+ 2pq +q2=1 Allele frequencies do not change from generation to generation in a population at Hardy-Weinberg equilibrium Equilibrium is equal to 1 A Hardy-Weinberg population achieves the genotype frequencies of p2, 2pq, and q2 in just one generation and maintains them in subsequent generations Measuring how mutation and selection cause changes in allele frequency Evolution – sometimes defined as change in allele frequency over multiple generations Macroevolution – changes that occur through geologic time among species Microevolution – changes that occur from generation to generation within a species Can use Hardy-Weinberg law to examine microevolution Violations of assumptions to Hardy-Weinberg can be used to analyze evolutionary forces causing deviations in allele frequencies Natural selection acts on differences in fitness to alter allele frequencies Fitness – individual’s relative ability to survive and transmit genes to next generation Viability and reproductive success Natural selection – individuals with higher fitness survive and reproduce more than individuals with lower fitness Summary of evolutionary equilibrium between mutation and selection New alleles arise in populations by mutation When the allele affects fitness, selection will drive frequency towards an equilibrium with wild-type allele Equilibrium value is determined by relative selection coefficients for heterozygous and homozygous individuals for new allele If new allele has no effect on fitness, genetic drift will determine its frequency Changes in allele and genotype frequency when selection acts on genotype-dependent differences in fitness Fig. 20.4 Frequency of q allele in next generation where q’ is frequency of q allele in generation after selection Fig. 20.6 How a recessive genetic condition influences allele frequency of a population rr genotype has decreased fitness Fitness or RR and Rr same WRR = 1, WRr = 1, Wrr = 1-s Fig. 20.7 s = selection coefficient against rr which varies from 0 (no affect) to 1 (lethal) P. 678 1, 2,