Presentation on theme: "Population Genetics What is population genetics?"— Presentation transcript:
1 Population Genetics What is population genetics? Studying VariationGene frequencies & allele frequenciesHardy-Weinberg equilibriumFactors that change allele frequencies in populations
2 I. What is Population Genetics Goal: understand the genetic composition of a population and the forces that determine and change that compositionFundamental measurement = allele frequencyForces that change allele frequency = mutation, selection, gene flow, genetic driftSide-blotched lizards (Uta stansburiana) in central California experience unusual patterns of throat color.
3 Terms for understanding genetic diversity Population, subpopulation, local populationGenetic structure = genetic composition of a given populationBased on analysis of Polymorphic lociOnly an estimate at that given moment in timeGenotype frequency = # individuals with a particular genotype in a pop / NAllele frequency = # of copies of an allele in a pop / total # alleles in a pop.
4 Genetic PolymorphismGenetic StructureAllele Frequency
5 Gene Frequencies & Allele Frequencies Gene frequency refers to proportion of particular allelic form among all copies of gene in populationUsually estimated by sampling populationdiploid: 2 copies of genehomozygotes: 2 copies of alleleheterozygotes: 1copy of each allelehaploid: 1 copy of alleleFor two alleles, p + q = 1, where p and q are frequencies of the two alleles
6 Calculating Genotype Frequencies Relative frequencies of genotypes – proportion of organisms that have the particular genotypeThe proportion of individuals in a population with a particular genotypefA/A = # of A/A divided by the totalfA/a = # of A/a divided by totalfa/a = # of a/a divided by totalA/AA/aa/aN404713f0.400.470.13
7 Calculating allele frequencies If fA/A, and fa/a are the proportions of the three genotypes at a locus with two alleles, then the frequency p(A) of the A allele and the frequency q(a) of the a allele are obtained by counting alleles:p = fA/A + ½ fA/aq = fa/a + ½ fA/ap + q = fA/A + fa/a + fA/a = 1.00q = 1 – p and p = 1 – q
8 AA Aa aa total N 40 47 13 100 # of A 80 127 # of a 26 73 Total 200 127# of a2673Total200Allele Frequency of A = 127/200 = 0.635p(A) = 0.635Allele Frequency of a = 73/200 = 0.365q(a) = = 1 - p
9 Mendelian considerations in population genetics…
11 II. Hardy Weinberg equilibrium Sexual reproduction does not cause a constant reduction in genetic variation in each generation; rather the amount of variation remains constant generation after generation in the absence of other disturbing forces.Model that shows what happens to allele and genotype in an “ideal” population using a set of simple assumptions
12 Populations in HW equilibrium have the following properties: The frequency of alleles does not change from generation to generationAfter one generation of random mating, offspring genotype frequencies can be predicted from the parent allele frequenciesWhy use HW?It Identifies the real-world forces that change allele frequencies
13 80% of all the gametes in the population carry a dominant allele for black coat (B) and 20% carry the recessive allele for gray coat (b).Random union of these gametes will produce a generation:p2 = 0.642pq = 0.32q2 = 0.04So 96% of this generation will have black coats; only 4% gray coats.Will the gray phenotype eventually be lost?
14 Testing for equilibrium Determine the genotype frequenciesDirectly from phenotypesAnalyzing DNA sequenceCalculate allele frequenciesPredict the offspring’s genotype frequencies using HW principle… does the prediction hold true? Are they similar to the observed frequencies?
16 The allele frequency for hemophilia (A) is 1/10,000 or 0.0001. What is the allele frequency for the normal allele in the human population?Among males, what is the frequency of affected individuals?Within a population of 100,000 people, what is the expected number of affected males? What is the number of expected carrier females?
17 III. Factors that change allele frequencies in populations: Disturbing forces MutationNon-random matingGene flowGenetic DriftNatural selection
18 1) mutationMutation is the Ultimate source of variation, playing a fundamental role in the process of evolutionMutation rate (μ)= probability that a copy of an allele changes to some other allelic form in one generationΔq = μpp = 0.8, q = 0.2, μ = 10-5,Δq = (10-5)(0.8) = )Next generation:qn+1 = =pn+1 = 0.8 – =Mutations don’t significantly alter allele freq. In 1 generation…Gets slower every generation
19 2) Gene flow Gene Flow = migration Gene flow - Genetic exchange between populations due to the migration of individuals between populationsCan offset the effects of genetic driftInhibited by isolation
20 3) Genetic Drift Genetic Drift Random fluctuations of allele frequencies between generationscompounded by small population sizealleles can become fixed
21 The genetic bottleneck effect Founder effect, similar outcome… due to chance, the allele frequency in the founding population may differ from the original population.
22 4) Inbreeding (non-random mating) Inbreeding = Mating between relativesIDB – identical by descent, the two alleles may be copies of the same gene in an earlier member of the line
23 5) Natural selection The force that can result in adaptation! Darwinian fitness – relative probability of survival and rate of reproduction of a phenotype or genotypeDifferential rates of survival and reproductionFitness is a consequence of the relation between the phenotype of the organism and the environment in which it lives, so the same genotype will have different fitnesses in different environmentsconsequence of relationship between phenotype and environmentsame genotype may have different fitness in different environments
24 Heritability of beak depth in medium ground finches Heritability of beak depth in medium ground finches. The red line and circles are data from 1978, and the blue line and circles are from 1976 data. The results from the two years are consistent. Both show a strong relationship between the beak depth of parents and their offspring
25 In every natural population studied, more offspring are produced each generation than survive to breed. The reproductive capacity or biotic potential of organisms is astonishing (Table 3.1). It has been shown that in most populations, some individuals are more successful at mating and producing offspring than others. Variation in reproductive success represents an opportunity for selection, as does variation in survival.
26 Combining forces shape genetic structure Natural selection, mutation and genetic drift all can combine to maintain allele frequenciesPopulations undergo evolution, not individuals"Evolution is evidenced by changes in the gene pool which includes all the genes of any population at any give time."