April 9, 20108.08Probability of fixation, starting at the frequencyKolmogorov Backward equationusing boundary conditionsLet's obtain probability of fixation uder drift only(no mutations, no selection)Consequences of Hrady-Weinberg equilibrium Homozygosity : Heterozygosity:Probability to pick two identical alleles in the population223. Estimating the timescale of genetic driftInfinite allele model 4. Mutations and drift 3344Conculsion:Balance between mutations and drfift leads to a constant(steady state) diversity of the populationHST.508/Biophysics 170 Lecture 4 9/20/2010Selection and driftsHaldein's equationHST.508/Biophysics 170 Lecture 4 9/15/2008Selection and driftsHaldein's equations - selectione coeffeceint (strength of selection), s>0 h determines the type of selection * Types of selectionh=0 A1 dominanth=1 A2 dominant0<h<1 incomplete dominance -> directional selectionh<0 overdominance -> balancing selectionh>1 underdominance -> disruptive selection* Special type of balancing selection-- frequency-dependent selection.* Let's calculate the effect of selection on allele freq. p (in one generation)1234Harmonic mean- the value is dominated by the smallest population size in historySensitive to bottlenecks5Selection and drift (Haldein's equation). Deleteriousallele A23)New mutation (new A2)New mutation (new A1)Effective population