遗传学英文课件：11 群体遗传学英文课件.pptVIP

* Population Genetics * Mutation-selection balance for a deleterious recessive allele with frequency q. Genetic equilibrium is reached when the introduction of the allele into the population by mutation at rate ? is balanced by the elimination of the allele by selection with intensity s against the recessive homozygotes. * Population Genetics * When these two processes are in balance, a dynamic equilibrium will be established. We can calculate the frequency of the mutate allele at the equilibrium created by mutation-selection balance by equating the rate of mutation to the rate of elimination by selection: ?=sq2, thus, * Population Genetics * Mutation-Drift Balance We have already seen that random genetic variability from a population. Without any counteracting force, this process would eventually make up population completely homozygous. However, mutation replenishes the variability that is lost by drift. At some point, the opposing forces of mutation and genetic drift come into balance and a dynamic equilibrium is established. * Population Genetics * Previously we saw that genetic variability can be quantified by calculating the frequency of heterozygotes in a population—a statistic called heterozygosity (杂合性), which is symbolized by the letter H. The frequency of homozygotes in a population—often called homozygosity (纯合性)—is equal to 1-H. Over time, genetic drift decreases H, and increases 1-H, and mutation does just the opposite. * Population Genetics * Mutation-drift balance for variability as measured by the frequency of heterozygotes H in a population of size N. An equilibrium frequency of heterozygotes is reached when the introduction by mutation at rate u is balanced by the elimination of variability by genetic drift at rate 1/2N. * Population Genetics * Let’s assume that each new mutation is selectively neutral. In a randomly mating population of size N, the rate which drift decreases H is (1/2N)H. The rate at which mutation increases H is proportional to