医学遗传学08基因的群体行为engv.ppt

08基因变异的群体行为 Population Genetics,,,,,What is a population from a genetic perspective? A population in the genetic sense, is not just a group of individuals, but a breeding group,,The genetics of a population is concerned not only with the genetic constitution of the individuals but also with the transmission of the genes from one generation to the next.,,In the transmission the genotypes of the parents are broken down and a new set of genotypes is constituted in the progeny, from the genes transmitted in the gametes. The genes carried by the population thus have continuity from generation to generation, but the genotypes in which they appear do not. The genetic constitution of a population, referring to the genes it carries, is described by the array of gene frequencies, that is by specification of the alleles present at every locus and the numbers or proportions of the different alleles at each locus.“,,Goals of Population Genetics To describe how the frequency of an allele which controls a trait changes over time； To analyze the factors that lead to changes in gene (allele) frequencies； To determine how changes in gene (allele) frequencies affect evolution and speciation.,,Why Study Populations and Gene Frequencies Genetic variability necessary for evolutionary success; Measuring genetic variability at many loci can characterize a population; Variability of phenotypic and molecular traits are analyzed.,1. Variability and Gene (or Allelic) Frequencies,Genetic data for a population can be expressed as gene or allelic frequencies; All genes have at least two alleles; Summation of all the allelic frequencies for a population can be considered a description of the population; Frequencies can vary widely among the alleles in a population; Two populations of the same species do not have to have the same allelic frequencies.,,Genotypic frequencies It describes the distribution of genotypes in a population.,,Example Blood type locus; two alleles , M or N, and three MM, MN, NN genotypes are possible (the following data was collected from a single human population).,,Deriving Gene (or Allelic) Frequencies To determine the allelic frequencies we simply count the number of M or N alleles and divide by the total number of alleles. f(M) = [(2 x 1787) + 3039]/12,258 = 0.5395 f(N) = [(2 x 1303) + 3039]/12,258 = 0.4605 By convention one of the alleles is given the designation p and the other q. Also p + q = 1. p=0.5395 and q=0.4605 Furthermore, a population is considered by population geneticists to be polymorphic if two alleles are segregating and the frequency of the most frequent allele is less than 0.99.,,Deriving allelic frequencies from genotypic frequencies The following example will illustrate how to calculate allelic frequencies from genotypic frequencies. It will also demonstrate that two different populations from the same species do not have to have the same allelic frequencies.,Let: p=f(M) and q=f(N) Thus: p=f(MM) + ½ f(MN) and q=f(NN) + ½ f(MN).,.,,So the results of the above data are: Greenland: p=0.835 + ½ (0.156)=0.913 and q=0.009 + ½ (0.156)=0.087 Iceland: p=0.312 + ½ (0.515)=0.569 and q=0.173 + ½ (0.515)=0.431 Clearly the allelic frequencies vary between these populations.,2. The Hardy-Weinberg Law,The Hardy-Weinberg Law The unifying concept of population genetics Named after the two scientists who simultaneously discovered the law The law predicts how gene frequencies will be transmitted from generation to generation given a specific set of assumptions.,,If an infinitely large, random mating population is free from outside evolutionary forces (i.e. mutation, migration and natural selection), then the gene frequencies will not change over time, and the frequencies in the next generation will be: p2 for the AA genotype 2pq for the Aa genotype, and q2 for the aa genotype.,,Let's examine the assumptions and conclusions in more detail starting first with the assumptions.,,A.Infinitely large population No such population actually exists. The effect that is of concern is genetic drift (a change in gene frequency that is the result of chance deviation from expected genotypic frequencies) a problem in small populations.,,B. Random mating Random mating - matings in a population that occur in proportion to their allelic frequencies.,,For example, if the allelic frequencies in a population are: f(M) = 0.91 f(N) = 0.09 then the probability of MM individuals occurring is 0.91 x 0.91 =0.828. If a significant deviation occurred, then random mating did not happen in this population.,,Within a population, random mating can be occurring at some loci but not at others. Examples of random mating loci - blood type, RFLP patterns Examples of non-random mating loci - intelligence , physical stature,,C. No evolutionary forces affecting the population The principal forces are: Mutation Migration Selection Some loci in a population may be affected by these forces, and others may not; those loci not affected by the for。