08基因变异的群体行为详解.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. 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. 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. 群体的遗传平衡 一、Hardy-Weinberg平衡律 在随机婚配的大群体中，没有受到外在因素影响的情况下，显性性状并没有随着隐性性状的减少而增加，不同基因型相互比例在一代代传递中保持稳定。 Hardy-Weinberg平衡律 Hardy-Weinberg平衡律 Hardy-Weinberg平衡律 Hardy-Weinberg平衡律 表中结果显示： AA后代 =p4+2p3q+p2q2=p2(p2+2pq+q2)=p2(p+q)2=p2； Aa后代 =2p3q+4p2q2+2pq3=2pq(p2+2pq+q2)=2pq(p+q)2=2pq； Aa后代 =p2q2+2pq3+ q4=q2(p2+2pq+q2)=q2(p+q)2=q2 可以看出在这一群体中第一代和第二代的基因型频率是一致的。实际上无论经过多少代，基因型频率将保持不变，每种基因型的个体数量随着群体大小而增减，但是相对频率不变，这就是Hardy-Weinberg平衡的推理。 Hardy-Weinberg平衡律的应用——Hardy-Weinberg平衡判定 例1：某一基因座的一对等位基因A和a，有三种基因型AA，Aa／aA和aa，在随机1000人的群体中，观察的基因型分布如下： AA为600人 Aa／aA为340人 aa为60人。 X2检验： 当p＞0.05时，表示预期值和观察值之间的差异无统计学意义，等位基因频率和基因型频率分布符合Hardy-Weinberg平衡， p＜0.05时，表示预期值和观察值