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1、医学遗传学基因的群医学遗传学基因的群体行为体行为第1页，本讲稿共33页第2页，本讲稿共33页第3页，本讲稿共33页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第4页，本讲稿共33页 The genetics of a population is concerned not only with the genetic constitution of the indivi
2、duals but also with the transmission of the genes from one generation to the next.第5页，本讲稿共33页 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 h
3、ave 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 propor
4、tions of the different alleles at each locus.第6页，本讲稿共33页Goals of Population Genetics 1.To describe how the frequency of an allele which controls a trait changes over time；2.To analyze the factors that lead to changes in gene(allele)frequencies；3.To determine how changes in gene(allele)frequencies af
5、fect evolution and speciation.第7页，本讲稿共33页Why Study Populations and Gene Frequencies 1.Genetic variability necessary for evolutionary success;2.Measuring genetic variability at many loci can characterize a population;3.Variability of phenotypic and molecular traits are analyzed.第8页，本讲稿共33页1.Variabili
6、ty and Gene(or Allelic)Frequencies1.Variability and Gene(or Allelic)Frequencies 1.Genetic data for a population can be expressed as gene or allelic frequencies;2.All genes have at least two alleles;3.Summation of all the allelic frequencies for a population can be considered a description of the pop
7、ulation;4.Frequencies can vary widely among the alleles in a population;5.Two populations of the same species do not have to have the same allelic frequencies.第9页，本讲稿共33页Genotypic frequencies It describes the distribution of genotypes in a population.第10页，本讲稿共33页Example Blood type locus;two alleles,
8、M or N,and three MM,MN,NN genotypes are possible(the following data was collected from a single human population).第11页，本讲稿共33页Genotye#of Individuals Genotypic Frequencies MM 1787 MM=1787/6129=0.289 MN 3039 MN=3039/6129=0.50 NN 1303 NN=1303/6129=0.21 Total 6129 第12页，本讲稿共33页Deriving Gene(or Allelic)Fr
9、equenciesTo 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 a
10、nd 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.第13页，本讲稿共33页Deriving allelic frequencies from genotypic frequencies The following example will illustrate how
11、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.第14页，本讲稿共33页Let:p=f(M)and q=f(N)Thus:p=f(MM)+f(MN)and q=f(NN)+f(MN).Percent Allelic Frequencies Location MM MN NN
12、 p q Greenland 83.5 15.6 0.90 0.913 0.087 Iceland 31.2 51.5 17.30 0.569 0.431 第15页，本讲稿共33页So the results of the above data are:Greenland:p=0.835+(0.156)=0.913 and q=0.009+(0.156)=0.087 Iceland:p=0.312+(0.515)=0.569 and q=0.173+(0.515)=0.431Clearly the allelic frequencies vary between these populatio
13、ns.第16页，本讲稿共33页2.The Hardy-Weinberg Law The Hardy-Weinberg LawThe 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.第
14、17页，本讲稿共33页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
15、q2 for the aa genotype.第18页，本讲稿共33页 Lets examine the assumptions and conclusions in more detail starting first with the assumptions.第19页，本讲稿共33页A.Infinitely large population 1.No such population actually exists.2.The effect that is of concern is genetic drift(a change in gene frequency that is the r
16、esult of chance deviation from expected genotypic frequencies)a problem in small populations.第20页，本讲稿共33页B.Random mating Random mating-matings in a population that occur in proportion to their allelic frequencies.第21页，本讲稿共33页For example,if the allelic frequencies in a population are:f(M)=0.91 f(N)=0
17、.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.第22页，本讲稿共33页 Within a population,random mating can be occurring at some loci but not at others.Examples of random mating loci-blood type,
18、RFLP patterns Examples of non-random mating loci-intelligence,physical stature 第23页，本讲稿共33页C.No evolutionary forces affecting the population The principal forces are:1.Mutation 2.Migration 3.Selection Some loci in a population may be affected by these forces,and others may not;those loci not affecte
19、d by the forces can by analyzed as a Hardy-Weinberg population 第24页，本讲稿共33页Mathematical Derivation of the Hardy-Weinberg Law If p equals the frequency of allele A in a population and q is the frequency of allele a in the same population,union of gametes would occur with the following genotypic frequ
20、encies:第25页，本讲稿共33页*The gamete and offspring genotypes are in parentheses.From the table,it is clear that the prediction regarding genotypic frequencies after one generation of random mating is correct.That is:AA=p2;Aa=2pq;and aa=q2 Female Gametes*p(A)q(a)MaleGametes p(A)p2(AA)pq(Aa)q(a)pq(Aa)q2(aa)
21、第26页，本讲稿共33页Prediction regarding stability of gene frequencies The following is a mathematical proof of the second prediction.To determine the allelic frequency,they can be derived from the genotypic frequencies as shown above.p=f(AA)+f(Aa)(substitute from the table on previous page)p=p2+(2pq)(facto
22、r out p and divide)p=p(p+q)(p+q=1;therefore q=1-p;make this substitution)p=p p+(1-p)(subtract and multiply)p=p 第27页，本讲稿共33页Evolutionary Genetics The Hardy-Weinberg Law described a population that exists in genetic equilibrium where allelic frequencies do not change from generation to generation.For
23、evolution of a population to occur,the gene frequencies of that population must undergo change.Several factors can act to change fitness or the ability to maintain allelic frequencies.Viability-ability to survive Fertility-ability to reproduce By altering the fitness of an individual,the mating dist
24、ribution will change,and consequently the allelic frequencies will change and the population will evolve.第28页，本讲稿共33页3.Factors that Affect Stability of Gene Frequencies3.Factors that Affect Stability of Gene FrequenciesA.Mutation1.Classified as beneficial,harmful or neutral;2.Can occur by point muta
25、tions;or small insertions or deletions of the nucleotide sequence;3.Harmful mutations are lost if they reduce fitness;4.If fitness is improved by a mutation,then the frequency of that allele will increase from generation to generation;5.The mutation could be a change in one allele to resemble one cu
26、rrently in the population,for example from a dominant to a recessive allele;第29页，本讲稿共33页6 The mutation could generate an entirely new allele.Most of these mutations though will be detrimental and lost.If the environment changes,the new mutant allele may be favored and eventually become the dominant
27、alelle in that population.If the mutation is beneficial to the species as a whole,migration must occur for it to spread to other populations of the species.7 Gene duplication favor mutational events.The duplicated gene can undergo mutations to generate a new gene that has a similar,but a slightly mo
28、dified function for the organism.This type of evolution generates multigene families.(Examples:hemoglobin and muscle genes in humans,and seed storage and photosynthetic genes in plants)第30页，本讲稿共33页B.Migration 1.The Hardy-Weinberg Law assumes the population is closed.But for many populations this is
29、not the case.2.Migration will change gene frequencies by bringing in more copies of an allele already in the population or by bringing in a new allele that has arisen by mutation.3.Because mutations do not occur in every population,migration will be required for that allele to spread throughout that
30、 species.第31页，本讲稿共33页4.In a genetic context,migration requires the introduction of new alleles into the population.This will only occur after the migrant has successfully mated with an individual in the population.The term that is used to described this introduction of new alleles is gene flow.5.The
31、 two effects of migration are to:(1)increase variability within a population(2)prevent a population of that species from diverging to the extent that it becomes a new species.第32页，本讲稿共33页C.Selection1.Mutation causes new functions for the individual.2.These new forms may or may not add to the fitness
32、 of the individual.3.If the fitness of the individual leads to a reproductive advantage,then the alleles present in that individual will be more prevalent in the next generation of the population.4.A population undergoes selection when certain alleles are preferentially found in a new generation because of the increased fitness of the parent.5.The alleles in the individual with increased fitness will increase in frequency in the population.6.In a Darwinian context,mutation,migration and selection lead to changes in gene frequencies,and the population evolves by natural selection.第33页，本讲稿共33页