遗传学的基本原理课件.ppt

1、2023-1-26The basic principles of inheritance1Chapter 3Mendelism:The Basic Principles of inheritance2023-1-26The basic principles of inheritance2Chapter OutlineMendels study of heredityApplication of Mendels principlesFormulating and testing genetic hypothesis(遗传学假说的形成和检验遗传学假说的形成和检验)Mendelian princip

2、les in human genetics(人人类遗传学中的孟德尔原理类遗传学中的孟德尔原理)2023-1-26The basic principles of inheritance3The Birth of Genetics:A Scientific RevolutionPisum sativumThe subject of Gregor Mendels experiments.2023-1-26The basic principles of inheritance4Section 1Mendels Study of HeredityGregor Mendels experiments wi

3、th peas elucidated how traits are inherited.2023-1-26The basic principles of inheritance51 Mendels Life Story1822-18842023-1-26The basic principles of inheritance62 Mendels Experimental Organism,the Garden PeaThe garden pea,Pisum sativum,is a dicot(双子叶双子叶)and a true-breeding(纯种纯种).One peculiarity of

4、 pea reproduction is that the petals of flower close down tightly,preventing pollen grains from entering or leaving.2023-1-26The basic principles of inheritance73 Monohybrid Crosses:the Principles of Dominance and Segregation2023-1-26The basic principles of inheritance8柱头柱头雌蕊雌蕊花药花药雄蕊雄蕊去势的去势的2023-1-2

5、6The basic principles of inheritance92023-1-26The basic principles of inheritance10Mendel inferred that these hybrids carried a latent genetic factor for dwarfness,one that was masked by the expression of another factor for tallness.Clearly,the hybrids that he had made by crossing tall and dwarf var

6、ieties had the ability to produce dwarf progeny even through they themselves were tall.2023-1-26The basic principles of inheritance11He said the latent factor was recessive(隐性隐性)and that the expressed factor was dominant(显性显性).These recessive and dominant factors separated from each other when the h

7、ybrid plants reproduced.This enable to explain the reappearance of the dwarf characteristic in the next generation.2023-1-26The basic principles of inheritance12These factors are now called genes;their dominant and recessive are called alleles.Alleles are alternate forms of a gene.Genes come in pair

8、s.Each of parental strains carried two identical copies of a genein modern terminology,they are diploid and homozygous(纯合子纯合子).2023-1-26The basic principles of inheritance13During the production of gametes,these two copies are reduced to one;that is,the gametes that emerge from meiosis carry a singl

9、e copy of a genein modern terminology,they are haploid.Diploid gene number would be restored when sperm and egg unite to form a zygote.2023-1-26The basic principles of inheritance14If the sperm and egg came from genetically different plants,the hybrid zygote would inherit two different alleles,one f

10、rom the mother and one from the father.Such an offspring is said to be heterozygous.The different alleles that are present in heterozygote must coexist even through one is dominant and the other recessive,and that each of these alleles would have an equal chance of entering a gamete when the heteroz

11、ygote reproduces.2023-1-26The basic principles of inheritance15Furthermore,random fertilizations with a mixed population of gameteshalf carrying the dominant allele and half carrying the recessive allelewould produce some zygotes in which both alleles were recessive,Thus,we could explain the reappea

12、rance the recessive characteristic in the progeny of the hybrid plants.2023-1-26The basic principles of inheritance16Mendel used symbols to present the hereditary factors that he postulateda methodological breakthrough.2023-1-26The basic principles of inheritance17The genotype is the genetic constit

13、ution of a cell,an organism,or an individual(i.e.the specific allele makeup of the individual)usually with reference to a specific character under consideration 基因型基因型是一个细胞、一个生物或一个个体是一个细胞、一个生物或一个个体的的遗传遗传构成形式构成形式(比如组成个体的特定等位比如组成个体的特定等位基因基因)，通常与其控制的特殊性状相关。，通常与其控制的特殊性状相关。2023-1-26The basic principles o

14、f inheritance18A phenotype is any observable characteristic or trait of an organism:such as its morphology,development,biochemical or physiological properties,or behavior.表型是表型是一个生物可以被观察到的特征或性状：一个生物可以被观察到的特征或性状：如形态、发育、生化或生理学的性质或行为。如形态、发育、生化或生理学的性质或行为。Phenotypes result from the expression of an organ

15、isms genes as well as the influence of environmental factors and possible interactions between the two.2023-1-26The basic principles of inheritance19The allele for dwarfness,being recessive,is symbolized by a lowercase letter d;the allele for tallness,being dominant,is symbolized by the correspondin

16、g uppercase letter D.As the parental strain,the tall and dwarf pea plants form the P generation of the experiment.Their hybrid progeny are referred to as the first filial(子女子女)generation,or F1.2023-1-26The basic principles of inheritance20Upon self-fertilization,the two kinds of gametes produced by

17、heterozygotes can unite in all possible ways.Thus,they produce four kinds of zygotes:DD,Dd,dD,and dd.However,because of dominance,three of the genotypes have the same phenotype.Thus,in next generation,called the F2,the plants are either tall or dwarf,in ratio of 3:1.2023-1-26The basic principles of

18、inheritance21Which results would happen when did this analysis one step further take?The F2 plants were self-fertilized to produce an F3.All the dwarf F2 plants produce only dwarf offspring,demonstrating that they were homozygous for the d allele,but the tall F2 plants comprised two categories.Appro

19、ximately 1/3 of them produced only tall offspring,whereas the other 2/3 produced a mixture of tall and dwarf offspring.2023-1-26The basic principles of inheritance22Mendel concluded:The third that were true-breeding were DD homozygotes and that the two-thirds that were segregating were Dd heterozygo

20、tes.These proportions,1/3 and 2/3,were exactly what his analysis predicted because,among the tall F2 plants,the DD and Dd genotypes occur in a ratio of 1:2.2023-1-26The basic principles of inheritance23Mendel discovered two key principles The Principle of Dominance:In a heterozygote,one allele may c

21、onceal the presence of another.This principle is a statement about genetic function.Some alleles evidently control the phenotype even when they are present in a single copy.2023-1-26The basic principles of inheritance24 The Principle of Segregation:In a heterozygote,two different alleles segregate f

22、rom each other during the formation of gametes.This principle is a statement about genetic transmission.An allele is transmitted faithfully to the next generation,even if it was present with a different allele in a heterozygote.原理原理2023-1-26The basic principles of inheritance254.Dihybrid Crosses:the

23、 Principle of Independent Assortment2023-1-26The basic principles of inheritance262023-1-26The basic principles of inheritance27Yellow,round9/16Yellow,wrinkled3/16Green,round3/16Green,wrinkled1/162023-1-26The basic principles of inheritance28 This analysis is predicated on two assumptions(以以2个假设为基础个

24、假设为基础):each gene segregates its alleles,and these segregations are independent of each other.The second assumption implies that there is no connection or linkage between the segregation events of the two genes.For example,a gamete that receives W through the segregation of the texture gene is just a

25、s likely to receive G as it is to receive g through the segregation of the color gene.2023-1-26The basic principles of inheritance29Do the experimental data fit with the predictions of our analysis?2023-1-26The basic principles of inheritance30Mendel conducted similar experiments with other combinat

26、ions of traits and in each case observed that the genes segregated independently.These results of theses experiments led him to a third key principle:3.The Principle of Independent Assortment:The alleles of different genes segregate,or as we sometimes say,assort,independently of each other.2023-1-26

27、The basic principles of inheritance31Key PointsMendel studied the inheritance of seven different traits in garden peas,each trait being controlled by a different gene.Mendels research led him to formulate three principles of inheritance:the alleles of a gene are either dominant or recessive,differen

28、t alleles of a gene segregate from each other during the formation of gametes,and the alleles of different genes assort independently.2023-1-26The basic principles of inheritance32Applications of Mendels PrinciplesMendels principles can be used to predict the outcomes of crosses between different st

29、rains of organisms.Section 32023-1-26The basic principles of inheritance33If the genetic basis of a strait is known,Mendels principles can be used to predict the outcome of crosses.There are three general procedures,two relying on the systematic enumeration of all the zygotic genotypes or phenotypes

30、 and one relying on mathematical insight.2023-1-26The basic principles of inheritance341 The Punnett Square Method(1875-1967)British GeneticistIntercrosse互交互交2023-1-26The basic principles of inheritance352 The Forked-Line MethodThe forked-line method for predicting the outcome of an intercross invol

31、ving three independently assorting genes in peas.2023-1-26The basic principles of inheritance36The forked-line method for predicting the outcome of a testcross involving three independently assorting genes in peas.2023-1-26The basic principles of inheritance373 The Probability MethodAn intercross sh

32、owing the probability method in the context of a Punnett square.2023-1-26The basic principles of inheritance38Application of the probability method to an intercross involving two genes.2023-1-26The basic principles of inheritance39Key PointThe outcome of a cross can be predicted by the systematic en

33、umeration of genotypes using a Punnett square.When more than two genes are involved,the forked-line or probability method is used to predict the outcome of a cross.2023-1-26The basic principles of inheritance40Section 4Formulating and Testing Genetic HypothesesThe chi-square test is a simple way of

34、evaluating whether the predictions of a genetic hypothesis agree with data from an experiment.2023-1-26The basic principles of inheritance41A well-formulated scientific idea is called a hypothesis.Data collected from observations or from experimentation enable scientists to test hypothesisthat is,to

35、 determine if a particular hypothesis should be accepted or rejected.2023-1-26The basic principles of inheritance42Snapdragons,Antirrhinum majus(金鱼草金鱼草)2023-1-26The basic principles of inheritance43We could hypothesize that flower color is controlled by a single gene with two alleles,W(for red)and w

36、(for white),and that the flowers of Ww heterozygotes are pink because the alleles are not strictly either dominant or recessive.According to this hypothesis,the P generation would be WW(red)X ww(white),producing F1 hybrids that would be Ww(pink),which when intercrossed,would yield WW(red),Ww(pink),a

37、nd ww(whiter)F2 progeny in a 1:2:1 ratio.2023-1-26The basic principles of inheritance44The actual numbers seem to bear this out,giving credence to the hypothesis.We may ask whether data really do support a particular hypothesis.This question is critical,since the value of a hypothesis depends on its

38、 ability to explain the data.A hypothesis that does not fit needs to be modified or discarded altogether in favor of something better.2023-1-26The basic principles of inheritance453 The Chi-Square Test(卡方检验卡方检验)2023-1-26The basic principles of inheritance46Comparison of the observed and expected res

39、ults and calculation of X2 for an intercross with hybrid snapdragons.2023-1-26The basic principles of inheritance47The degree of freedom(自由度自由度)Critical value(可信区间可信区间)2023-1-26The basic principles of inheritance48Key PointsThe chi-square statistic is calculated as X2=(observed number expected numbe

40、r)2/expected number,with the sum computed over all categories comprising the data.Each chi-square statistic is associated with an index,the degree of freedom,which is equal to the number of data categories minus one.2023-1-26The basic principles of inheritance49Section 5Mendelian Principles in Human

41、 GeneticsMendels principles can be applied to study the inheritance of traits in humans.2023-1-26The basic principles of inheritance50显性：显性：软骨发育不全、软骨发育不全、短指、先天性夜盲、皮短指、先天性夜盲、皮肤弹性过度综合征、肤弹性过度综合征、Huntington氏病、马凡氏病、马凡氏综合症、神经纤维瘤、氏综合症、神经纤维瘤、苯硫脲尝味试验、克夫苯硫脲尝味试验、克夫相、羊毛状发相、羊毛状发隐性：隐性：白化病、尿黑酸白化病、尿黑酸症、共济失调毛细血管症、共济

42、失调毛细血管扩张、囊性纤维瘤、杜扩张、囊性纤维瘤、杜氏肌无力、半乳糖血症、氏肌无力、半乳糖血症、糖原存储症、苯丙酸尿糖原存储症、苯丙酸尿症、链状细胞贫血、症、链状细胞贫血、Tay-Sachs 病等病等2023-1-26The basic principles of inheritance51This is widows peak!这是一个长寿的显性表型？这是一个长寿的显性表型？中国人的美人尖！中国人的美人尖！2023-1-26The basic principles of inheritance521 Pedigrees(家系家系)Pedigrees are diagrams that sho

43、w the relationships among the members of a family.Pedigree conventions2023-1-26The basic principles of inheritance53Which is dominance?Which is recessive?2023-1-26The basic principles of inheritance542 Mendelian Segregation in Human FamiliesIn human beings,the number of children produced by a couple

44、 is typically small.Today in China,one child one family!In USA,the average is around two!In developing countries,it is six to seven!Such numbers provide nothing close to the statistical power that Mendel had in his experiments with peas.2023-1-26The basic principles of inheritance55Consequently,phen

45、otypic ratios in human families often deviate significantly from their Mendelian expectationAs an example,let us consider a couple who are each heterozygous for a recessive allele that,in homozygous condition,causes cystic fibrosis(囊性纤维瘤囊性纤维瘤).If the couple were to have four children,would we expect

46、 exactly three to be unaffected and one to be affected by cystic fibrosis?2023-1-26The basic principles of inheritance56 The answer is no.Although this is possible outcome,it is not only one.There are,in fact,five distinct possibilities:1.Four unaffected,none affected.2.Three unaffected,one affected

47、.3.Two unaffected,two affected.4.One unaffected,three affected.5.None unaffected,four affected.2023-1-26The basic principles of inheritance57Intuitively,the second outcome seems to be the most likely,since it conforms to Mendels 3:1 ratio.We can calculate the probability of this outcome,and each of

48、the others,by using Mendels principles and by treating each birth as an independent event.2023-1-26The basic principles of inheritance582023-1-26The basic principles of inheritance594 Genetic Counseling(遗传咨询遗传咨询)2023-1-26The basic principles of inheritance60Pedigree showing the inheritance of heredi

49、tary nonpolypoid colorectal cancer.2023-1-26The basic principles of inheritance61In this example the children fall into two possible phenotypic classes.Because there are only two classes,the probabilities associated with the various outcomes are called binomial probabilities(二项可能性二项可能性).2023-1-26The

50、 basic principles of inheritance62a)A Cuna Indian child from Panama with Albinismb)Pedigree showing the inheritance of albinismc)Punnett square showing that among offspring without albinism,the frequency of heterozygotes is 2/3.？2023-1-26The basic principles of inheritance63In summary,the risk that