遗传学的基本原理课件

1、2022/7/29The basic principles of inheritance1Chapter 3Mendelism: The Basic Principles of inheritance2022/7/29The 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)Mendelian principles in hu

2、man genetics (人类遗传学中的孟德尔原理) 2022/7/29The basic principles of inheritance3The Birth of Genetics: A Scientific RevolutionPisum sativumThe subject of Gregor Mendels experiments.2022/7/29The basic principles of inheritance4Section 1Mendels Study of HeredityGregor Mendels experiments with peas elucidated

3、 how traits are inherited.2022/7/29The basic principles of inheritance51 Mendels Life Story1822-18842022/7/29The basic principles of inheritance62 Mendels Experimental Organism, the Garden PeaThe garden pea, Pisum sativum, is a dicot (双子叶) and a true-breeding (纯种).One peculiarity of pea reproduction

4、 is that the petals of flower close down tightly, preventing pollen grains from entering or leaving.2022/7/29The basic principles of inheritance73 Monohybrid Crosses: the Principles of Dominance and Segregation2022/7/29The basic principles of inheritance8柱头雌蕊花药雄蕊去势的2022/7/29The basic principles of i

5、nheritance92022/7/29The 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 varieties had the ability t

6、o produce dwarf progeny even through they themselves were tall.2022/7/29The basic principles of inheritance11He said the latent factor was recessive (隐性) and that the expressed factor was dominant (显性).These recessive and dominant factors separated from each other when the hybrid plants reproduced.T

7、his enable to explain the reappearance of the dwarf characteristic in the next generation.2022/7/29The 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 pairs.Each of parental strai

8、ns carried two identical copies of a genein modern terminology, they are diploid and homozygous(纯合子). 2022/7/29The 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 single copy of a genein mod

9、ern terminology, they are haploid.Diploid gene number would be restored when sperm and egg unite to form a zygote.2022/7/29The basic principles of inheritance14If the sperm and egg came from genetically different plants, the hybrid zygote would inherit two different alleles, one from the mother and

10、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 heterozygote reproduces.2

11、022/7/29The 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 reappearance the recess

12、ive characteristic in the progeny of the hybrid plants.2022/7/29The basic principles of inheritance16Mendel used symbols to present the hereditary factors that he postulateda methodological breakthrough. 这是一个创举2022/7/29The basic principles of inheritance17The genotype is the genetic constitution of

13、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 基因型是一个细胞、一个生物或一个个体的遗传构成形式(比如组成个体的特定等位基因)，通常与其控制的特殊性状相关。2022/7/29The basic principles of inheritance18A phenotype is any observable characterist

14、ic or trait of an organism: such as its morphology, development, biochemical or physiological properties, or behavior. 表型是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。Phenotypes result from the expression of an organisms genes as well as the influence of environmental factors and possible interactions betwe

15、en the two. 2022/7/29The 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 corresponding uppercase letter D.As the parental strain, the tall and dwarf pea plants form the P

16、generation of the experiment.Their hybrid progeny are referred to as the first filial (子女) generation, or F1.2022/7/29The basic principles of inheritance20Upon self-fertilization, the two kinds of gametes produced by heterozygotes can unite in all possible ways.Thus, they produce four kinds of zygot

17、es: 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 3:1.2022/7/29The basic principles of inheritance21Which results would happen when did this analysis one step

18、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.Approximately 1/3 of them produced only tall offspring, whereas the other 2

19、/3 produced a mixture of tall and dwarf offspring.2022/7/29The basic principles of inheritance22Mendel concluded:The third that were true-breeding were DD homozygotes and that the two-thirds that were segregating were Dd heterozygotes.These proportions, 1/3 and 2/3, were exactly what his analysis pr

20、edicted because, among the tall F2 plants, the DD and Dd genotypes occur in a ratio of 1:2.2022/7/29The basic principles of inheritance23Mendel discovered two key principlesThe Principle of Dominance: In a heterozygote, one allele may conceal the presence of another. This principle is a statement ab

21、out genetic function. Some alleles evidently control the phenotype even when they are present in a single copy.2022/7/29The basic principles of inheritance24The Principle of Segregation: In a heterozygote, two different alleles segregate from each other during the formation of gametes. This principl

22、e 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2022/7/29The basic principles of inheritance254. Dihybrid Crosses: the Principle of Independent Assortment2022/7/29The basic pri

23、nciples of inheritance262022/7/29The basic principles of inheritance27Yellow, round9/16Yellow, wrinkled3/16Green, round3/16Green, wrinkled1/162022/7/29The basic principles of inheritance28This analysis is predicated on two assumptions(以2个假设为基础):each gene segregates its alleles, andthese segregations

24、 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 as likely to receive G as it is to receive g through the segre

25、gation of the color gene.2022/7/29The basic principles of inheritance29Do the experimental data fit with the predictions of our analysis?2022/7/29The basic principles of inheritance30Mendel conducted similar experiments with other combinations of traits and in each case observed that the genes segre

26、gated independently. These results of theses experiments led him to a third key principle:The Principle of Independent Assortment: The alleles of different genes segregate, or as we sometimes say, assort, independently of each other. 2022/7/29The basic principles of inheritance31Key PointsMendel stu

27、died 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, different alleles of a gene segregate from each other during t

28、he formation of gametes, and the alleles of different genes assort independently.2022/7/29The basic principles of inheritance32Applications of Mendels PrinciplesMendels principles can be used to predict the outcomes of crosses between different strains of organisms.Section 32022/7/29The basic princi

29、ples 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 and one relying on mathematical insight.2022/7/29T

30、he basic principles of inheritance341 The Punnett Square Method(1875-1967) British GeneticistIntercrosse互交2022/7/29The basic principles of inheritance352 The Forked-Line MethodThe forked-line method for predicting the outcome of an intercross involving three independently assorting genes in peas.202

31、2/7/29The basic principles of inheritance36The forked-line method for predicting the outcome of a testcross involving three independently assorting genes in peas.2022/7/29The basic principles of inheritance373 The Probability MethodAn intercross showing the probability method in the context of a Pun

32、nett square.2022/7/29The basic principles of inheritance38Application of the probability method to an intercross involving two genes.2022/7/29The basic principles of inheritance39Key PointThe outcome of a cross can be predicted by the systematic enumeration of genotypes using a Punnett square.When m

33、ore than two genes are involved, the forked-line or probability method is used to predict the outcome of a cross.2022/7/29The basic principles of inheritance40Section 4Formulating and Testing Genetic HypothesesThe chi-square test is a simple way of evaluating whether the predictions of a genetic hyp

34、othesis agree with data from an experiment.2022/7/29The 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 determine if a particular hypothesis should be ac

35、cepted or rejected.2022/7/29The basic principles of inheritance42Snapdragons, Antirrhinum majus (金鱼草)Do this color differences have a genetic basis?2022/7/29The 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),

37、 Ww (pink), and ww (whiter) F2 progeny in a 1:2:1 ratio.2022/7/29The 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 hypothes

38、is depends on its ability to explain the data.A hypothesis that does not fit needs to be modified or discarded altogether in favor of something better.2022/7/29The basic principles of inheritance453 The Chi-Square Test (卡方检验)2022/7/29The basic principles of inheritance46Comparison of the observed an

39、d expected results and calculation of X2 for an intercross with hybrid snapdragons.2022/7/29The basic principles of inheritance47The degree of freedom (自由度)Critical value (可信区间)2022/7/29The basic principles of inheritance48Key PointsThe chi-square statistic is calculated as X2=(observed number expec

40、ted number)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.2022/7/29The basic principles of inheritance49Section 5Mendelian Princip

41、les in Human GeneticsMendels principles can be applied to study the inheritance of traits in humans.2022/7/29The basic principles of inheritance50显性：软骨发育不全、短指、先天性夜盲、皮肤弹性过度综合征、Huntington氏病、马凡氏综合症、神经纤维瘤、苯硫脲尝味试验、克夫相、羊毛状发隐性：白化病、尿黑酸症、共济失调毛细血管扩张、囊性纤维瘤、杜氏肌无力、半乳糖血症、糖原存储症、苯丙酸尿症、链状细胞贫血、Tay-Sachs 病等2022/7/29Th

42、e basic principles of inheritance51This is widows peak!这是一个长寿的显性表型？中国人的美人尖！2022/7/29The basic principles of inheritance521 Pedigrees (家系)Pedigrees are diagrams that show the relationships among the members of a family.Pedigree conventions2022/7/29The basic principles of inheritance53Which is dominan

43、ce? Which is recessive?2022/7/29The basic principles of inheritance542 Mendelian Segregation in Human FamiliesIn human beings, the number of children produced by a couple is typically small.Today in China, one child one family!In USA, the average is around two!In developing countries, it is six to s

44、even!Such numbers provide nothing close to the statistical power that Mendel had in his experiments with peas.2022/7/29The basic principles of inheritance55Consequently, phenotypic ratios in human families often deviate significantly from their Mendelian expectationAs an example, let us consider a c

45、ouple who are each heterozygous for a recessive allele that, in homozygous condition, causes cystic fibrosis (囊性纤维瘤).If the couple were to have four children, would we expect exactly three to be unaffected and one to be affected by cystic fibrosis?2022/7/29The basic principles of inheritance56The an

46、swer is no.Although this is possible outcome, it is not only one.There are, in fact, five distinct possibilities:Four unaffected, none affected.Three unaffected, one affected.Two unaffected, two affected.One unaffected, three affected.None unaffected, four affected.2022/7/29The basic principles of inherita