孟德爾理論的延伸課件

1、2022/7/27Extensions of Mendelism1Chapter 4Extensions of Mendelism2022/7/27Extensions of Mendelism2Chapter OutlineAllelic Variation and Gene FunctionGene Action: from Genotype to Phenotype2022/7/27Extensions of Mendelism3遺傳學發(fā)展中的災難李森科2022/7/27Extensions of Mendelism4Section1 Allelic Variation and Gene

2、 FunctionThe diverse kinds of alleles of genes affect phenotypes in different ways.2022/7/27Extensions of Mendelism5Mendels experiments established that genes can exist in alternate forms.For each of seven traits that he studiedseed color, seed texture, plants height, flower color, flower position,

3、pod shape, and pod colorMendel identified two alleles, one dominant, the other recessive.This discovery suggested a simple functional dichotomy between alleles, as if one allele did nothing and the other did every thing to determine the phenotype.2022/7/27Extensions of Mendelism6However, research ea

4、rly in the twentieth century demonstrated this to be an oversimplification.Genes can exist in more than two allelic states, and each allele can have a different effect on the phenotype.2022/7/27Extensions of Mendelism7Incomplete Dominance and CodominanceIncomplete, or partially dominant 不完全顯性或部分顯性Se

5、midominant 半顯性Codominant 共顯性2022/7/27Extensions of Mendelism8semidominant2022/7/27Extensions of Mendelism9Detection of the M and N antigens on blood cells by agglutination (凝集) with specific anti-sera. With the anti-M and anti-N sera, three blood types can be identified.Codominant人的血型系統(tǒng)2022/7/27Exte

6、nsions of Mendelism10Multiple Alleles (復等位基因)The color-determining gene of rabbits, c, has four alleles, three of which are distinguished by a superscript: c (albino), ch (himalayan), cch (chinchilla, 栗色), and c+ (wild-type)Coat color in rabbits2022/7/27Extensions of Mendelism112022/7/27Extensions o

7、f Mendelism12Allelic Series (等位基因系列)The functional relationship among the members of a series of multiple alleles can be studied by making heterozygous combination through crosses between homozygotes.The four alleles of the c gene in rabbits can be combined with each other to make six different kind

8、s of heterozygotes: chc, cchc, c+c, cchch, c+ch, and c+cch.2022/7/27Extensions of Mendelism13The wild-type allele is completely dominant over all the other alleles in the series; these chinchilla allele is partially dominant over the himalayan and albino alleles, and the himalayan allele is complete

9、ly dominant over the albino allele.These dominance relations can be summarized as c+cchchc.2022/7/27Extensions of Mendelism14Nonfunctional alleles are said to be null or amorphic (無效等位的); they are almost always completely recessive.Partially functional alleles are said to be hypomorphic (亞等位的); they

10、 are recessive to alleles that are more functional, including (usually) the wild-type allele.2022/7/27Extensions of Mendelism15Testing Gene Mutations for Allelism(等位性的基因突變測定)A mutant allele is created when an excising allele changes to a new genetic statea process called mutation.2022/7/27Extensions

11、 of Mendelism16這種方法僅能檢測隱性突變！Dominant mutations cannot be tested in this way because they exert their effects even if a wild-type copy of the gene is present.2022/7/27Extensions of Mendelism17Cinnabar 朱砂色Scarlet 深紅色2022/7/27Extensions of Mendelism18Variation Among the Effects of MutationsMutations th

12、at alter some aspect of morphology, such as seed texture or color, are called visible mutations (可見突變). Most visible mutations are recessive, but a few are dominant.2022/7/27Extensions of Mendelism19Mutations that limit reproduction are called sterile mutations(不育性突變).Some sterile mutations affect b

13、oth sexes, but most affect either males or females.As with visible mutations, sterile can be either dominant or recessive.Some sterile completely prevent reproduction, whereas others only impair it slightly.2022/7/27Extensions of Mendelism20Mutations that interfere with necessary vital functions are

14、 called lethal mutations.Their phenotypic effect is death.Dominant lethals that act early in life are lost one generation after they occur because the individuals that carry them die; however, dominant lethal that act later in life, after reproduction, can be passed to the next generation.2022/7/27E

15、xtensions of Mendelism21Recessive lethal may linger a long time in a population because they can be hidden in heterozygous condition by a wild-type allele.Recessive lethal mutations are detected by observing unusual segregation ratios in the progeny of heterozygous carriers.2022/7/27Extensions of Me

16、ndelism22黃色致死突變不同的遺傳學家用不同的字母表示不同的基因！2022/7/27Extensions of Mendelism23Genes Function to Produce PolypeptidesThe extensive variation revealed by mutations indicates that organisms contain many different genes and that these genes can exist in multiple allelic states.How genes actually affect the phen

17、otype?What is it about a gene that enables it to influence a trait such as eye color, seed texture, or plant height?2022/7/27Extensions of Mendelism24One gene, one enzyme2022/7/27Extensions of Mendelism25Each gene specifies a different polypeptide. These polypeptides then function to influence the o

18、rganisms phenotype.2022/7/27Extensions of Mendelism26Why Are Some Mutations Dominant and others Recessive? 2022/7/27Extensions of Mendelism27Recessive mutations often involve a loss of gene function, that is, when the gene no longer specifies a polypeptide or when it specifies a nonfunctional or und

19、erfunctional polypeptide. Recessive mutations are therefore typically loss-of-function alleles.2022/7/27Extensions of Mendelism28Some recessive mutations result in a partial loss of gene function.In the extremities of himalayan homozygous rabbits, the polypeptide specified by this allele is function

20、al, whereas in the rest of the body, it is not.2022/7/27Extensions of Mendelism29Some dominant mutations may also involve a loss of gene function.If the phenotype controlled by a gene is sensitive to the amount of gene product, a loss-of-function mutation can evoke a mutant phenotype in heterozygous

21、 condition with a wild-type allele.In such cases, the wild-type allele, by itself, is not able to supply enough gene product to provide full, normal function.2022/7/27Extensions of Mendelism30Other dominant mutations actually interfere with the functions of the wild-type allele by specifying polypep

22、tides that inhibit, antagonize, or limit the activity of the wild-type polypeptide.Such mutations called dominant-negative mutations.例如：老鼠的短尾基因T2022/7/27Extensions of Mendelism31Some dominant mutations cause a mutant phenotype in heterozygous condition with a wild-type allele because they enhance th

23、e function of the gene product. The enhanced function may arise because the mutation specifies a novel polypeptide or because it causes the wild-type polypeptide to be produced where or when it should not be.Dominant mutations that work in these ways are called gain-of-function mutations.2022/7/27Ex

24、tensions of Mendelism32Antennapedia (Antp)2022/7/27Extensions of Mendelism33Key PointsGenes often have multiple alleles.Mutant alleles may be dominant, recessive, incompletely dominant, or codominant.If a hybrid that inherited a recessive mutation from each of its parents has a mutant phenotype, the

25、n the recessive mutations are alleles of the same gene; if the hybrid has a wild phenotype, then the recessive mutations are alleles of different genes.2022/7/27Extensions of Mendelism34Most gene encode polypeptides.In homozygous condition, recessive mutations often abolish or diminish polypeptide a

26、ctivity.Some dominant mutations produce a polypeptide that interferes with the activity of the polypeptide encoded by the wild-type allele of a gene.Key Points2022/7/27Extensions of Mendelism35Section 2Gene Action: From Genotype to PhenotypePhenotypes depend on both environmental and genetic factors

27、.2022/7/27Extensions of Mendelism36Influence of the EnvironmentDrosophila shibire (麻痹) mutationAt the normal cultural temperature, 25C, shibire flies are viable and fertile, but are extremely sensitive to a sudden shock.When a shibire culture is shaken, the fliestemporarily paralyzedfall to the bott

28、om of the culture.Shibire (Japanese) = paralysis2022/7/27Extensions of Mendelism37However, if a culture of shibire flies is placed at a slightly higher temperature, 29C, all the flies fall to the bottom and die, even without a shock.Thus the phenotype of the shibire mutation is temperature-sensitive

29、.At 25C, the mutation is viable, but at 29C it is lethal.A plausible explanation is that at 25C the mutant gene makes a partially functional protein, but at 29C this protein is totally nonfunctional.2022/7/27Extensions of Mendelism38environment often influences phenotypethe norm of reaction = phenot

30、ypic range due to environmental effectsnorms of reactions are often broadest for polygenic characters.Blue require low pHEnvironmental effects2022/7/27Extensions of Mendelism39Environmental effects: effect of temperatureon pigment expression in Siamese catsEnvironmental effects2022/7/27Extensions of

31、 Mendelism40Environmental Effects on the Expression of Human genes學生：“老師，你為什么是光頭？”老師：“這叫聰明絕頂！”學生：“那有的人剃了頭就變得聰明絕頂了？是嗎？”老師：“那叫自作聰明！”testosterone2022/7/27Extensions of Mendelism41Penetrance and Expressivity 外顯率和表現(xiàn)度When individuals do not show a trait even though they have the appropriate genotype, the

32、trait is said to exhibit incomplete penetrance (不完全外顯).For example: polydactyly (多指)2022/7/27Extensions of Mendelism42The term expressivity (表現(xiàn)度) is used if a trait is not manifested uniformly among the individuals that show it.The dominant Lobe eye mutation in Drosophila is and example.The phenotyp

33、e associated with this mutation is extremely variable.The Lobe mutation is therefore said to have variable expressivity (可變表現(xiàn)度).Each fly is heterozygous for this dominant mutation, however, the phenotypes vary from complete absence of the eye to nearly wild-type eye.2022/7/27Extensions of Mendelism4

34、3Gene InteractionsComp shapes in chickens of different breeds.Rose, Wyandottes; Pea, Brahmas;Walnut, hybrid from cross between chickens with rose and pea combs;Single, Lefhorns.2022/7/27Extensions of Mendelism44The comb type is determined by two independently assorting genes, R and P, each with two

35、alleles.The work of Bateson & Punnette demonstrated that two independently assorting genes can affect a trait.2022/7/27Extensions of Mendelism45Epistasis (上位性)When two or more genes influence a trait, an allele of one of them may have an overriding effect on the phenotype.When an allele has such an

36、overriding effect, it is said to be epistatic to the other genes that are involved.2022/7/27Extensions of Mendelism46Bateson and Punnetts experiment on the genetic control of flower color in sweet peas. White and purple flowers of the sweet pea. 2022/7/27Extensions of Mendelism47Enzyme 1Enzyme 2Colo

37、r genetics in Cape Primrose - an explanation for how epistatis works.Enzyme 1Enzyme 2Color genetics in Cape Primrose - an explanation for how epistatis works.GenotypeF functionsf does notF/F or F/fCould be mauve or blue, but not roseEnzyme 1Enzyme 2Color genetics in Cape Primrose - an explanation fo

38、r how epistatis works.GenotypeF functionsf does notf/fXWill be rose regardless of genotype at gene D - gene O is epistatic to gene D 2022/7/27Extensions of Mendelism50GenePrecursorintermediateAnthocyaninCPGenotypeC- P-cc- P-C- pp+-+-2022/7/27Extensions of Mendelism512022/7/27Extensions of Mendelism5

39、2Triangular seed capsules of shepherds purse, Bursa bursa-pastoris (芥菜).2022/7/27Extensions of Mendelism532022/7/27Extensions of Mendelism54西葫蘆的色素調控2022/7/27Extensions of Mendelism55西葫蘆的色素調控2022/7/27Extensions of Mendelism56Pleiotropy (基因多效性)Not only is it true that a phenotype can be influenced by

40、many genes, but it is also true that a gene can influence many phenotypes,When a gene affects many aspects of a phenotype, it is said to be pleiotropic.2022/7/27Extensions of Mendelism57Key PointsGene actions is affected by biological and physical factors in the environment.Two or more genes may inf

41、luence a trait.A mutant allele is epistatic to a mutant allele of another gene if it has an overriding effect on the phenotype.A gene is pleiotropic if it influences many different phenotypes.2022/7/27Extensions of Mendelism58Section 3Inbreeding: Another Look at PedigreesGeneticists use a simple sta

42、tistic, the inbreeding coefficient, to analyze the effects of mating between relatives.2022/7/27Extensions of Mendelism59Geneticists have always been interested in the phenomenon of inbreeding, whether to make true-breeding strains or to reveal the homozygous effects of recessive alleles.In addition

43、, when inbreeding occurs in nature, it can affect the character of plant and animal populations.2022/7/27Extensions of Mendelism601. Effects of InbreedingInbreeding occurs when mates are related to each other by virtue of common ancestry.A mating between relatives is often referred to as a consangui

44、neous mating (of the same blood).In human population, these types of matings are rare, with the incidence depending on cultural and ethnic traditions and on geography.2022/7/27Extensions of Mendelism61In many cultures, marriages between close relativesfor example, between siblings or half siblingsar

45、e expressly forbidden, and marriage between more distant relatives, though allowed, must be approved by civil or religious authorities before they can occur.The season for these restrictions is that inbreeding increases the frequency of individuals who are homozygous for recessive alleles that cause

46、 disorders such as mental retardation, Phenylketonuria, and cystic fibrosis.2022/7/27Extensions of Mendelism62近親結婚家系圖近親結婚的危害在遺傳學建立之前人們就已經(jīng)知道了！2022/7/27Extensions of Mendelism63The effects of inbreeding are also evident in experimental species where it is possible to arrange matings between relatives.

47、For example, animals such as rats, mice, and guinea pigs can be mated brother to sister, generation after generation, to create an inbred line (近交系).2022/7/27Extensions of Mendelism64Although these lines are genetically quite purethat is, they do not segregate different alleles of particular genesth

48、ey often are less vigorous than lines maintained by matings between unrelated individuals.We refer to this loss of vigor as inbreeding depression (近交抑制).In plants where self-fertilization is possible, very highly inbred lines can be created by repeated self-fertilization over several generations.202

49、2/7/27Extensions of Mendelism652022/7/27Extensions of Mendelism66Increase in frequency of homozygotes with repeated self-fertilizationInbreeding creates genotypes that are homozygotes for many of their genes.2022/7/27Extensions of Mendelism67Inbred strains that have been produced by repeated self-fe

50、rtilization would be expected to be homozygous for different alleles of the genes that were segregating in the original population.One strain might have be AA bb CC dd, and another might be aa BB cc DD.A cross between two such strains would produce offspring heterozygotes for many different genes.20

51、22/7/27Extensions of Mendelism68Such multiple-gene heterozygotes often exhibit much greater vigor than their inbred parents.Geneticists call this phenomenon of increased vigor in the hybrid offspring of inbred parents heterosis (雜種優(yōu)勢).2022/7/27Extensions of Mendelism692. Genetic Analysis of Inbreedi

52、ngHalf siblings半同胞Full siblings全同胞兄弟姐妹間的婚配在這種情況下，有3個后代，即：n=32022/7/27Extensions of Mendelism70First, we identify the common ancestor(s) of the inbred individuals.The second step in calculating the inbreeding coefficient is to count the number of individuals (n) in each inbreeding loop defined by a c

53、ommon ancestor.The third step in the procedure to calculate the inbreeding coefficient is to compute the quantity (1/2)n for each inbreeding loop and then sum the result.2022/7/27Extensions of Mendelism71T攜帶特殊基因的幾率是多大呢？2022/7/27Extensions of Mendelism72三代血親近親婚配的形式2022/7/27Extensions of Mendelism733.

54、 Measuring Genetic RelationshipThe inbreeding coefficient can also be used to measure the closeness of genetic relationships.Full siblings are obviously more closely related than half siblings.Aunt and nephew more closely related than half siblings?Half siblings are more closely related than first cousins (堂兄弟)?Are they more closely related than double first cousins?2022/7/27Extensions of Mendelism74To answer these questions, we must determine the fraction of genes that two relatives share by virtue of comm