Bài giảng Biology - Chapter 14: Mendel and the Gene Idea

Chapter 14Mendel and the Gene IdeaOverview: Drawing from the Deck of GenesWhat genetic principles account for the transmission of traits from parents to offspring?One possible explanation of heredity is a “blending” hypothesisThe idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make greenAn alternative to the blending model is the “particulate” hypothesis of inheritance: the gene ideaParents pass on discrete heritable units, genesGregor MendelDocumented a particulate mechanism of inheritance through his experiments with garden peasFigure 14.1Concept 14.1: Mendel used the scientific approach to identify two laws of inheritanceMendel discovered the basic principles of heredityBy breeding garden peas in carefully planned experimentsMendel’s Experimental, Quantitative ApproachMendel chose to work with peasBecause they are available in many varietiesBecause he could strictly control which plants mated with whichCrossing pea plantsFigure 14.215432Removed stamensfrom purple flowerTransferred sperm-bearing pollen fromstamens of white flower to egg-bearing carpel of purple flower Parentalgeneration(P)Pollinated carpelmatured into podCarpel(female)Stamens(male)Planted seedsfrom podExaminedoffspring:all purpleflowersFirstgenerationoffspring(F1)APPLICATION By crossing (mating) two true-breedingvarieties of an organism, scientists can study patterns ofinheritance. In this example, Mendel crossed pea plantsthat varied in flower color.TECHNIQUETECHNIQUE When pollen from a white flower fertilizeseggs of a purple flower, the first-generation hybrids all have purpleflowers. The result is the same for the reciprocal cross, the transferof pollen from purple flowers to white flowers.TECHNIQUERESULTSSome genetic vocabularyCharacter: a heritable feature, such as flower colorTrait: a variant of a character, such as purple or white flowersMendel chose to trackOnly those characters that varied in an “either-or” mannerMendel also made sure thatHe started his experiments with varieties that were “true-breeding”In a typical breeding experimentMendel mated two contrasting, true-breeding varieties, a process called hybridizationThe true-breeding parentsAre called the P generationThe hybrid offspring of the P generationAre called the F1 generationWhen F1 individuals self-pollinateThe F2 generation is producedThe Law of SegregationWhen Mendel crossed contrasting, true-breeding white and purple flowered pea plantsAll of the offspring were purpleWhen Mendel crossed the F1 plantsMany of the plants had purple flowers, but some had white flowersMendel discoveredA ratio of about three to one, purple to white flowers, in the F2 generationFigure 14.3P Generation(true-breeding parents) PurpleflowersWhiteflowersF1 Generation (hybrids)All plants hadpurple flowersF2 Generation EXPERIMENT True-breeding purple-flowered pea plants andwhite-flowered pea plants were crossed (symbolized by ). Theresulting F1 hybrids were allowed to self-pollinate or were cross-pollinated with other F1 hybrids. Flower color was then observedin the F2 generation.RESULTS Both purple-flowered plants and white-flowered plants appeared in the F2 generation. In Mendel’sexperiment, 705 plants had purple flowers, and 224 had whiteflowers, a ratio of about 3 purple : 1 white.Mendel reasoned thatIn the F1 plants, only the purple flower factor was affecting flower color in these hybridsPurple flower color was dominant, and white flower color was recessiveMendel observed the same patternIn many other pea plant charactersTable 14.1Mendel’s ModelMendel developed a hypothesisTo explain the 3:1 inheritance pattern that he observed among the F2 offspringFour related concepts make up this modelFirst, alternative versions of genesAccount for variations in inherited characters, which are now called allelesFigure 14.4Allele for purple flowersLocus for flower-color geneHomologouspair ofchromosomesAllele for white flowers Second, for each characterAn organism inherits two alleles, one from each parentA genetic locus is actually represented twiceThird, if the two alleles at a locus differThen one, the dominant allele, determines the organism’s appearanceThe other allele, the recessive allele, has no noticeable effect on the organism’s appearanceFourth, the law of segregationThe two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametesDoes Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses?We can answer this question using a Punnett squareMendel’s law of segregation, probability and the Punnett squareFigure 14.5P GenerationF1 GenerationF2 GenerationPpPpPpPpPpPPppPpAppearance:Genetic makeup:Purple flowersPPWhite flowersppPurple flowersPpAppearance:Genetic makeup:Gametes:Gametes:F1 spermF1 eggs1/21/2Each true-breeding plant of the parental generation has identicalalleles, PP or pp.Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple-flower allele is dominant, allthese hybrids have purple flowers.When the hybrid plants producegametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.3: 1Random combination of the gametesresults in the 3:1 ratio that Mendelobserved in the F2 generation.This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1  F1 (Pp  Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottomleft box shows the genetic combinationresulting from a p egg fertilized bya P sperm. Useful Genetic VocabularyAn organism that is homozygous for a particular geneHas a pair of identical alleles for that geneExhibits true-breedingAn organism that is heterozygous for a particular geneHas a pair of alleles that are different for that geneAn organism’s phenotypeIs its physical appearanceAn organism’s genotypeIs its genetic makeupPhenotype versus genotypeFigure 14.631121PhenotypePurplePurplePurpleWhiteGenotypePP(homozygous)Pp(heterozygous)Pp(heterozygous)pp(homozygous)Ratio 3:1Ratio 1:2:1The TestcrossIn pea plants with purple flowersThe genotype is not immediately obviousA testcrossAllows us to determine the genotype of an organism with the dominant phenotype, but unknown genotypeCrosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait The testcrossFigure 14.7 Dominant phenotype,unknown genotype:PP or Pp? Recessive phenotype,known genotype:ppIf PP,then all offspringpurple:If Pp,then 1⁄2 offspring purpleand 1⁄2 offspring white:ppPPPpPpPpPpppppPpPpPpppAPPLICATION An organism that exhibits a dominant trait,such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organism’sgenotype, geneticists can perform a testcross.TECHNIQUE In a testcross, the individual with theunknown genotype is crossed with a homozygous individualexpressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.RESULTSThe Law of Independent AssortmentMendel derived the law of segregationBy following a single traitThe F1 offspring produced in this crossWere monohybrids, heterozygous for one characterMendel identified his second law of inheritanceBy following two characters at the same timeCrossing two, true-breeding parents differing in two charactersProduces dihybrids in the F1 generation, heterozygous for both charactersHow are two characters transmitted from parents to offspring?As a package?Independently?YYRRP GenerationGametesYRyryyrrYyRrHypothesis ofdependentassortmentHypothesis ofindependentassortmentF2 Generation(predictedoffspring)1⁄2YRYRyr1 ⁄21 ⁄21⁄2yrYYRRYyRryyrrYyRr3 ⁄41 ⁄4SpermEggsPhenotypic ratio 3:1YR1 ⁄4Yr1 ⁄4yR1 ⁄4yr1 ⁄49 ⁄163 ⁄163 ⁄161 ⁄16YYRRYYRrYyRRYyRrYyrrYyRrYYrrYYrrYyRRYyRryyRRyyRryyrryyRrYyrrYyRrPhenotypic ratio 9:3:3:131510810132Phenotypic ratio approximately 9:3:3:1F1 GenerationEggsYRYryRyr1 ⁄41 ⁄41 ⁄41 ⁄4SpermRESULTSCONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. EXPERIMENT Two true-breeding pea plants—one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.A dihybrid crossIllustrates the inheritance of two charactersProduces four phenotypes in the F2 generationFigure 14.8Using the information from a dihybrid cross, Mendel developed the law of independent assortmentEach pair of alleles segregates independently during gamete formationConcept 14.2: The laws of probability govern Mendelian inheritanceMendel’s laws of segregation and independent assortmentReflect the rules of probabilityThe Multiplication and Addition Rules Applied to Monohybrid CrossesThe multiplication ruleStates that the probability that two or more independent events will occur together is the product of their individual probabilities Probability in a monohybrid crossCan be determined using this ruleRrSegregation ofalleles into eggsRrSegregation ofalleles into spermRrrRRRR1⁄21⁄21⁄21⁄41⁄41⁄41⁄41⁄2rrRrrSpermEggsFigure 14.9The rule of additionStates that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilitiesSolving Complex Genetics Problems with the Rules of ProbabilityWe can apply the rules of probabilityTo predict the outcome of crosses involving multiple charactersA dihybrid or other multicharacter crossIs equivalent to two or more independent monohybrid crosses occurring simultaneouslyIn calculating the chances for various genotypes from such crossesEach character first is considered separately and then the individual probabilities are multiplied togetherConcept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian geneticsThe relationship between genotype and phenotype is rarely simpleExtending Mendelian Genetics for a Single GeneThe inheritance of characters by a single geneMay deviate from simple Mendelian patternsThe Spectrum of Dominance Complete dominanceOccurs when the phenotypes of the heterozygote and dominant homozygote are identicalIn codominanceTwo dominant alleles affect the phenotype in separate, distinguishable waysThe human blood group MNIs an example of codominanceIn incomplete dominanceThe phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varietiesFigure 14.10P GenerationF1 GenerationF2 GenerationRedCRCRGametesCRCWWhiteCWCWPinkCRCWSpermCRCRCRCwCRCRGametes1⁄21⁄21⁄21⁄21⁄2Eggs1⁄2CR CRCR CWCW CWCR CWThe Relation Between Dominance and PhenotypeDominant and recessive allelesDo not really “interact”Lead to synthesis of different proteins that produce a phenotypeFrequency of Dominant AllelesDominant allelesAre not necessarily more common in populations than recessive allelesMultiple AllelesMost genes exist in populationsIn more than two allelic formsThe ABO blood group in humansIs determined by multiple allelesTable 14.2PleiotropyIn pleiotropyA gene has multiple phenotypic effectsExtending Mendelian Genetics for Two or More GenesSome traitsMay be determined by two or more genesEpistasisIn epistasisA gene at one locus alters the phenotypic expression of a gene at a second locusAn example of epistasisFigure 14.11BCbCBcbc1⁄41⁄41⁄41⁄4BCbCBcbc1⁄41⁄41⁄41⁄4BBCcBbCcBBccBbccBbccbbccbbCcBbCcBbCCbbCCBbCcbbCcBBCCBbCCBBCcBbCc9⁄163⁄164⁄16BbCcBbCcSpermEggsPolygenic InheritanceMany human charactersVary in the population along a continuum and are called quantitative charactersAaBbCcAaBbCcaabbccAabbccAaBbccAaBbCcAABbCcAABBCcAABBCC20⁄6415⁄646⁄641⁄64Fraction of progenyQuantitative variation usually indicates polygenic inheritanceAn additive effect of two or more genes on a single phenotypeFigure 14.12Nature and Nurture: The Environmental Impact on PhenotypeAnother departure from simple Mendelian genetics arisesWhen the phenotype for a character depends on environment as well as on genotypeThe norm of reactionIs the phenotypic range of a particular genotype that is influenced by the environmentFigure 14.13Multifactorial charactersAre those that are influenced by both genetic and environmental factorsIntegrating a Mendelian View of Heredity and VariationAn organism’s phenotypeIncludes its physical appearance, internal anatomy, physiology, and behaviorReflects its overall genotype and unique environmental historyEven in more complex inheritance patternsMendel’s fundamental laws of segregation and independent assortment still applyConcept 14.4: Many human traits follow Mendelian patterns of inheritanceHumans are not convenient subjects for genetic researchHowever, the study of human genetics continues to advancePedigree AnalysisA pedigreeIs a family tree that describes the interrelationships of parents and children across generationsInheritance patterns of particular traitsCan be traced and described using pedigreesFigure 14.14 A, BWwwwwwWwwwWwWwwwwwWwWWorWwwwFirst generation(grandparents)Second generation(parents plus auntsand uncles)Thirdgeneration(two sisters)FfFfffFfffFfFfffFfFF or FfffFForFfWidow’s peakNo Widow’s peakAttached earlobeFree earlobe(a) Dominant trait (widow’s peak)(b) Recessive trait (attached earlobe)PedigreesCan also be used to make predictions about future offspringRecessively Inherited DisordersMany genetic disordersAre inherited in a recessive mannerRecessively inherited disordersShow up only in individuals homozygous for the alleleCarriersAre heterozygous individuals who carry the recessive allele but are phenotypically normalCystic FibrosisSymptoms of cystic fibrosis includeMucus buildup in the some internal organsAbnormal absorption of nutrients in the small intestineSickle-Cell DiseaseSickle-cell diseaseAffects one out of 400 African-AmericansIs caused by the substitution of a single amino acid in the hemoglobin protein in red blood cellsSymptoms includePhysical weakness, pain, organ damage, and even paralysisMating of Close RelativesMatings between relativesCan increase the probability of the appearance of a genetic disease Are called consanguineous matingsDominantly Inherited DisordersSome human disordersAre due to dominant allelesOne example is achondroplasiaA form of dwarfism that is lethal when homozygous for the dominant alleleFigure 14.15Huntington’s diseaseIs a degenerative disease of the nervous systemHas no obvious phenotypic effects until about 35 to 40 years of ageFigure 14.16Multifactorial DisordersMany human diseasesHave both genetic and environment componentsExamples includeHeart disease and cancerGenetic Testing and CounselingGenetic counselorsCan provide information to prospective parents concerned about a family history for a specific diseaseCounseling Based on Mendelian Genetics and Probability RulesUsing family historiesGenetic counselors help couples determine the odds that their children will have genetic disordersTests for Identifying CarriersFor a growing number of diseasesTests are available that identify carriers and help define the odds more accuratelyFetal TestingIn amniocentesisThe liquid that bathes the fetus is removed and testedIn chorionic villus sampling (CVS)A sample of the placenta is removed and testedFetal testingFigure 14.17 A, B(a) AmniocentesisAmnioticfluidwithdrawnFetusPlacentaUterusCervixCentrifugationA sample ofamniotic fluid canbe taken starting atthe 14th to 16thweek of pregnancy.(b) Chorionic villus sampling (CVS)FluidFetalcellsBiochemical tests can bePerformed immediately onthe amniotic fluid or lateron the cultured cells.Fetal cells must be culturedfor several weeks to obtainsufficient numbers forkaryotyping.SeveralweeksBiochemicaltestsSeveralhoursFetalcellsPlacentaChorionic viIIiA sample of chorionic villustissue can be taken as earlyas the 8th to 10th week ofpregnancy.Suction tubeInserted throughcervixFetusKaryotyping and biochemicaltests can be performed onthe fetal cells immediately,providing results within a dayor so.KaryotypingNewborn ScreeningSome genetic disorders can be detected at birthBy simple tests that are now routinely performed in most hospitals in the United States