Chapter 14Slide 2Slide 3Slide 4Slide 5Mendel’s ModelSlide 7Slide 9Slide 10Slide 11Dihybrid CrossIncomplete DominanceSlide 14EpistasisPolygenic InheritanceMultifactorial charactersPedigree AnalysisDominantly Inherited DisordersSlide 20Fetal testingSlide 22Slide 23You should now be able to:Slide 25PowerPoint Lectures for Biology, Eighth EditionNeil Campbell and Jane ReeceChapter 14Chapter 14Mendel and the Gene Idea•Gregor Mendel documented a particulate mechanism of inheritance through his experiments with garden peas•Crossing 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.TECHNIQUETECHNIQUEWhen 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.TECHNIQUERESULTS•Mendel discovered–A ratio of about three to one, purple to white flowers, in the F2 generationFigure 14.3P Generation(true-breeding parents) PurpleflowersWhiteflowersF1 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 observed the same pattern–In many other pea plant characters1) Alternative versions of genes–Account for variations in inherited characters, which are now called allelesFigure 14.4Allele for purple flowersLocus for flower-color geneHomologouspair ofchromosomesAllele for white flowersMendel’s Model–Using a Punnett square allows us to see the possible offspring outcomes from a particular cross.AAaaAA AaAa aaPossible Alleles in gametes from Parent 1Possible Alleles in Gametes from Parent 2•Probability in a monohybrid crossRrSegregation ofalleles into eggsRrSegregation ofalleles into spermRrrRRRR1⁄21⁄21⁄21⁄41⁄41⁄41⁄41⁄2rrRrrSpermEggs•Mendel’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 eggs1/21/2Each 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.•Phenotype versus genotypeFigure 14.631121PhenotypePurplePurplePurpleWhiteGenotypePP(homozygous)Pp(heterozygous)Pp(heterozygous)pp(homozygous)Ratio 3:1Ratio 1:2:1•The testcrossFigure 14.7Dominant 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:ppPPPpPpPpPpppppPpPpPpp pAPPLICATION 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.RESULTSYYRRP GenerationGametes YR yryyrrYyRrHypothesis ofdependentassortmentHypothesis ofindependentassortmentF2 Generation(predictedoffspring)1⁄2YRYRyr1 ⁄21 ⁄21⁄2yrYYRR YyRryyrrYyRr3 ⁄41 ⁄4SpermEggsPhenotypic ratio 3:1YR1 ⁄4Yr1 ⁄4yR1 ⁄4yr1 ⁄49 ⁄163 ⁄163 ⁄161 ⁄16YYRRYYRrYyRRYyRrYyrrYyRrYYrrYYrrYyRRYyRryyRR yyRryyrryyRrYyrrYyRrPhenotypic ratio 9:3:3:1315 10810132Phenotypic ratio approximately 9:3:3:1F1 GenerationEggsYRYryR yr1 ⁄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.Dihybrid Cross•A dihybrid cross illustrates the inheritance of two characters•Produces four phenotypes in the F2 generationFigure 14.8Incomplete Dominance•In incomplete dominance the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varietiesFigure 14.10P GenerationF1 GenerationF2 GenerationRedCRCRGametesCRCWWhiteCWCWPinkCRCWSpermCRCRCRCwCRCRGametes1⁄21⁄21⁄21⁄21⁄2Eggs1⁄2CR CRCR CWCW CWCR CW•The ABO blood group in humans–Is determined by multiple allelesTable 14.2Epistasis•An example of epistasisFigure 14.11BCbC
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