3 Figure 14.2 Research Method Crossing Pea Plants 5432Removed stamensfrom purple flowerTransferred sperm-bearing pollen fromstamens of whiteflower to egg-bearing carpel ofpurple flowerParentalgeneration(P)Pollinated carpelmatured into podCarpel(female)Stamens(male)Planted seedsfrom podExaminedoffspring:all purpleflowersFirstoffspring(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.TECHNIQUEWhen 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.RESULTS
4 Figure 14.3 When F1 pea plants with purple flowers are allowed to self-pollinate, what flower color appears in the F2 generationP Generation(true-breedingparents)PurpleflowersWhiteF1 Generation(hybrids)All plants hadpurple flowersF2 GenerationEXPERIMENT 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.
5 Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
6 Figure 14.4 Alleles, alternative versions of a gene Allele for purple flowersLocus for flower-color geneHomologouspair ofchromosomesAllele for white flowers
7 Figure 14.5 Mendel’s law of segregation (layer 1) P GenerationF1 GenerationPpAppearance: Genetic makeup:Purple flower PPWhite flowers ppPurple flowers PpGametes:Each true-breeding plant of theparental generation has identicalalleles, PP or pp.Gametes (circles) each contain onlyone allele for the flower-color gene.In this case, every gamete producedby one parent has the same allele.Union of the parental gametesproduces F1 hybrids having a Ppcombination. 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 Pallele and the other half the p allele.
8 Figure 14.5 Mendel’s law of segregation (layer 2) P GenerationF1 GenerationF2 GenerationPpPpPPppAppearance: Genetic makeup:Purple flower PPWhite flowers ppPurple flowers PpGametes:F1 spermF1 eggs1/2Each true-breeding plant of theparental generation has identicalalleles, PP or pp.Gametes (circles) each contain onlyone allele for the flower-color gene.In this case, every gamete producedby one parent has the same allele.Union of the parental gametesproduces F1 hybrids having a Ppcombination. 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 Pallele and the other half the p allele.This box, a Punnett square, showsall possible combinations of allelesin offspring that result from anF1 F1 (Pp Pp) cross. Each squarerepresents an equally probable productof fertilization. For example, the bottomleft box shows the genetic combinationresulting from a p egg fertilized bya P sperm.Random combination of the gametesresults in the 3:1 ratio that Mendelobserved in the F2 generation.3: 1
9 Figure 14.6 Phenotype versus genotype 312PhenotypePurpleWhiteGenotypePP(homozygous)Pp(heterozygous)ppRatio 3:1Ratio 1:2:1
10 then 1⁄2 offspring purple Figure 14.7 The TestcrossDominant 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:pPPpAPPLICATION An organism that exhibits a dominant trait,such as purple flowers in pea plants, can be either homozygous for the 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.RESULTS
11 Phenotypic ratio approximately 9:3:3:1 Figure 14.8 Do the alleles for seed color and seed shape sort into gametes dependently (together) or independently?YYRRP GenerationGametesYRyryyrrYyRrHypothesis ofdependentassortmentindependentF2 Generation(predictedoffspring)1 ⁄23 ⁄41 ⁄4SpermEggsPhenotypic ratio 3:1YryR9 ⁄163 ⁄161 ⁄16YYRrYyRRYyrrYYrryyRRyyRrPhenotypic ratio 9:3:3:131510810132Phenotypic ratio approximately 9:3:3:1F1 GenerationRESULTSCONCLUSION 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.
12 Figure 14.9 Segregation of alleles and fertilization as chance events RrSegregation ofalleles into eggsalleles into spermRr1⁄21⁄4SpermEggs
13 Figure 14.10 Incomplete dominance in snapdragon color P GenerationF1 GenerationF2 GenerationRedCRCRGametesCRCWWhiteCWCWPinkCRCWSpermCw1⁄2EggsCR CRCR CWCW CW
14 Table 14.2 Determination of ABO Blood Group by Multiple Alleles
15 Figure 14.11 An example of epistasis BCbCBcbc1⁄4BBCcBbCcBBccBbccbbccbbCcBbCCbbCCBBCC9⁄163⁄164⁄16SpermEggs
16 Figure 14.12 A simplified model for polygenic inheritance of skin color AaBbCcaabbccAabbccAaBbccAABbCcAABBCcAABBCC20⁄6415⁄646⁄641⁄64Fraction of progeny
17 Figure 14.13 The effect of environment on phenotype
20 Figure 14.16 Large families as excellent case studies of human genetics
21 Figure 14.17 Testing a fetus for genetic disorders (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.SeveralweeksBiochemicaltestshoursChorionic viIIiA sample of chorionic villustissue can be taken as earlyas the 8th to 10th week ofpregnancy.Suction tubeInserted throughcervixKaryotyping and biochemicaltests can be performed onthe fetal cells immediately,providing results within a dayor so.Karyotyping
22 Unnumbered Figure p.273 George Arlene Sandra Tom Sam Wilma Ann Michael DanielAlanTinaChristopherCarla
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