Chapter 15 chromosomes

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Chapter 15 chromosomes

  1. 1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsPowerPoint® Lecture Presentations forBiologyEighth EditionNeil Campbell and Jane ReeceLectures by Chris Romero, updated by Erin Barley with contributions from Joan SharpChapter 15The Chromosomal Basis ofInheritance
  2. 2. Overview: Locating Genes Along Chromosomes• Mendel’s “hereditary factors” weregenes, though this wasn’t known at the time• Today we can show that genes are located onchromosomes• The location of a particular gene can be seenby tagging isolated chromosomes with afluorescent dye that highlights the geneCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  3. 3. Fig. 15-1
  4. 4. Concept 15.1: Mendelian inheritance has itsphysical basis in the behavior of chromosomes• Mitosis and meiosis were first described in thelate 1800s• The chromosome theory of inheritancestates:– Mendelian genes have specific loci (positions) onchromosomes– Chromosomes undergo segregation and independentassortment• The behavior of chromosomes during meiosiswas said to account for Mendel’s laws ofsegregation and independent assortmentCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  5. 5. Fig. 15-2P Generation Yellow-roundseeds (YYRR)YF1 GenerationYR RR Y rrryyyMeiosisFertilizationGametesGreen-wrinkledseeds (yyrr)All F1 plants produceyellow-round seeds (YyRr)R RYYr ry yMeiosisR RY Yr ry yMetaphase IY YR Rrry yAnaphase Ir ry YMetaphase IIRYRyyyyRRYYrrrryYYR RyRYryrYR1/41/41/41/4F2 GenerationGametesAn F1  F1 cross-fertilization9 : 3 : 3 : 1LAW OF INDEPENDENTASSORTMENT Alleles of geneson nonhomologouschromosomes assortindependently during gameteformation.LAW OF SEGREGATIONThe two alleles for each geneseparate during gameteformation.123321
  6. 6. Fig. 15-2aP GenerationGametesMeiosisFertilizationYellow-roundseeds (YYRR)Green-wrinkledseeds ( yyrr)All F1 plants produceyellow-round seeds (YyRr)yyyrrrYYYRRR
  7. 7. Fig. 15-2b0.5 mmMeiosisMetaphase IAnaphase IMetaphase IIGametesLAW OF SEGREGATIONThe two alleles for each geneseparate during gameteformation.LAW OF INDEPENDENTASSORTMENT Alleles of geneson nonhomologouschromosomes assortindependently during gameteformation.14yr 1 4Yr14 YR3 3F1 Generation1 4yRRRRRRRRR R R RRYYY YYYYYYYYYyr rrrr rrrr r r ryyyyyy yyyyyAll F1 plants produceyellow-round seeds (YyRr)12 21
  8. 8. Fig. 15-2cF2 GenerationAn F1  F1 cross-fertilization9 : 3 : 3 : 13 3
  9. 9. Morgan’s Experimental Evidence: ScientificInquiry• The first solid evidence associating a specificgene with a specific chromosome came fromThomas Hunt Morgan, an embryologist• Morgan’s experiments with fruit flies providedconvincing evidence that chromosomes are thelocation of Mendel’s heritable factorsCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  10. 10. Morgan’s Choice of Experimental Organism• Several characteristics make fruit flies aconvenient organism for genetic studies:– They breed at a high rate– A generation can be bred every two weeks– They have only four pairs of chromosomesCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  11. 11. • Morgan noted wild type, ornormal, phenotypes that were common in thefly populations• Traits alternative to the wild type are calledmutant phenotypesCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  12. 12. Fig. 15-3
  13. 13. Correlating Behavior of a Gene’s Alleles withBehavior of a Chromosome Pair• In one experiment, Morgan mated male flieswith white eyes (mutant) with female flies withred eyes (wild type)– The F1 generation all had red eyes– The F2 generation showed the 3:1 red:whiteeye ratio, but only males had white eyes• Morgan determined that the white-eyed mutantallele must be located on the X chromosome• Morgan’s finding supported the chromosometheory of inheritanceCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  14. 14. Fig. 15-4PGenerationGenerationGenerationGenerationGenerationGenerationF1F2All offspringhad red eyesSpermEggsF1F2PSpermEggsXXXYCONCLUSIONEXPERIMENTRESULTSwwwwwww w++++ +www wwwwww++++ ++
  15. 15. Fig. 15-4aEXPERIMENTPGenerationGenerationF1 All offspringhad red eyes
  16. 16. Fig. 15-4bRESULTSGenerationF2
  17. 17. Fig. 15-4cEggsF1CONCLUSIONGenerationGenerationP XXwSpermXY++++ +EggsSperm+++ ++GenerationF2wwwwwwwwwwwwwww
  18. 18. Concept 15.2: Sex-linked genes exhibit uniquepatterns of inheritance• In humans and some other animals, there is achromosomal basis of sex determinationCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  19. 19. The Chromosomal Basis of Sex• In humans and other mammals, there are twovarieties of sex chromosomes: a larger Xchromosome and a smaller Y chromosome• Only the ends of the Y chromosome haveregions that are homologous with the Xchromosome• The SRY gene on the Y chromosome codes forthe development of testesCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  20. 20. Fig. 15-5XY
  21. 21. • Females are XX, and males are XY• Each ovum contains an X chromosome, whilea sperm may contain either an X or a Ychromosome• Other animals have different methods of sexdeterminationCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  22. 22. Fig. 15-644 +XYParents44 +XX22 +X22 +X22 +Yor +44 +XX orSperm Egg44 +XYZygotes (offspring)(a) The X-Y system22 +XX22 +X(b) The X-0 system76 +ZW76 +ZZ(c) The Z-W system32(Diploid)16(Haploid)(d) The haplo-diploid system
  23. 23. Fig. 15-6a(a) The X-Y system44 +XY44 +XXParents44 +XY44 +XX22 +X22 +X22 +YororSperm Egg+Zygotes (offspring)
  24. 24. Fig. 15-6b(b) The X-0 system22 +XX22 +X
  25. 25. Fig. 15-6c(c) The Z-W system76 +ZW76 +ZZ
  26. 26. Fig. 15-6d(d) The haplo-diploid system32(Diploid)16(Haploid)
  27. 27. Inheritance of Sex-Linked Genes• The sex chromosomes have genes for manycharacters unrelated to sex• A gene located on either sex chromosome iscalled a sex-linked gene• In humans, sex-linked usually refers to a geneon the larger X chromosomeCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  28. 28. • Sex-linked genes follow specific patterns ofinheritance• For a recessive sex-linked trait to be expressed– A female needs two copies of the allele– A male needs only one copy of the allele• Sex-linked recessive disorders are much morecommon in males than in femalesCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  29. 29. Fig. 15-7(a) (b) (c)XNXN XnY XNXn  XNY XNXn XnYYXnSpermYXNSpermYXnSpermXNXnEggs XNXNXNXnXNYXNYEggs XNXnXNXNXnXNXNYXnYEggs XNXnXNXnXnXnXNYXnY
  30. 30. • Some disorders caused by recessive alleles onthe X chromosome in humans:– Color blindness– Duchenne muscular dystrophy– HemophiliaCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  31. 31. X Inactivation in Female Mammals• In mammalian females, one of the two Xchromosomes in each cell is randomlyinactivated during embryonic development• The inactive X condenses into a Barr body• If a female is heterozygous for a particulargene located on the X chromosome, she will bea mosaic for that characterCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  32. 32. Fig. 15-8X chromosomesEarly embryo:Allele fororange furAllele forblack furCell division andX chromosomeinactivationTwo cellpopulationsin adult cat:Active XActive XInactive XBlack fur Orange fur
  33. 33. • Each chromosome has hundreds or thousandsof genes• Genes located on the same chromosome thattend to be inherited together are called linkedgenesConcept 15.3: Linked genes tend to be inheritedtogether because they are located near each other onthe same chromosomeCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  34. 34. How Linkage Affects Inheritance• Morgan did other experiments with fruit flies tosee how linkage affects inheritance of twocharacters• Morgan crossed flies that differed in traits ofbody color and wing sizeCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  35. 35. Fig. 15-UN1b+ vg+Parentsin testcrossMostoffspringb+ vg+b vgb vgb vgb vgb vgb vgor
  36. 36. Fig. 15-9-1EXPERIMENTP Generation (homozygous)Wild type(gray body,normal wings)Double mutant(black body,vestigial wings)b b vg vgb+ b+ vg+ vg+
  37. 37. Fig. 15-9-2EXPERIMENTP Generation (homozygous)Wild type(gray body,normal wings)Double mutant(black body,vestigial wings)b b vg vgb b vg vgDouble mutantTESTCROSSb+ b+ vg+ vg+F1 dihybrid(wild type)b+ b vg+ vg
  38. 38. Fig. 15-9-3EXPERIMENTP Generation (homozygous)Wild type(gray body,normal wings)Double mutant(black body,vestigial wings)b b vg vgb b vg vgDouble mutantTESTCROSSb+ b+ vg+ vg+F1 dihybrid(wild type)b+ b vg+ vgTestcrossoffspring Eggs b+ vg+ b vg b+ vg b vg+Black-normalGray-vestigialBlack-vestigialWild type(gray-normal)b vgSpermb+ b vg+ vg b b vg vg b+ b vg vgb b vg+ vg
  39. 39. Fig. 15-9-4EXPERIMENTP Generation (homozygous)RESULTSWild type(gray body,normal wings)Double mutant(black body,vestigial wings)b b vg vgb b vg vgDouble mutantTESTCROSSb+ b+ vg+ vg+F1 dihybrid(wild type)b+ b vg+ vgTestcrossoffspring Eggs b+ vg+ b vg b+ vg b vg+Black-normalGray-vestigialBlack-vestigialWild type(gray-normal)b vgSpermb+ b vg+ vg b b vg vg b+ b vg vgb b vg+ vgPREDICTED RATIOSIf genes are located on different chromosomes:If genes are located on the same chromosome andparental alleles are always inherited together:11111 10 0965 944 206 185:::::::::
  40. 40. • Morgan found that body color and wing sizeare usually inherited together in specificcombinations (parental phenotypes)• He noted that these genes do not assortindependently, and reasoned that they were onthe same chromosomeCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  41. 41. • However, nonparental phenotypes were alsoproduced• Understanding this result involves exploringgenetic recombination, the production ofoffspring with combinations of traits differingfrom either parentCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  42. 42. Genetic Recombination and Linkage• The genetic findings of Mendel and Morganrelate to the chromosomal basis ofrecombinationCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  43. 43. Recombination of Unlinked Genes: IndependentAssortment of Chromosomes• Mendel observed that combinations of traits insome offspring differ from either parent• Offspring with a phenotype matching one of theparental phenotypes are called parental types• Offspring with nonparental phenotypes (newcombinations of traits) are called recombinanttypes, or recombinants• A 50% frequency of recombination is observedfor any two genes on different chromosomesCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  44. 44. Fig. 15-UN2YyRrGametes from green-wrinkled homozygousrecessive parent (yyrr)Gametes from yellow-roundheterozygous parent (YyRr)Parental-typeoffspringRecombinantoffspringyryyrr Yyrr yyRrYR yr Yr yR
  45. 45. Recombination of Linked Genes: Crossing Over• Morgan discovered that genes can belinked, but the linkage was incomplete, asevident from recombinant phenotypes• Morgan proposed that some process mustsometimes break the physical connectionbetween genes on the same chromosome• That mechanism was the crossing over ofhomologous chromosomesCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin CummingsAnimation: Crossing Over
  46. 46. Fig. 15-10TestcrossparentsReplicationof chromo-somesGray body, normal wings(F1 dihybrid)Black body, vestigial wings(double mutant)Replicationof chromo-somesb+ vg+b+ vg+b+ vg+b vgb vgb vgb vgb vgb vgb vgb vgb vgb+ vg+b+ vgb vg+b vgRecombinantchromosomesMeiosis I and IIMeiosis IMeiosis IIb vg+b+ vgb vgb+ vg+EggsTestcrossoffspring965Wild type(gray-normal)944Black-vestigial206Gray-vestigial185Black-normalb+ vg+b vg b vgb vg b+ vgb vg b vgb vg+Spermb vgParental-type offspring Recombinant offspringRecombinationfrequency =391 recombinants2,300 total offspring 100 = 17%
  47. 47. Fig. 15-10aTestcrossparentsReplicationof chromo-somesGray body, normal wings(F1 dihybrid)Black body, vestigial wings(double mutant)Replicationof chromo-somesb+ vg+b+ vg+b+ vg+b vgb vgb vgb vgb vgb vgb vgb vgb vgb+ vg+b+ vgb vg+b vgRecombinantchromosomesMeiosis I and IIMeiosis IMeiosis IIEggs Spermb+ vg+b vg b+ vg b vgb vg+
  48. 48. Fig. 15-10bTestcrossoffspring965Wild type(gray-normal)944Black-vestigial206Gray-vestigial185Black-normalb+ vg+b vg b vgb vg b+ vgb vgb vgb+ vg+Spermb vgParental-type offspring Recombinant offspringRecombinationfrequency=391 recombinants2,300 total offspring 100 = 17%b vgb+ vg b vg+b vg+EggsRecombinantchromosomes
  49. 49. Mapping the Distance Between Genes UsingRecombination Data: Scientific Inquiry• Alfred Sturtevant, one of Morgan’sstudents, constructed a genetic map, anordered list of the genetic loci along a particularchromosome• Sturtevant predicted that the farther apart twogenes are, the higher the probability that acrossover will occur between them andtherefore the higher the recombinationfrequencyCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  50. 50. • A linkage map is a genetic map of achromosome based on recombinationfrequencies• Distances between genes can be expressed asmap units; one map unit, orcentimorgan, represents a 1% recombinationfrequency• Map units indicate relative distance andorder, not precise locations of genesCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  51. 51. Fig. 15-11RESULTSRecombinationfrequenciesChromosome9% 9.5%17%b cn vg
  52. 52. • Genes that are far apart on the samechromosome can have a recombinationfrequency near 50%• Such genes are physically linked, butgenetically unlinked, and behave as if found ondifferent chromosomesCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  53. 53. • Sturtevant used recombination frequencies tomake linkage maps of fruit fly genes• Using methods like chromosomalbanding, geneticists can develop cytogeneticmaps of chromosomes• Cytogenetic maps indicate the positions ofgenes with respect to chromosomal featuresCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  54. 54. Fig. 15-12Mutant phenotypesShortaristaeBlackbodyCinnabareyesVestigialwingsBrowneyesRedeyesNormalwingsRedeyesGraybodyLong aristae(appendageson head)Wild-type phenotypes0 48.5 57.5 67.0 104.5
  55. 55. Concept 15.4: Alterations of chromosome numberor structure cause some genetic disorders• Large-scale chromosomal alterations oftenlead to spontaneous abortions (miscarriages)or cause a variety of developmental disordersCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  56. 56. Abnormal Chromosome Number• In nondisjunction, pairs of homologouschromosomes do not separate normally duringmeiosis• As a result, one gamete receives two of thesame type of chromosome, and anothergamete receives no copyCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  57. 57. Fig. 15-13-1Meiosis I(a) Nondisjunction of homologouschromosomes in meiosis I(b) Nondisjunction of sisterchromatids in meiosis IINondisjunction
  58. 58. Fig. 15-13-2Meiosis INondisjunction(a) Nondisjunction of homologouschromosomes in meiosis I(b) Nondisjunction of sisterchromatids in meiosis IIMeiosis IINondisjunction
  59. 59. Fig. 15-13-3Meiosis INondisjunction(a) Nondisjunction of homologouschromosomes in meiosis I(b) Nondisjunction of sisterchromatids in meiosis IIMeiosis IINondisjunctionGametesNumber of chromosomesn + 1 n + 1 n + 1n – 1 n – 1 n – 1 n n
  60. 60. • Aneuploidy results from the fertilization ofgametes in which nondisjunction occurred• Offspring with this condition have an abnormalnumber of a particular chromosomeCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  61. 61. • A monosomic zygote has only one copy of aparticular chromosome• A trisomic zygote has three copies of aparticular chromosomeCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  62. 62. • Polyploidy is a condition in which an organismhas more than two complete sets ofchromosomes– Triploidy (3n) is three sets of chromosomes– Tetraploidy (4n) is four sets of chromosomes• Polyploidy is common in plants, but not animals• Polyploids are more normal in appearance thananeuploidsCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  63. 63. Fig. 15-14
  64. 64. Alterations of Chromosome Structure• Breakage of a chromosome can lead to fourtypes of changes in chromosome structure:– Deletion removes a chromosomal segment– Duplication repeats a segment– Inversion reverses a segment within achromosome– Translocation moves a segment from onechromosome to anotherCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  65. 65. Fig. 15-15DeletionA B C D E F G H A B C E F G H(a)(b)(c)(d)DuplicationInversionReciprocaltranslocationA B C D E F G HA B C D E F G HA B C D E F G HA B C B C D E F G HA D C B E F G HM N O C D E F G HM N O P Q R A B P Q R
  66. 66. Human Disorders Due to ChromosomalAlterations• Alterations of chromosome number andstructure are associated with some seriousdisorders• Some types of aneuploidy appear to upset thegenetic balance less than others, resulting inindividuals surviving to birth and beyond• These surviving individuals have a set ofsymptoms, or syndrome, characteristic of thetype of aneuploidyCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  67. 67. Down Syndrome (Trisomy 21)• Down syndrome is an aneuploid condition thatresults from three copies of chromosome 21• It affects about one out of every 700 childrenborn in the United States• The frequency of Down syndrome increaseswith the age of the mother, a correlation thathas not been explainedCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  68. 68. Fig. 15-16
  69. 69. Fig. 15-16a
  70. 70. Fig. 15-16b
  71. 71. Aneuploidy of Sex Chromosomes• Nondisjunction of sex chromosomes producesa variety of aneuploid conditions• Klinefelter syndrome is the result of an extrachromosome in a male, producing XXYindividuals• Monosomy X, called Turnersyndrome, produces X0 females, who aresterile; it is the only known viable monosomy inhumansCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  72. 72. Disorders Caused by Structurally AlteredChromosomes• The syndrome cri du chat (“cry of thecat”), results from a specific deletion inchromosome 5• A child born with this syndrome is mentallyretarded and has a catlike cry; individualsusually die in infancy or early childhood• Certain cancers, including chronicmyelogenous leukemia (CML), are caused bytranslocations of chromosomesCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  73. 73. Fig. 15-17Normal chromosome 9Normal chromosome 22Reciprocaltranslocation Translocated chromosome 9Translocated chromosome 22(Philadelphia chromosome)
  74. 74. Concept 15.5: Some inheritance patterns areexceptions to the standard chromosome theory• There are two normal exceptions to Mendeliangenetics• One exception involves genes located in thenucleus, and the other exception involvesgenes located outside the nucleusCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  75. 75. Genomic Imprinting• For a few mammalian traits, the phenotypedepends on which parent passed along thealleles for those traits• Such variation in phenotype is called genomicimprinting• Genomic imprinting involves the silencing ofcertain genes that are “stamped” with animprint during gamete productionCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  76. 76. Fig. 15-18Normal Igf2 alleleis expressedPaternalchromosomeMaternalchromosomeNormal Igf2 alleleis not expressedMutant Igf2 alleleinherited from mother(a) HomozygoteWild-type mouse(normal size)Mutant Igf2 alleleinherited from fatherNormal size mouse(wild type)Dwarf mouse(mutant)Normal Igf2 alleleis expressedMutant Igf2 alleleis expressedMutant Igf2 alleleis not expressedNormal Igf2 alleleis not expressed(b) Heterozygotes
  77. 77. Fig. 15-18aNormal Igf2 alleleis expressedPaternalchromosomeMaternalchromosome(a) HomozygoteWild-type mouse(normal size)Normal Igf2 alleleis not expressed
  78. 78. Fig. 15-18bMutant Igf2 alleleinherited from motherMutant Igf2 alleleinherited from fatherNormal size mouse(wild type)Dwarf mouse(mutant)Normal Igf2 alleleis expressedMutant Igf2 alleleis expressedMutant Igf2 alleleis not expressedNormal Igf2 alleleis not expressed(b) Heterozygotes
  79. 79. • It appears that imprinting is the result of themethylation (addition of –CH3) of DNA• Genomic imprinting is thought to affect only asmall fraction of mammalian genes• Most imprinted genes are critical for embryonicdevelopmentCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  80. 80. Fig. 15-UN3
  81. 81. Inheritance of Organelle Genes• Extranuclear genes (or cytoplasmic genes) aregenes found in organelles in the cytoplasm• Mitochondria, chloroplasts, and other plantplastids carry small circular DNA molecules• Extranuclear genes are inherited maternallybecause the zygote’s cytoplasm comes fromthe egg• The first evidence of extranuclear genes camefrom studies on the inheritance of yellow orwhite patches on leaves of an otherwise greenplantCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  82. 82. Fig. 15-19
  83. 83. • Some defects in mitochondrial genes preventcells from making enough ATP and result indiseases that affect the muscular and nervoussystems– For example, mitochondrial myopathy andLeber’s hereditary optic neuropathyCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  84. 84. Fig. 15-UN4EggSpermP generationgametesCBAD EFDF EABCedfc badfecbaThis F1 cell has 2n = 6chromosomes and isheterozygous for all sixgenes shown (AaBbCcDdEeFf).Red = maternal; blue = paternal.+Each chromosomehas hundreds orthousands of genes.Four (A, B, C, F) areshown on this one.The alleles of unlinkedgenes are either onseparate chromosomes(such as d and e) or sofar apart on the samechromosome (c and f)that they assortindependently.Genes on the same chromo-some whose alleles are soclose together that they donot assort independently(such as a, b, and c) are saidto be linked.
  85. 85. Fig. 15-UN5
  86. 86. Fig. 15-UN6
  87. 87. Fig. 15-UN7
  88. 88. Fig. 15-UN8
  89. 89. Fig. 15-UN9
  90. 90. You should now be able to:1. Explain the chromosomal theory ofinheritance and its discovery2. Explain why sex-linked diseases are morecommon in human males than females3. Distinguish between sex-linked genes andlinked genes4. Explain how meiosis accounts forrecombinant phenotypes5. Explain how linkage maps are constructedCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  91. 91. 6. Explain how nondisjunction can lead toaneuploidy7. Define trisomy, triploidy, and polyploidy8. Distinguish amongdeletions, duplications, inversions, andtranslocations9. Explain genomic imprinting10.Explain why extranuclear genes are notinherited in a Mendelian fashionCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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