BIOL 102 Chp 14 PowerPoint


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BIOL 102 Chp 14 PowerPoint

  1. 1. Chapter 14 Mendel and the Gene IdeaBIOL 102: Rob Swatski Assoc. Prof. BiologyGeneral Biology II HACC- HACC-York 1
  2. 2. What genetic principles account for the passing of traits from parents to offspring? The “blending” hypothesis: 2
  3. 3. The “particulate” hypothesis:parents pass on discrete heritable units (genes) 3
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  5. 5. Augustinian AbbeyBrno, Czech Republic 5
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  7. 7. Mendel SculptureVillanova University, Phila. 7
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  9. 9. Mendel used the scientific methodto identify 2 laws of inheritance- Experimentation- Quantitative data 9
  10. 10. Advantages of Pea Plants for Genetic Study: – Many varieties with distinct heritable features (characters), such as flower color - character variants = traits – Mating of plants can be controlled – Each flower produces sperm (stamens) & eggs (carpels) – Cross-pollination 10
  11. 11. TECHNIQUE 1 2 Parental generation Stamens (P) Carpel 3 4 RESULTS First 5 filialgeneration offspring (F1) 11
  12. 12. Mendel tracked only those characters that varied in an “either-or” manner - used pairs of traitsHe also used varieties that were true-breeding - plants that produce offspring of the same variety when they self-pollinate 12
  13. 13. In a typical experiment, Mendel mated 2 contrasting, true-breeding varieties (hybridization)- P generation: the true-breeding parents- F1 generation: the hybrid offspring of the P generation- F2 generation: produced when F1 individuals self- pollinate 13
  14. 14. EXPERIMENTP Generation(true-breeding parents) Purple White flowers flowersF1 Generation (hybrids) All plants had purple flowersF2 Generation 705 purple-flowered 224 white-flowered plants plants 14
  15. 15. Mendel’s Experiments• When Mendel crossed true-breeding white & purple flowered plants, all of the F1 hybrids were purple• When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white• Mendel discovered a ratio of about 3:1 (purple to white flowers) in the F2 generation 15
  16. 16. • Mendel reasoned that only the purple flower “factor” was affecting flower color in the F1 hybrids• Mendel called purple flower color a dominant trait & white flower color a recessive trait• He observed the same pattern of inheritance in 6 other pea plant characters, each represented by 2 traits - what Mendel called a “heritable factor” is what we now call a gene 16
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  19. 19. Mendel’s Model• Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring• 4 related concepts make up Mendel’s model - we can now describe these concepts in light of what we know about genes & chromosomes 19
  20. 20. Concept 1: Alternative versions of genes 1: account for variations in inherited characters- Ex: the gene for flower color in pea plants exists in 2versions - purple flowers & white flowers- Alleles- each allele is found at a specific locus on a specificchromosome 20
  21. 21. Allele for purple flowers Pair ofLocus for flower-color gene homologous chromosomes Allele for white flowers 21
  22. 22. Concept 2: For each character, an 2: organism inherits 2 alleles, one from each parent- the 2 alleles at a locus may be identical, as in the true- breeding P generation- the 2 alleles at a locus may also differ, as in the F1 hybrids 22
  23. 23. Concept 3: If the 2 alleles at a locus differ, 3: then one dominant allele determines the organism’s appearance, while the recessive allele has no noticeable effect - Ex: the F1 plants had purple flowers because the allele for that trait is dominant 23
  24. 24. Concept 4: The 2 alleles for a heritable 4: character separate (segregate) during gamete formation & end up in different gametes = The Law of Segregation- An egg or sperm receives only 1 of the 2 alleles present in the somatic cells of an organism- Segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis 24
  25. 25. - Mendel’s segregation model accounts for the 3:1 ratio observed in his F2 generations- The possible combinations of sperm & egg can be shown using a Punnett square - helps predict the results of a genetic cross between individuals of known genetic makeupSymbols: - Capital letter represents a dominant allele - lowercase letter represents a recessive allele 25
  26. 26. P GenerationAppearance: Purple flowers White flowersGenetic makeup: PP ppGametes: P p 26
  27. 27. P GenerationAppearance: Purple flowers White flowersGenetic makeup: PP ppGametes: P pF1 GenerationAppearance: Purple flowersGenetic makeup: Pp 2 p 1/ 1/Gametes: 2 P 27
  28. 28. P GenerationAppearance: Purple flowers White flowersGenetic makeup: PP ppGametes: P pF1 GenerationAppearance: Purple flowersGenetic makeup: Pp 2 p 1/ 1/Gametes: 2 P Sperm from F1 (Pp) plantF2 Generation P p P Eggs from PP Pp F1 (Pp) plant p Pp pp 3 :1 28
  29. 29. Useful Genetic VocabularyHomozygous:Heterozygous: 29
  30. 30. An organism’s traits do not always reveal its genetic composition- due to different effects of dominant & recessive allelesPhenotype:Genotype:- PP- Pp 30
  31. 31. Phenotype Genotype Purple PP 1 (homozygous)3 Purple Pp (heterozygous) 2 Purple Pp (heterozygous) White pp1 1 (homozygous) Ratio 3:1 Ratio 1:2:1 31
  32. 32. The Testcross How can we determine the genotype of an individual with a dominant phenotype? Testcross: breed the “mystery” individual with a known homozygous recessive individual- if any offspring display the recessive phenotype, the mystery parent must be heterozygous 32
  33. 33. TECHNIQUE Dominant phenotype, Recessive phenotype, unknown genotype: known genotype: PP or Pp? pp Predictions If purple-flowered or If purple-flowered parent is PP parent is Pp Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p Pp Pp pp ppRESULTS or All offspring purple 1/ 2 offspring purple and 1/ offspring white 2 33
  34. 34. Mendel derived the Law of Segregation by following one character- the F1 offspring produced were monohybrids, individuals that are heterozygous for one characterMonohybrid cross: between F1 monohybrids 34
  35. 35. Mendel derived his 2nd law of inheritance by following two characters at the same time- crossing 2 true-breeding parents differing in 2 characters produces dihybrids in the F1 generation, heterozygous for both characters Dihybrid cross: between F1 dihybrids- can determine whether 2 characters are transmitted to offspring as a package or independently 35
  36. 36. EXPERIMENT P Generation YYRR yyrr Gametes YR yr F1 Generation YyRr Predictions Hypothesis of Hypothesis of dependent assortment independent assortment Sperm Predicted or 1/ 1/ 1/ 1/ offspring of 4 YR 4 Yr 4 yR 4 yr Sperm F2 generation 1/ YR 1/ 2 2 yr 1/ YR 4 YYRR YYRr YyRR YyRr 1/ 2 YR YYRR YyRr 1/ Yr Eggs 4 YYRr YYrr YyRr Yyrr 1/ Eggs 2 yr YyRr yyrr 1/ yR 4 YyRR YyRr yyRR yyRr 3/ 1/ 4 4 1/ 4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/ 3/ 3/ 1/ 16 16 16 16 Phenotypic ratio 9:3:3:1RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 36
  37. 37. Law of Independent Assortment- each pair of alleles segregates independently of other pairs of alleles during gamete formationApplies only to genes on different non-homologous chromosomes - genes located near each other on the same chromosome tend to be inherited together 37
  38. 38. Mendel’s laws reflect the rules of probability- when tossing a coin, the outcome of 1 toss has no impact on the outcome of the next toss - the alleles of one gene segregate into gametes independently of another gene’s alleles 38
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  40. 40. The Multiplication RuleThe probability of 2 or more independent eventsoccurring together is the product of their individual probabilities- determines probability in an F1 monohybrid cross Each gamete has a: ½ chance of carrying the dominant allele & ½ chance of carrying the recessive allele 40
  41. 41. Rr Rr Segregation of Segregation ofalleles into eggs alleles into sperm Sperm 1/ 2 R 1/ 2 r R R 1/ 2 R R r 1/ 1/ 4 4Eggs r r 1/ r R r 2 1/ 1/ 4 4 41
  42. 42. The Addition RuleThe probability of any one exclusive event occurring (out of 2 or more) is calculated by adding their individual probabilities - determines the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous 42
  43. 43. Rr Rr Segregation of Segregation ofalleles into eggs alleles into sperm Sperm 1/ 2 R 1/ 2 r R R 1/ 2 R R r 1/ 1/ 4 4Eggs r r 1/ r R r 2 1/ 1/ 4 4 43
  44. 44. Inheritance patterns are often more complexthan predicted by simple Mendelian genetics Many heritable characters are not determined by only 1 gene with 2 allelesBut…the basic Laws of Segregation & Independent assortment do apply to more complex patterns of inheritance 44
  45. 45. Extending Mendelian Genetics for a Single GeneInheritance of characters by one gene may deviate from simple Mendelian patterns: - When alleles are not completely dominant or recessive - When a gene has more than 2 alleles - When a gene produces multiple phenotypes 45
  46. 46. Degrees of Dominance• Complete dominance: phenotypes of the heterozygote & dominant homozygote are identical• Incomplete dominance: the phenotype of F1 hybrids is somewhere between the 2 parental phenotypes• Codominance: the 2 dominant alleles affect the phenotype in separate & distinguishable ways 46
  47. 47. P Generation Red White CRCR CWCW Gametes CR CW F1 Generation Pink CRCW 1/ Gametes 1/2 CR 2 CW Sperm F2 Generation 1/ 2 CR 1/ 2 CW 1/ 2 CR Eggs CRCR CRCWIncomplete 1/ 2 CW CRCW CWCWDominance 47
  48. 48. A dominant allele does not subdue a recessive allele - alleles don’t interact: they are simply variations in a gene’s nucleotide sequenceFor any character, dominance/recessive relationships of alleles depends on the level at which one examines the phenotype 48
  49. 49. Tay- Tay-Sachs Disease- result of dysfunctional enzyme - lipids accumulate in the brain At the organismal level: - the allele is recessive At the biochemical level:- the phenotype (enzyme activity) is incompletely dominant At the molecular level: - the alleles are codominant 49
  50. 50. Frequency of Dominant AllelesDominant alleles are not necessarily more common in populations than recessive alleles - Ex: 1 in 400 U.S. babies is born with extra fingers or toes (polydactyly) 50
  51. 51. Multiple AllelesMost genes exist in more than 2 allelic forms Ex: ABO blood group phenotypes (A, B, AB, O) - determined by 3 alleles for an enzyme (I) thatattaches A or B carbohydrates to red blood cells: IA, IB, and i - IA allele: enzyme adds the A carbohydrate - IB allele: enzyme adds the B carbohydrate - i allele: enzyme adds neither carbohydrate 51
  52. 52. (a) The three alleles for the ABO blood groups and their carbohydrates Allele IA IB iCarbohydrate A B none(b) Blood group genotypes and phenotypes Genotype IAIA or IAi IBIB or IBi IAIB iiRed blood cellappearancePhenotype A B AB O(blood group) 52
  53. 53. B = brown eye colorB = blue eye colorG = green or hazel eyesg = lighter colored eyes 53
  54. 54. PleiotropyMost genes have multiple phenotypic effects- Ex: pleiotropic alleles are responsible for multiple symptoms of hereditary diseases like cystic fibrosis & sickle-cell disease 54
  55. 55. Relationship among Description Examplealleles of a single geneComplete dominance Heterozygous phenotypeof one allele same as that of homo- PP Pp zygous dominantIncomplete dominance Heterozygous phenotypeof either allele intermediate between the two homozygous phenotypes CRCR CRCW CWCWCodominance Both phenotypes expressed in IAIB heterozygotesMultiple alleles In the whole population, ABO blood group alleles some genes have more than two alleles IA, IB, iPleiotropy One gene is able to affect Sickle-cell disease multiple phenotypic characters 55
  56. 56. Epistasis Some traits may be determined by 2 or more genesEpistasis: a gene at 1 locus alters the phenotypic expression of a gene at a 2nd locus Ex: Coat color in dogs depends on 2 genes: - one gene determines pigment color (B = black, b = brown) - 2nd gene determines if pigment will be deposited in hair (E = color, e = no color) 56
  57. 57. BbEe BbEe Sperm 1/ BE 1/ 1/ 1/ 4 4 bE 4 Be 4 beEggs1/ 4 BE BBEE BbEE BBEe BbEe1/ 4 bE BbEE bbEE BbEe bbEe1/ 4 Be BBEe BbEe BBee Bbee1/ be 4 BbEe bbEe Bbee bbee 9 : 3 : 4 57
  58. 58. Polygenic Inheritance- an additive effect of 2 or more genes on a single phenotype- Quantitative characters: vary along a continuum - Ex: skin color in humans 58
  59. 59. AaBbCc AaBbCc Sperm 1/ 1/ 1/ 1/ 1/ 1/ 1/ 1/ 8 8 8 8 8 8 8 8 1/ 8 1/ 8 1/ 8 1/ 8 Eggs 1/ 8 1/ 8 1/ 8 1/ 8Phenotypes: 1/ 64 6/ 64 15/ 64 20/ 64 15/ 64 6/ 64 1/ 64Number ofdark-skin alleles: 0 1 2 3 4 5 6 59
  60. 60. Relationship among Description Exampletwo or more genesEpistasis The phenotypic BbEe BbEe expression of one gene affects that BE bE Be be of another BE bE Be be 9 :3 :4Polygenic inheritance A single phenotypic character is affected AaBbCc AaBbCc by two or more genes 60
  61. 61. The Environmental Impact on PhenotypeAnother variation from Mendelian genetics occurs when a character’s phenotype depends on the environment in addition to the genotype = Norm of reaction - Ex: hydrangea flowers of same genotype can range in color from blue-violet to pink, depending on soil acidity 61
  62. 62. Integrating a Mendelian View of Heredity & Variation An organism’s phenotype includes its: - physical appearance - internal anatomy - physiology - behavior Phenotype reflects an organism’s overall genotype and unique environmental history 62
  63. 63. Humans are not good subjects for genetic research: - our generation time is too long - parents produce relatively few offspring - ethics of breeding experimentsHowever, basic Mendelian genetics is the foundation of human genetics 63
  64. 64. Pedigree AnalysisPedigree: a family tree describing the interrelationships of parents & children across generations- trace & describe inheritance patterns of certain traits - can make predictions about future offspring- use the multiplication & addition rules to predict the probability of specific phenotypes 64
  65. 65. Key Male Affected Mating male Offspring, in Female Affected birth order female (first-born on left) 65
  66. 66. 1st generation(grandparents) Ww ww ww Ww2nd generation(parents, aunts,and uncles) Ww ww ww Ww Ww ww3rd generation(two sisters) WW ww or Ww Widow’s peak No widow’s peak(a) Is a widow’s peak a dominant or recessive trait? 66
  67. 67. 1st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) ff FF or Ff Attached earlobe Free earlobe(b) Is an attached earlobe a dominant or recessive trait? 67
  68. 68. Recessively Inherited Disorders - appear only in individuals homozygous for the allele - Carriers: heterozygous individuals who have the recessive allele, but are phenotypically normal- Ex: Albinism is a recessive condition characterized by a lack of pigmentation in skin & hair 68
  69. 69. Parents Normal Normal Aa Aa Sperm A aEggs Aa AA Normal A Normal (carrier) Aa Normal aa a Albino (carrier) 69
  70. 70. If a recessive allele causing a disease is rare, then the chance of 2 carriers meeting & mating is lowConsanguineous matings (between close relatives) increase the chance of mating between 2 carriers of the same rare allele - most societies & cultures have laws or taboos against marriages between close relatives 70
  71. 71. Dominantly Inherited DisordersResult from dominant alleles that cause a lethal disease - rare & arise by mutation Achondroplasia: a form of dwarfism due to a rare dominant allele 71
  72. 72. Parents Dwarf Normal Dd dd Sperm D dEggs Dd dd d Dwarf Normal Dd dd d Dwarf Normal 72
  73. 73. Huntington’s DiseaseDegenerative disease of the nervous system showing no obvious phenotypic effects until person is approx. 35-40 years of age 73
  74. 74. Multifactorial DisordersDiseases having both genetic & environmental components - Ex: most diseases (heart disease, cancer)- little is understood about the genetic contribution to most multifactorial diseases 74
  75. 75. Genetic Testing & Counseling Genetic counselors: inform prospective parents concerned about family history for specific disease- use family histories & carrier ID tests to help couples determine odds of their children having genetic disorders 75
  76. 76. Fetal Testing- Amniocentesis: removes & tests amniotic fluid that bathes fetus- Chorionic villus sampling (CVS): removes & tests a sample of placenta- Ultrasound & fetoscopy: visually assesses fetal health in utero 76
  77. 77. (a) Amniocentesis (b) Chorionic villus sampling (CVS) 1 Ultrasound monitor Ultrasound Amniotic monitor fluid withdrawn Fetus 1 Placenta SuctionFetusPlacenta Chorionic villi tube Cervix insertedUterus Cervix through Uterus cervix Centrifugation Fluid Several hours Several Biochemical hours Fetal 2 and genetic Several Fetal cells cells tests weeks 2 Several weeks Several hours 3 Karyotyping 77
  78. 78. Credits by Rob Swatski, 2013 Visit my website for more Biology study resources! send your comments and feedback to: rjswatsk@hacc.eduImages used in this work bear a This work bears an Creative Commons license and Attribution-Noncommercial are attributed to their original Share Alike Creative authors. Commons license. 78
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