BIOL 102 Chp 14 PowerPoint


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BIOL 102 Chp 14 PowerPoint

  1. 1. BIOL 102: General Biology II Rob Swatski Assoc. Prof. Biology HACC-York Chapter 14 Mendel and the Gene Idea 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
  4. 4. 4
  5. 5. Augustinian Abbey Brno, Czech Republic 5
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  7. 7. Mendel Sculpture Villanova University, Phila. 7
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  9. 9. Mendel used the scientific method to 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 RESULTS Parental generation (P) Stamens Carpel 1 2 3 4 First filial generation offspring (F1) 5 11
  12. 12. Mendel tracked only those characters that varied in an “either-or” manner - used pairs of traits He 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. EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers  F1 Generation (hybrids) All plants had purple flowers F2 Generation 705 purple-flowered plants 224 white-flowered 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 account for variations in inherited characters - Ex: the gene for flower color in pea plants exists in 2 versions - purple flowers & white flowers - Alleles - each allele is found at a specific locus on a specific chromosome 20
  21. 21. Allele for purple flowers Locus for flower-color gene Allele for white flowers Pair of homologous chromosomes 21
  22. 22. Concept 2: For each character, an 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, 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 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 makeup Symbols: - Capital letter represents a dominant allele - lowercase letter represents a recessive allele 25
  26. 26. P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers PP pp P p 26
  27. 27. P Generation F1 Generation Appearance: Genetic makeup: Gametes: Appearance: Genetic makeup: Gametes: Purple flowers White flowers Purple flowers Pp PP pp P P p p1/2 1/2 27
  28. 28. P Generation F1 Generation F2 Generation Appearance: Genetic makeup: Gametes: Appearance: Genetic makeup: Gametes: Purple flowers White flowers Purple flowers Sperm from F1 (Pp) plant Pp PP pp P P P P p p p p Eggs from F1 (Pp) plant PP ppPp Pp 1/2 1/2 3 : 1 28
  29. 29. Useful Genetic Vocabulary Homozygous: Heterozygous: 29
  30. 30. An organism’s traits do not always reveal its genetic composition - due to different effects of dominant & recessive alleles Phenotype: Genotype: - PP - Pp 30
  31. 31. Phenotype Purple Purple Purple White 3 1 1 1 2 Ratio 3:1 Ratio 1:2:1 Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) 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. Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If purple-flowered parent is PP If purple-flowered parent is Pp or Sperm Sperm Eggs Eggs or All offspring purple 1/2 offspring purple and 1/2 offspring white Pp Pp Pp Pp Pp Pp pp pp p p p p P P P p TECHNIQUE RESULTS 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 character Monohybrid 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. P Generation F1 Generation Predictions Gametes EXPERIMENT RESULTS YYRR yyrr yrYR YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predicted offspring of F2 generation Sperm Sperm or Eggs Eggs Phenotypic ratio 3:1 Phenotypic ratio 9:3:3:1 Phenotypic ratio approximately 9:3:3:1315 108 101 32 1/2 1/2 1/2 1/2 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 9/16 3/16 3/16 1/16 YR YR YR YR yr yr yr yr 1/4 3/4 Yr Yr yR yR YYRR YyRr YyRr yyrr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr 36
  37. 37. Law of Independent Assortment - each pair of alleles segregates independently of other pairs of alleles during gamete formation Applies 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
  39. 39. 39
  40. 40. The probability of 2 or more independent events occurring 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 The Multiplication Rule 40
  41. 41. Segregation of alleles into eggs Segregation of alleles into sperm Sperm Eggs 1/2 1/2 1/2 1/2 1/4 1/4 1/4 1/4 Rr Rr R R R R R R r r r r r  r 41
  42. 42. The 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 The Addition Rule 42
  43. 43. Segregation of alleles into eggs Segregation of alleles into sperm Sperm Eggs 1/2 1/2 1/2 1/2 1/4 1/4 1/4 1/4 Rr Rr R R R R R R r r r r r  r 43
  44. 44. Inheritance patterns are often more complex than predicted by simple Mendelian genetics Many heritable characters are not determined by only 1 gene with 2 alleles But…the basic Laws of Segregation & Independent Assortment do apply to more complex patterns of inheritance 44
  45. 45. Extending Mendelian Genetics for a Single Gene Inheritance 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 F1 Generation F2 Generation 1/2 1/2 1/2 1/2 1/2 1/2 Red White Gametes Pink Gametes Sperm Eggs CWCW CRCR CR CW CRCW CR CW CWCR CR CW CRCR CRCW CRCW CWCW Incomplete Dominance 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 sequence For any character, dominance/recessive relationships of alleles depends on the level at which one examines the phenotype 48
  49. 49. Frequency of Dominant Alleles Dominant 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) 49
  50. 50. Multiple Alleles Most 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) that attaches 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 - IA IB = Codominant (both phenotypes expressed) 50
  51. 51. Carbohydrate Allele (a) The three alleles for the ABO blood groups and their carbohydrates (b) Blood group genotypes and phenotypes Genotype Red blood cell appearance Phenotype (blood group) A A B B AB none O IA IB i iiIAIBIAIA or IAi IBIB or IBi 51
  52. 52. Pleiotropy Most genes have multiple phenotypic effects - Ex: Pleiotropic alleles are responsible for multiple symptoms of hereditary diseases like cystic fibrosis & sickle-cell disease 52
  53. 53. Complete dominance of one allele Relationship among alleles of a single gene Description Example Incomplete dominance of either allele Codominance Multiple alleles Pleiotropy Heterozygous phenotype same as that of homo- zygous dominant Heterozygous phenotype intermediate between the two homozygous phenotypes Both phenotypes expressed in heterozygotes In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters ABO blood group alleles Sickle-cell disease PP Pp CRCR CRCW CWCW IAIB IA, IB, i 53
  54. 54. Epistasis Some traits may be determined by 2 or more genes Epistasis: 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) 54
  55. 55. Sperm Eggs 9 : 3 : 4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 BbEe BbEe BE BE bE bE Be Be be be BBEE BbEE BBEe BbEe BbEE bbEE BbEe bbEe BBEe BbEe BBee Bbee BbEe bbEe Bbee bbee 55
  56. 56. 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 56
  57. 57. Eggs Sperm Phenotypes: Number of dark-skin alleles: 0 1 2 3 4 5 6 1/64 6/64 15/64 20/64 15/64 6/64 1/64 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 AaBbCc AaBbCc  57
  58. 58. Epistasis Polygenic inheritance Relationship among two or more genes Description Example The phenotypic expression of one gene affects that of another A single phenotypic character is affected by two or more genes 9 : 3 : 4 BbEe BbEe BE BE bE bE Be Be be be AaBbCc AaBbCc 58
  59. 59. The Environmental Impact on Phenotype Another 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 59
  60. 60. 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 60
  61. 61. Humans are not good subjects for genetic research: - our generation time is too long - parents produce relatively few offspring - ethics of breeding experiments However, basic Mendelian genetics is the foundation of human genetics 61
  62. 62. Pedigree Analysis Pedigree: 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 62
  63. 63. Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left) 63
  64. 64. 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Widow’s peak No widow’s peak (a) Is a widow’s peak a dominant or recessive trait? Ww ww Ww Wwww ww ww wwWw Ww wwWW Ww or 64
  65. 65. Attached earlobe 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Free earlobe (b) Is an attached earlobe a dominant or recessive trait? Ff Ff Ff Ff Ff ff Ff ff ff ff ff FF or or FF Ff 65
  66. 66. 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 66
  67. 67. Parents Normal Normal Sperm Eggs Normal Normal (carrier) Normal (carrier) Albino Aa Aa A A AA Aa a Aa aa a  67
  68. 68. 68 Albinism
  69. 69. 69
  70. 70. If a recessive allele causing a disease is rare, then the chance of 2 carriers meeting & mating is low Consanguineous 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. 71 Charles and Emma Darwin (1st cousins)
  72. 72. 72 Edgar Allan and Virginia Clemm Poe (also 1st cousins) Married at age 13
  73. 73. Dominantly Inherited Disorders Result from dominant alleles that may cause a lethal disease - rare & arise by mutation Achondroplasia (Achon’s): a form of dwarfism due to a rare dominant allele 73
  74. 74. Parents Dwarf Dd Sperm Eggs Dd Dwarf dd Normal Dd Dwarf dd Normal D d d d Normal dd 74
  75. 75. 75 Peter Dinklage as Tyrion Lassiter in Game of Thrones - Achon’s is the most common form of short-limbed dwarfism (1 in 15,000- 40,000) - 4ft-4in average height - Cannot form bone out of cartilage, esp. in long bones - Average-sized trunk - Enlarged head (macrocephaly)
  76. 76. 76 Jason Acuna, aka “Wee Man” of Jackass fame Potential health concerns of achon: - Apnea - Obesity - Bowed legs - Back problems - Spinal problems (stenosis) – upper spine compressed
  77. 77. Degenerative disease of the nervous system showing no obvious phenotypic effects until person is approx. 35-40 years of age - also due to rare dominant allele Huntington’s Disease 77
  78. 78. 78
  79. 79. 79 Effects of Huntington’s Disease - Caused by repeated sections of Huntingtin gene that forms altered form of Huntingtin protein - Early symptoms: changes in mood, cognition, unsteady gait, coordination - Later: jerky body movements, mental decline, dementia - Shortened life span, pneumonia, heart disease
  80. 80. Multifactorial Disorders Diseases having both genetic & environmental components - Ex: most diseases (heart disease, cancer) - little is understood about the genetic contribution to most multifactorial diseases 80
  81. 81. 81
  82. 82. 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 82
  83. 83. Fetal Testing - Amniocentesis: removes & tests amniotic fluid that bathes fetus (slow… results take weeks; must culture fetal cells) - Chorionic villus sampling (CVS): removes & tests a sample of placenta (fast… hours) - Ultrasound & fetoscopy: visually assesses fetal health in utero 83
  84. 84. (a) Amniocentesis (b) Chorionic villus sampling (CVS) Ultrasound monitor Amniotic fluid withdrawn Fetus Placenta Uterus Cervix Centrifugation Fluid Fetal cells Several hours Several weeks Several weeks Biochemical and genetic tests Karyotyping Ultrasound monitor Fetus Placenta Chorionic villi Uterus Cervix Suction tube inserted through cervix Several hours Fetal cells Several hours 1 1 2 2 3 84