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Chapter 14. Mendel & Genetics

Chapter 14. Mendel & Genetics. 1. Gregor Mendel. Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas used experimental method used quantitative analysis collected data & counted them

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Chapter 14. Mendel & Genetics

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  1. Chapter 14. Mendel & Genetics 1

  2. Gregor Mendel • Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas • used experimental method • usedquantitative analysis • collected data & counted them • excellent example of scientific method 2

  3. Mendel’s work • Bred pea plants • cross-pollinated true breeding parents (P) • raised seed & then observed traits (F1) • filial • allowed offspring to cross-pollinate & observed next generation (F2) 3

  4. Mendel collected data for 7 pea traits 4

  5. 100% purple-flower peas F1 generation (hybrids) 100% self-pollinate 75% purple-flower peas 25% white-flower peas 3:1 F2 generation Looking closer at Mendel’s work true-breeding purple-flower peas true-breeding white-flower peas X P 5

  6. What did Mendel’s findings mean? • Traits come in alternative versions • purple vs. white flower color • alleles • different alleles vary in the sequence of nucleotides at the specific locus of a gene purple-flower allele & white-flower allele are 2 DNA variations at flower-color locus different versions of gene on homologous chromosomes 6

  7. What are theadvantages ofbeing diploid? Traits are inherited as discrete units • For each characteristic, an organism inherits 2 alleles, 1 from each parent • diploid organism • inherits 2 sets of chromosomes, 1 from each parent • homologous chromosomes • like having 2 editions of encyclopedia • Encyclopedia Britannica • Encyclopedia Americana 7

  8. What did Mendel’s findings mean? • Some traits mask others • purple & white flower colors are separate traits that do not blend • purple x white ≠ light purple • purple masked white • dominant allele • fully expressed • recessive allele • no noticeable effect • the gene makes a non-functional protein 8

  9. P F1 Genotype vs. phenotype • difference between how an organism “looks” & its genetics • phenotype • description of an organism’s trait • genotype • description of an organism’s genetic makeup Explain Mendel’s results using …dominant & recessive …phenotype & gentotype 9

  10. PP pp x Making crosses • using representative letters • flower color alleles  P or p • true-breeding purple-flower peas  PP • true-breeding white-flower peas  pp Pp 10

  11. self-pollinate Looking closer at Mendel’s work true-breeding purple-flower peas true-breeding white-flower peas X P PP pp phenotype 100% purple-flower peas 100% purple-flower peas F1 generation (hybrids) 100% 100% Pp Pp Pp Pp 75% purple-flower peas 75% purple-flower peas 3:1 3:1 25% white-flower peas F2 generation ? ? ? ? 11

  12. PP 25% male / sperm P p Pp 50% 75% P Pp female / eggs pp p 25% 25% Punnett squares Pp x Pp % genotype % phenotype PP Pp Pp pp 1:2:1 3:1 12

  13. Genotypes • Homozygous = same alleles = PP, pp • Heterozygous = different alleles = Pp homozygousdominant homozygousrecessive 13

  14. purple PP homozygous dominant purple Pp heterozygous Phenotype vs. genotype • 2 organisms can have the same phenotype but have different genotypes 14

  15. So howdo you figure outthe genotype? Dominant phenotypes • It is not possible to determine the genotype of an organism with a dominant phenotype by looking at it. PP? Pp? 15

  16. Test cross • Cross-breed the dominant phenotype — unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele x is itPP or Pp? pp 16

  17. x x Test cross PP pp Pp pp p p p p Pp Pp Pp Pp P P 50%:50%1:1 100% P Pp Pp p pp pp 17

  18. P P P p p PP Pp pp p Mendel’s laws of heredity (#1) • Law of segregation • when gametes are produced during meiosis, homologous chromosomes separate from each other • each allele for a trait is packaged into a separate gamete 18

  19. Meiosis 1 And Mendeldidn’t even knowDNA or genesexisted! Law of Segregation • What meiotic event creates the law of segregation? 19

  20. Monohybrid cross • Some of Mendel’s experiments followed the inheritance of single characters • flower color • seed color • monohybrid crosses 20

  21. This helped Mendelunderstand othergenetic “rules” Dihybrid cross • Other of Mendel’s experiments followed the inheritance of 2 different characters • seed color andseed shape • dihybrid crosses 21

  22. yellow, round peas F1 generation (hybrids) 100% self-pollinate 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas 9:3:3:1 F2 generation Dihybrid cross true-breeding yellow, round peas true-breeding green, wrinkled peas P x YYRR yyrr Y = yellow R = round y = green r = wrinkled YyRr 22

  23. YyRr YyRr YR YR yR Yr yr yr Which systemexplains the data? What’s going on here? • How are the alleles on different chromosomes handed out? • together or separately? 23

  24. 9/16 yellow round 3/16 green round YR Yr 3/16 yellow wrinkled yR 1/16 green wrinkled yr Dihybrid cross YyRr x YyRr YR Yr yR yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr 24

  25. Can youthinkof anexceptionto this? Mendel’s laws of heredity (#2) • Law of independent assortment • each pair of alleles segregates into gametes independently • 4 classes of gametes are produced in equal amounts • YR, Yr, yR, yr • only true for genes on separate chromosomes YyRr Yr Yr yR yR YR YR yr yr 25

  26. Remember…Mendel didn’t even know DNA —or genes—existed! Meiosis 1 Law of Independent Assortment • What meiotic event creates the law of independent assortment? 26

  27. The chromosomal basis of Mendel’s laws… Trace the genetic events through meiosis, gamete formation & fertilization to offspring One Option The Other Option 27

  28. Review: Mendel’s laws of heredity • Law of segregation • monohybrid cross • single trait • each allele segregates into separate gametes • established by Meiosis 1 • Law of independent assortment • dihybrid (or more) cross • 2 or more traits • each pair of alleles for genes on separate chromosomes segregates into gametes independently • established by Meiosis 1 28

  29. Probability & Genetics

  30. Genetics & Probability • Mendel’s laws: • segregation • independent assortment reflect same laws of probability that apply to tossing coins or rolling dice

  31. B B B BB Bb b Probability & genetics • Calculating probability of making a specific gamete is just like calculating the probability in flipping a coin • probability of tossing heads? • probability making a B gamete? 100% 50%

  32. B Bb b Probability & genetics • Outcome of 1 toss has no impact on the outcome of the next toss • probability of tossing heads each time? • probability making a B gamete each time? 50% 50%

  33. Probability • Likelihood that a specific event will occur • Probability= # of one kind of possible outcome total # of all possible outcomes

  34. P Pp p Rule of multiplication (“AND”) • Chance that 2 or more independent events will occur together • probability that 2 coins tossed at the same time will land heads up • probability of Pp x Pppp 1/2 x 1/2 = 1/4 1/2 x 1/2 = 1/4

  35. Multiplication • Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive? • Answer: • Probability that an egg from the F1 (Pp) will receive a p allele = ½ • Probability that a sperm from the F1 will receive a p allele = ½ • Overall probability that 2 recessive alleles will unite at fertilization: ½ x ½ = ¼

  36. YyRr YyRr Yy Yy Rr Rr x x yyrr yy rr Calculating probability in crosses Use rule of multiplication to predict crosses x 1/16 ?% 1/4 1/4 x

  37. Apply the Rule of Multiplication AABbccDdEEFf x AaBbccDdeeFf AabbccDdEeFF AAxAa Aa 1/2 BbxBb bb 1/4 Got it? Try this! ccxcc cc 1 DdxDd Dd 1/2 EExee Ee 1 1/64 FfxFf FF 1/4

  38. B b Bb b B Bb 1/4 1/2 x 1/2 = 1/4 1/4 + 1/2 x 1/2 = 1/4 1/2 Rule of addition “OR” • Chance that an event can occur 2 or more different ways • sum of the separate probabilities • probability of BbxBbBb sperm egg offspring

  39. Addition • Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will being a heterozygote? • Answer: • There are 2 ways in which a heterozygote may be produced: the dominant allele may be in the egg and the recessive allele in the sperm, or the dominant allele may be in the sperm and the recessive allele in the egg. • Probability that the dominant allele will be in the egg with the recessive in the sperm is ½ x ½ = ¼ • Probability that the dominant allele will be in the sperm with the recessive in the egg is ½ x ½ = ¼ • Therefore, the overall probability that a heterozygote offspring will be produced is ¼ + ¼ = ½

  40. male / sperm P P PP P p P p p P p p Pp pp P female / eggs p Calculating probability Pp x Pp sperm egg offspring 1/2 x 1/2 = 1/4 1/2 x 1/2 = 1/4 + PP Pp 1/4 1/2 x 1/2 = 1/2 Pp pp 1/2 x 1/2 = 1/4

  41. Chi-square test • Test to see if your data supports your hypothesis • Compare “observed” vs. “expected” data • is variance from expected due to “random chance”? • or is there another factor influencing data? • null hypothesis • degrees of freedom • statistical significance

  42. Any Questions??

  43. Mendel chose peas wisely • Pea plants are good for genetic research • available in many varieties with distinct heritable features with different variations • flower color, seed color, seed shape, etc. • Mendel had strict control over which plants mated with which • each pea plant has male & female structures • pea plants can self-fertilize • Mendel could also cross-pollinate plants: moving pollen from one plant to another 44

  44. Mendel chose peas luckily • Pea plants are good for genetic research • relatively simple genetically • most characters are controlled by a single gene • each gene has only 2 alleles, one of which is completely dominant over the other 45

  45. Any Questions?? 46

  46. Extending Mendelian genetics • Mendel worked with a simple system • peas are genetically simple • most traits are controlled by a single gene • each gene has only 2 alleles, 1 of which is completely dominant to the other • The relationship between genotype & phenotype is rarely that simple

  47. Exceptions to Medelian Genetics • Incomplete Dominance • Codominance • Epistasis • Pleiotrophy • Polygenetic Inheritance • Multiple Alleles • Sex-linked traits • Environmental Effects

  48. The Spectrum of Dominance • Diseases/Disorders are not just the result of the presence of the dominant allele • Tay-Sachs disease: homozygous recessive • Polydactyly: dominant allele that is more common than the allele for 5 digits (399/400 people are recessive to this disorder)

  49. 1) Incomplete dominance • Heterozygotes show an intermediate phenotype • RR = red flowers • rr = white flowers • Rr = pink flowers • make 50% less color

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