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Mendelian Genetics

Mendelian Genetics. Simple Probabilities & a Little Luck. Genetics. the study of heredity & its mechanisms Gregor Mendel reported experimental results in 1865/66 rediscovered in 1903 by de Vries, Correns & von Tschermak. Genetics. Before Mendel, heredity was seen as

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Mendelian Genetics

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  1. Mendelian Genetics Simple Probabilities & a Little Luck

  2. Genetics • the study of heredity & its mechanisms • Gregor Mendel • reported experimental results in 1865/66 • rediscovered in 1903 by de Vries, Correns & von Tschermak

  3. Genetics • Before Mendel, heredity was seen as • the blending of parental contributions • unpredictable • Mendel demonstrated that heredity • involves distinct particles • is statistically predictable

  4. Cross pollinationFigure 10.1

  5. Mendel’s Experiments • the model system • garden pea varieties • easy to grow • short generation time • many offspring • bisexual • reciprocal cross-pollination • self-compatible • self-pollination

  6. Mendel’s Experiments • garden pea varieties • many variable characters • a character is a heritable feature • flower color • a trait is a character state • blue flowers, white flowers, etc. • a heritable trait is reliably passed down • a true-breeding variety produces the same trait each generation

  7. 7 characters, 14 traitsTable 10.1

  8. one of Mendel’s charactersFigure 10.2

  9. Mendel’s Experiments • Mendel’s experimental design • selected 7 characters with distinct traits • crossed plants with one trait to plants with the alternate trait (P = “parental” generation) • self-pollinated offspring of P (F1 = first filial generation) • scored traits in F1 and F2 generations

  10. Mendel’s Experiments • Mendel’s experimental design • Protocol#1: monohybrid crosses • parents were true-breeding for alternate traits of one character • parents were reciprocally cross-pollinated • F1 progeny were self-pollinated • traits of F1 & F2 progeny were scored

  11. Mendel’s Experiments • Mendel’s experimental design • Protocol#1: monohybrid crosses • Results • all F1 progeny exhibited the same trait • F2 progeny exhibited both parental traits in a 3:1 ratio (F1 trait: alternate trait)

  12. Mendel’s Experiments • Mendel’s experimental design • Protocol#1: monohybrid crosses • Analysis • F1 trait is dominant • alternate trait is recessive • disappears from the F1 generation • reappears, unchanged, in F2 • Relevance • all seven characters have dominant and recessive traits appearing 3:1 in F2

  13. seven traits were inherited similarlyTable 10.1

  14. Mendel’s interpretation:inheritance does not involve blendingFigure 10.3

  15. Mendel’s Experiments • Mendel’s experimental design • Protocol#1: monohybrid crosses • Interpretation • inheritance is by discrete units (particles) • hereditary particles occur in pairs • particles segregate at gamete formation • particles are unaffected by combination • =>Mendel’s particles are genes<=

  16. Mendel’s Experiments • Mendel’s experimental design • Protocol#1: monohybrid crosses • symbolic representation • P: SS x ss • F1: Ss • each parent packages one gene in each gamete • gametes combine randomly

  17. recessive traits disappear in the F1 generationFigure 10.4

  18. Mendel’s Experiments • Mendel’s experimental design • Protocol#1: monohybrid crosses • [terminology • different versions of a gene = alleles • two copies of an allele = homozygous • one copy of each allele = heterozygous • genetic constitution = genotype • round or wrinkled seeds = phenotype • the genotype is not always seen in the phenotype]

  19. Mendel’s Experiments • Mendel’s experimental design • Protocol#1: monohybrid crosses • symbolic representation P: SS x ss F1: Ss gamete formation S or s self pollination: S with S s with s S with s or s with S F2: SS, ss, Ss, sS

  20. Punnett to the rescueFigure 10.4

  21. P: (SS or ss) p(S)=1 x p(s)=1F1: (Ss) p(Ss) =1 x 1=1 p(S)=1/2, p(s)=1/2, so F2: p(SS) =1/2 x 1/2=1/4 p(ss) =1/2 x 1/2=1/4 p(Ss)=[1/2x1/2=1/4] x 2=1/2

  22. Punnett explained by meiosisFigure 10.5 F1: Ss replication S-S &s-s anaphase I S-S or s-s anaphase II S or S or s or s

  23. Mendel’s Experiments • Mendel’s experimental design • Protocol#1: monohybrid crosses • if you know the genotypes of the parental generation you can predict the phenotypes of the F1 & F2 generations P: Round x wrinkled F1: 1/2 Round, 1/2 wrinkled F2: 3/4 Round, 1/4 wrinkled OR all wrinkled

  24. Mendel’s Experiments • Mendel’s experimental design • Protocol#1: monohybrid crosses • if you know the genotypes of the parental generation you can predict the phenotypes of the F1 & F2 generations P: Round (Rr) x wrinkled (rr) F1: 1/2 Round (Rr), 1/2 wrinkled (rr) F2: 3/4 Round, 1/4 wrinkled OR all wrinkled (RR,Rr,rR,rr) (rr)

  25. a test cross distinguishes between a homozygous dominantand a heterozygous parentFigure 10.6

  26. Mendel’s Experiments • Mendel’s experimental design • Protocol#2: dihybrid crosses • P: crossed true breeding plants with different traits for two characters • F1: scored phenotypes & self-pollinated • F2: scored phenotypes

  27. Mendel’s Experiments • Protocol#2: dihybrid crosses • results • F1: all shared the traits of one parent • F2: • traits of both parents occurred in 5/8 of F2 at a 9:1 ratio • non-parental pairs of traits appeared in 3/8 of F2 at a 1:1 ratio

  28. combining probabilities of two charactersFigure 10.7

  29. four different gametes by meiosis in F1dihybrid progenyFigure 10.8 or

  30. Mendel’s Experiments • Protocol#2: dihybrid crosses • results • F1: all shared traits of one parent • F2: • traits of both parents occurred in 5/8 of F2 at a 9:1 ratio • nonparental pairs of traits appeared in 3/8 of F2 at a 1:1 ratio • phenotypic ratios: 9:3:3:1

  31. Mendel’s Experiments • Protocol#2: dihybrid crosses • phenotypic ratios: 9:3:3:1 • predictable if alleles assort independently • character A - 3:1 dominant:recessive • character B - 3:1 dominant:recessive • characters A & B - • 9 dominant A & dominant B • 3 dominant A & recessive B • 3 recessive A & dominant B • 1 recessive A & recessive B

  32. Mendel’s Experiments • Protocol#2: dihybrid crosses • a dihybrid test cross (A_B_ x aabb) • F1 all with dominant parent phenotype, or • 1:1:1:1 phenotypes

  33. Mendel without the experiments: pedigrees • tracking inheritance patterns in human populations • uncontrolled experimentally • small progenies • unknown parental genotypes • Mendelian principles can interpret phenotypic inheritance patterns

  34. a pedigree of Huntington’s diseaseFigure 10.10

  35. a pedigree of albinismFigure 10.11

  36. some Mendelian luck • Multiple alleles • a single gene may have more than two alleles and multiple phenotypes

  37. One Character, Four Alleles, Five PhenotypesFigure 10.12

  38. incomplete dominance: intermediate phenotypesFigure 10.13

  39. some Mendelian luck • Incomplete Dominance • alters creates new intermediate phenotypes • reveals genotypes • Co-dominance • creates new dominant phenotypes

  40. co-dominance produces additional phenotypesFigure 10.14

  41. some Mendelian luck • genes may interact • epistasis • for mouse coat color • BB or Bb => agouti, bb => black • AA or Aa => colored, aa => white • AaBb x AaBb => 9 agouti, 3 black, 4 white • 9 AA or Aa with BB or Bb • 3 AA or Aa with bb • 3 aa with BB, Bb; 1 aa with bb = 4 white

  42. white, black & agouti Figure 10.15

  43. some Mendelian luck • genes may interact • hybrid vigor (heterosis) • hybrids are more vigorous than either inbred parent

  44. hybrid vigor in maizeFigure 10.16

  45. some Mendelian luck • genes may interact • quantitative traits • some traits are determined by many genes, each of which may have many alleles

  46. some Mendelian luck • environment may alter phenotype • some traits are altered by the environment of the organism • penetrance: proportion of a population expressing the phenotype • expressivity: degree of expression of the phenotype

  47. variation in heterozygotes due to differences in penetrance & expressivityvariation in the population due todifferences in penetrance, expressivity & genotypeFigure 10.17

  48. Drosophila melanogasterFigure 10.18

  49. More Mendelian luck: gene linkage • gene linkage was first demonstrated in Drosophila melanogaster • some genes do not assort independently • F2 phenotype ratios are not 9:3:3:1 • F1 test cross ratios are not 1:1:1:1 • more parental combinations appear than are expected • fewer recombinant combinations appear than are expected

  50. Mendel’s luck: some genes are linkedFigure 10.18 2300 test cross progeny

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