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 pollination Figure 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 traits Table 10.1

8. one of Mendel’s characters Figure 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 (F 1 = first filial generation) –scored traits in F 1 and F 2 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 F 1 progeny were self-pollinated traits of F 1 & F 2 progeny were scored

11. Mendel’s Experiments Mendel’s experimental design –Protocol #1: monohybrid crosses –Results all F 1 progeny exhibited the same trait F 2 progeny exhibited both parental traits in a 3:1 ratio (F 1 trait: alternate trait)

12. Mendel’s Experiments Mendel’s experimental design –Protocol #1: monohybrid crosses –Analysis F 1 trait is dominant alternate trait is recessive –disappears from the F 1 generation –reappears, unchanged, in F 2 –Relevance all seven characters have dominant and recessive traits appearing 3:1 in F 2

13. seven traits were inherited similarly Table 10.1

14. Mendel’s interpretation: inheritance does not involve blending Figure 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 –F 1 :Ss each parent packages one gene in each gamete gametes combine randomly

17. recessive traits disappear in the F1 generation Figure 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 F 1 :Ss gamete formation S or s self pollination: S with S s with s S with s or s with S F 2 : SS, ss, Ss, sS

20. Punnett to the rescue Figure 10.4

21. P: (SS or ss) p(S)=1 x p(s)=1 F 1 : (Ss) p(Ss) =1 x 1=1 p(S)=1/2, p(s)=1/2, so F 2 : 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 meiosis Figure 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 F 1 & F 2 generations P: Roundx wrinkled F 1 :1/2 Round, 1/2 wrinkled F 2 :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 F 1 & F 2 generations P: Round (Rr)x wrinkled (rr) F 1 :1/2 Round (Rr), 1/2 wrinkled (rr) F 2 :3/4 Round, 1/4 wrinkled OR all wrinkled (RR,Rr,rR,rr) (rr)

25. a test cross distinguishes between a homozygous dominant and a heterozygous parent Figure 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 F 1 : scored phenotypes & self-pollinated F 2 : scored phenotypes

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

28. combining probabilities of two characters Figure 10.7

29. four different gametes by meiosis in F 1 dihybrid progeny Figure 10.8 or

30. Mendel’s Experiments Protocol #2: dihybrid crosses –results F 1 : all shared traits of one parent F 2 : –traits of both parents occurred in 5/8 of F 2 at a 9:1 ratio –nonparental pairs of traits appeared in 3/8 of F 2 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 disease Figure 10.10

35. a pedigree of albinism Figure 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 Phenotypes Figure 10.12

38. incomplete dominance: intermediate phenotypes Figure 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 phenotypes Figure 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 maize Figure 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 & expressivity variation in the population due to differences in penetrance, expressivity & genotype Figure 10.17

48. Drosophila melanogaster Figure 10.18

49. More Mendelian luck: gene linkage gene linkage was first demonstrated in Drosophila melanogaster –some genes do not assort independently F 2 phenotype ratios are not 9:3:3:1 F 1 test cross ratios are not 1:1:1:1 –more parental combinations appear than are expected –fewer recombinant combinations appear than are expected

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

51. hypothetical reproduction without crossing over at prophase I of meiosis

52. crossing over can change allele combinations of linked loci Figure 10.19

53. recombination frequency depends on distance Figure 10.20 391/2300=0.17 17 map units

54. More Mendelian luck: gene linkage if genes were completely linked, only parental phenotypes would result if genes assort independently phenotypes arise in 9:3:3:1 ratio in F 2 when genes are linked, recombinant phenotypes are fewer than expected recombinant frequencies depend on distance –distances can be estimated from recombination rates (1% = 1 map unit)

55. chromosome mapping Figure 10.21 YyMm x yymm wt yell. min. y/m expected/1000 250 250 250 250 actual/1000 323 178 177 322

56. Mendel’s luck: sex-linked genes Sex determination –honey bees: diploid female, haploid male –grasshopper: XX female, XO male –mammals: XX female, XY male SRY gene determines maleness –Drosophila: XX female, XY male ratio of X:autosomes determines sex –birds, moths & butterflies: ZZ male, ZW female

57. Mendel’s luck: sex-linked genes genes carried on X chromosome are absent from the Y chromosome a recessive sex-linked allele is expressed in the phenotype of a male –females may be “carriers” –males express the single allele

58. sex-linked genes Figure 10.23

59. Mendel’s luck: sex-linked genes human sex-linked inheritance can be deduced from pedigree analysis

60. inheritance of X-linked gene Figure 10.24

61. Mendel’s Principles Principle of segregation –two alleles for a character are not altered by time spent together in a diploid nucleus Principle of independent assortment –segregation of alleles for one character does not affect segregation of alleles for another character unless both reside on the same chromosome