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Chapter 8 - Patterns of Inheritance

Chapter 8 - Patterns of Inheritance. CHAPTER 8 GENETICS AND HEREDITY. CHAPTER OUTLINE. Outline. I. THE CONCEPT OF A GENE (MENDEL AND HIS WORK) II. THE PRINCIPLE OF SEGREGATION Monohybrid cross Test cross Dihybrid cross III. THE CHROMOSOMAL BASIS OF MENDEL’S LAWS IV. LINKAGE

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Chapter 8 - Patterns of Inheritance

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  1. Chapter 8 - Patterns of Inheritance CHAPTER 8 GENETICS AND HEREDITY

  2. CHAPTER OUTLINE Outline • I. THE CONCEPT OF A GENE (MENDEL AND HIS WORK) • II. THE PRINCIPLE OF SEGREGATION • Monohybrid cross • Test cross • Dihybrid cross • III. THE CHROMOSOMAL BASIS OF MENDEL’S LAWS • IV. LINKAGE • V. MUTATIONS • BROADENING THE CONCEPT OF A GENE • Incomplete dominance • Multiple alleles • Epistasis • Polygenic inheritance • Pleiotropy • Cytoplasmic inheritance • Phenotype is the result of genotype interacting with environment • VII. THE STRUCTURE OF DNA • DNA REPLICATION • DNA AS A CARRIER OF INFORMATION

  3. Chapter 8 - Patterns of Inheritance Mendel

  4. Garden pea traits

  5. Pea flower

  6. MENDEL'S WORK Mendel’s postulates • Postulates (Modernized) • Traits are controlled by heritable factors (genes) • Factors (genes) are passed from parent to child in reproductive cells • When there are two or more traits, they segregate independently of each other

  7. Chapter 8 - Patterns of Inheritance Terminology

  8. PHENOTYPE = the appearance of an organism or trait Phenotype Purple flower White flower Phenotypes are purple and white flowers In this case, phenotype is controlled by a single gene with two alleles, W and w

  9. Monohybrid cross W and w are alleles of the same gene W w They are located at the same position or locus on homologous chromosomes • Postulates (Modernized) • Each individual contains pairs of factors (alleles) in every cell except reproductive cells

  10. Dominant alleles mask the expression of recessive alleles Monohybrid cross - alleles W is dominant (pink) w is recessive(White) recessive is usually indicated by lower case letters W w Note: The names of alleles vary from organism to organism. Here, alleles are indicated by a single letter representing the recessive form of the gene, “white”

  11. Homozygous = both alleles are the same Heterozygous = alleles are different Monohybrid cross - alleles Homozygous Homozygous Heterozygous W W w w W w

  12. GENOTYPE = the genetic composition of an organism or trait Phenotype Purple flower = WW or Ww White flower = ww WW = homozygous dominant Ww = heterozygous ww = homozygous recessive

  13. How to do a cross MONOHYBRID CROSS (one gene cross)

  14. A monohybrid cross is a standard cross used to teach genetic principles A. Monohybrid cross - terms A monohybrid cross starts with “true breeding” = homozygous parents PURPLE FLOWER = WW WHITE FLOWER = ww

  15. The role of meiosis!!! • Mendels First Law of Segregation (Modernized) • Paired factors (alleles / chromosomes) segregate during the formation of reproductive cells (meiosis) so that each cell gets one factor (allele) of a pair.

  16. Monohybrid Cross

  17. F1 generation is Pink (dominant color) and is heterozygous (Ww) F2 generation: Phenotype 3 purple:1 white Genotype 1 WW:2Ww:1ww

  18. LAWS OF PROBABILITY 1. IF TWO EVENTS ARE INDEPENDENT, MULTIPLY THEIR INDIVIDUAL PROBABILITIES TO GET THE PROBABILITY THAT THEY WILL OCCUR TOGETHER 2. IF TWO EVENTS ARE NOT INDEPENDENT, ADD THEIR INDIVIDUAL PROBABILITIES 1/2 1/2 1/2 1/4 1/4 1/2 X 1/2 = 1/4 1/4+1/4+1/4=3/4 1/2 1/4 1/4

  19. Chapter 8 - Patterns of Inheritance Test Cross

  20. A test cross is a cross between an unknown genotype and a homozygous recessive Test cross ww Ww or WW X Note: Crosses between a heterozygote and a homozygote recessive always produce a 1:1 progeny ratio. Test crosses using more than one gene are used to determine if genes are linked on the same chromosome See next page for cross

  21. try this cross ww WW X Test cross Gametes Next generation

  22. W W w ww WW X Test cross gametes w 1 all Ww 1

  23. try this cross ww Ww X Test cross Gametes Next generation

  24. W W w ww Ww X Test cross gametes w w 1 1/2 Ww 1/2 1/2 w 1/2 ww

  25. Note: Crosses between a heterozygote and a homozygote recessive always produce a 1:1 progeny ratio. Test crosses using more than one gene are used to determine if genes are linked on the same chromosome Aa X aa = 1/2Aa:1/2aa AaBb x aabb = 1/4 AaBb:1/4 Aabb:1/4 aaBb:1/4 aabb

  26. Chapter 8 - Patterns of Inheritance Dihybrid Cross

  27. ROUND SEEDS = R WRINKLED = r Dihybrid crosses R and r are alleles of the same gene R r They are located at the same position or locus on homologous chromosomes

  28. ROUND SEEDS = R WRINKLED SEEDS = r Dihybrid crosses YELLOW SEEDS = Y GREEN SEEDS = y R r Y y

  29. When considering two or more traits, each trait (chromosome) assorts independently of the others [Mendels second law of independent assortment] y R Y R or r Y r y

  30. P1 generation = Parental generation RRYY X rryy Dihybrid crosses Phenotype r y Y R Chromosomes and alleles r y Y R Gametes F1 generation

  31. P1 generation = Parental generation RRYY X rryy Dihybrid crosses Phenotype r y Y R Chromosomes and alleles r y Y R Meiosis R Y Gametes ry RrYy = Round Yellow Seeds F1 generation

  32. F1 generation RrYy RrYy X Dihybrid crosses R R R Y R Y Chromosomes and alleles y r y r meiosis (independent assortment) 1/4 RY 1/4 RY Gametes 1/4 ry ry 1/4 1/4 Ry 1/4 Ry 1/4 1/4 rY rY

  33. F1 generation RrYy RrYy X Dihybrid crosses R R R Y R Y Chromosomes and alleles y r y r meiosis 1/4 RY 1/4 RY Gametes 1/4 ry ry 1/4 1/4 Ry 1/4 Ry 1/4 1/4 rY rY

  34. Dihybrid crosses

  35. AN EASIER (OR AT LEAST ALTERNATE) WAY TO CALCULATE PROGENY Dihybrid crosses • Segregation of chromosome 1 is independent of chromosome 2 • When two events are independent, multiply their inndividual probabilities to get the probability that they will occur together

  36. F1 generation RrYy RrYy X Dihybrid crosses R R R Y R Y Chromosomes and alleles y r y r This dihybrid cross can be expressed as two independent monohybrid crosses R r Yy X Yy Rr X Rr 3/4 Yellow 1/4 Green 3/4 Round 1/4 Wrinkled Y y

  37. F1 generation RrYy RrYy X Dihybrid crosses R R R Y R Y Chromosomes and alleles y r y r This dihybrid cross can be expressed as two independent monohybrid crosses 9/16 Round Yellow Yy X Yy Rr X Rr 3/16 Round Green X 3/16 Wrinkled Green 3/4 Yellow 1/4 Green 1/16 Wrinkled Green 3/4 Round 1/4 Wrinkled

  38. Dihybrid cross – much faster method AbBb X AaBb Aa X Aa Bb X Bb 9/16 A-B- 3/16 A-bb 3/16 aaB- 1/16 aabb ¾ A- ¼ aa ¾ B- ¼ bb =

  39. Chapter 8 - Patterns of Inheritance Linkage

  40. A two-gene test cross involving genes on different chromosomes AaBb Linkage aabb a b A B X a a b b all ab 1/4 AB 1/4 Ab 1/4 aB 1/4 ab gametes If the A and B genes are on two different chromosomes, the progeny are equal in frequency 1:1:1:1 1/4 AaBb 1/4 Aabb 1/4 aaBb 1/4 aabb progeny

  41. Linkage A B a b Genes on the same chromosome are “linked”

  42. AaBb parent of the test cross Linkage chromosomes shown after formation of chromatids in Interphase S phase A B A B a b a b Without crossing over, they would be inherited as a single unit Crossing over (recombination) breaks up sets of linked alleles on a chromosome i.e., A would not always go with B and a with b AB ab AB ab 4 products of meiosis but only two kinds

  43. Recombination can occur between any two chromatids Link and crossing overage A B a b With crossing over, additional gametes are possible AB ab More frequent (non-crossovers) Ab aB Less frequent (crossovers)

  44. Gametes from AaBb heterozygous parent – Unlinked genes = Frequencies equal AB ab Link and crossing overage Ab aB AB ab Gametes from AaBb heterozygous parent – Linked genes = Frequencies unequal Recombinants are less frequent Ab aB

  45. The distance between genes is proportional to the amount of crossing over between them Close genes have little or no crossing over Distant genes have a high level of crossing over Therefore, the percentage of crossovers is used to determine the distance between genes Percent crossing over = gene distance Linkage 10% 5% a B c 5% 10% A b C

  46. Linkage Results of a 2 gene test cross (CcGg X ccgg) Curved green = 1150 Straight yellow = 1152 Curved yellow = 38 Straight green = 39 Are the genes linked? Yes because the ratio is not 1:1:1:1 The distance between the genes is (38 + 39)/2380 = 3.24% The distance between C and G = 3.24 map units

  47. Linkage From the lab book Results of a 2 gene test cross (CcGg X ccgg) Curved green = 1150 Straight yellow = 1152 Curved yellow = 38 Straight green = 39 Are the genes linked? Yes because the ratio is not 1:1:1:1 The distance between the genes is (38 + 39)/2380 = 3.24% The distance between C and G = 3.24 map units

  48. Chapter 8 - Patterns of Inheritance Mutations

  49. Frame shift mutations are an insertion or deletion of a single or few bases. It results in a change the amino acids translated from the DNA starting with the mutation

  50. MUTATIONS INVOLVING ONE OR A FEW BASES • POINT MUTATIONS • FRAME SHIFT MUTATIONS MUTATIONS POINT MUTATIONS – CHANGE IN A SINGLE BASE AACCTGTCAC AACTTGTCAC

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