Genetics chapter 2 part 1 (1)


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  • Figure 2.5
  • Figure 2.19
  • 3.4 Mendel’s monohybrid crosses revealed the principle of segregation and the concept of dominance.
  • Genetics chapter 2 part 1 (1)

    1. 1. Genetics Chapter 2 Part 1 Dr. Tricia Hardt Smith
    2. 2. State of genetics in early 1800’s What is inherited? How is it inherited? What is the role of chance in heredity?
    3. 3. Johann Gregor Mendel (1822-1884)  Born to simple farmers in the Czeh Republic  1843: Augustinian monastery (Brno)  1851-53: University of Vienna; Physics Institute  Mathmatics, chemistry, entomology, paleotology, botany, plant physiology  1856-1863: Pea plant breeding experiments  1866: Published his findings  1900: Three botanists (De Vries, von Tschermak & Correns) independently conduct the same experiments, come across Mendel’s paper and draw attention to his work.
    4. 4. Mendelian genetics • Mendel’s work unnoticed until 1900’s • Introduced concept of “units of inheritance” • When correlated with cytological data → Transmission genetics was born
    5. 5. Mendel’s workplace Fig. 2.5
    6. 6. Chapter 2 Opener
    7. 7. Why pea plants? Easy to grow and hybridize artificially • Reproduce well. • Each seed is a new individual, can measure the characteristics of a large number of offspring after one breeding season • Grow to maturity in single season
    8. 8. Mendel’s Approach • Mendel obtained 34 different varieties of peas from local suppliers and examined the characteristics of each • He identified 14 strains representing seven specific traits each with two forms that could be easily distinguished. He spent two years making sure these varities bred true. Jos A. Smith • He worked with these strains for 5 years, determining how each character was inherited
    9. 9. 1900 - Carl Correns, Hugo deVries, and Erich von Tschermak rediscover and confirm Mendel’s laws. Mendel published in 1866, was not appreciated in his lifetime.
    10. 10. Mendel’s Approach Followed the Modern Scientific Method 1. Make initial observations about a phenomenon or process 2. Formulate a testable hypothesis 3. Design a controlled experiment to test the hypothesis 4. Collect data from the experiment 5. Interpret the experimental results, comparing them to those expected under the hypothesis 6. Draw a conclusion and reformulate the hypothesis if necessary One of Mendel’s strengths was his careful experimental design
    11. 11. Five Critical Experimental Innovations • There were five features of Mendel’s breeding experiments that were critical to his success • Controlled crosses • Use of pure breeding strains • Selection of dichotomous traits • Quantification of results • Use of replicate (repeated), reciprocal, and test crosses • Luck?
    12. 12. Controlled Crosses Between Plants • Pea plants are capable of self-fertilization and artificial crossfertilization • Self-fertilization occurs naturally • Cross-fertilization involves removing the anthers from a flower and introducing pollen of the desired type with a small brush From Peirce Genetics
    13. 13. Pure-Breeding Strains to Begin Experimental Crosses • Mendel took 2 years prior to beginning his experiments to establish purebreeding (or truebreeding) strains • Each experiment began with crosses between two pure-breeding parental generation plants (P generation) that produced offspring called F1 (first filial generation)
    14. 14. Monohybrid Crosses Monohybrid Cross: a cross-pollination involving two true-breeding lines that differ for only one trait “Parental Female Male generation” Parents: Smooth Seeds “P” Wrinkled Seeds Progeny: All progeny had smooth seed! All progeny had same PHENOTYPE: “the form that is shown” “F1” “First Filial generation”
    15. 15. Monohybrid Crosses Female Male “P” Smooth Seeds Wrinkled Seeds “F1” Two possible Hypotheses How could you differentiate between these possibilities? Hypothesis 1: The smooth phenotype is “dominant” to the wrinkled phenotype Hypothesis 2: The child’s phenotype is determined by the mother’s phenotype
    16. 16. Mendel Made Reciprocal Crosses Reciprocal Cross: Repeating a particular genetic cross but with the sexes of the two parents switched Female Male Wrinkled Seeds Smooth Seeds “P” All F1 had smooth seed. “F1” Conclusion - Phenotype is not determined by the mother’s phenotype - The smooth trait is “dominant” to the wrinkled trait
    17. 17. • The trait shown by the F1 offspring was called the dominant phenotype (round peas, e.g.) • The other trait not apparent in the F1 was called the recessive phenotype (wrinkled) • When F1 were crossed, 75% of the resulting F2 had the dominant trait, but the recessive trait reappeared in the other 25%
    18. 18. Alleles • Mendel’s results rejected the blending theory of heredity • Theorized that plants carry two discrete hereditary units for each trait, alleles; a plant receives one of these in the egg and the second in pollen • Together the two alleles for each trait determine the phenotype of the individual Alleles Phenotype
    19. 19. Homozygous and Heterozygous Individuals Homozygous (TT & tt) Heterozygous (Tt) • Pure-breeding individuals, like Mendel’s parent plants, have identical copies of the two alleles for a trait (homozygous individual) • The F1 plants had different alleles from each parent and were heterozygous
    20. 20. Now that we have a Now that we have a theory, we can do theory, we can do some real predicting! some real predicting! • A 3:1 phenotypic ratio is predicted for the F2 produced by a monohybrid cross • A 1:2:1 genotypic ratio is also predicted (¼ G/G, ½ G/g, ¼ g/g)
    21. 21. Punnett Square • The alleles (in gametes) carried by one parent are arranged along the top of the square and those of the other parent, down the side • The results expected from random fusion of the gametes are placed within the square Punnett Square R r RR Rr Rr rr R r
    22. 22. Mendel’s Results Revisited: F1 ♀ ♂ Genotype? Genotype? “AA” Smooth Seeds Wrinkled Seeds Gametes possible: “A” or “A” “aa” Gametes possible: “a” or “a” ♀ Gametes Smooth “Aa” A ♂ Gametes a a Aa Aa A Aa Aa Use a “Punnett Square” to determine all possible progeny genotypes Explains why all progeny were smooth
    23. 23. Br A B C D E rr
    24. 24. What is the predicted cross of homozygous recessive red and heterozygous dominant 20% 20% brown? 20% 20% 20% 1 ed lr Al re d br o w n, 3 re d n, 2 br o w n, 1 2 3 br o w lb Al re d brown B. 3 brown, 1 red C. 2 brown, 2 red D. 1 brown, 3 red E. All red ro w n A. All
    25. 25. B r r Br rr r Br rr Br A B C D E rr
    26. 26. Red + Brown = Blond! (sometimes?) Not all traits are dominant/reccessive! More in upcoming lectures!
    27. 27. Punnett Square Practice Problems! Chapter 2: Problems 2 & 3
    28. 28. Mendel’s First Law • Mendel used his theory of particulate inheritance to formulate the law of segregation (Mendel’s first law) • Alleles are separated into gametes. Gametes randomly combine to create progeny in predictable proportions. • Hypothesis!: Mendel expected that half of the gametes of heterozygous F1 individuals would carry the dominant allele and half the recessive • How can we test this? Genotype? Genotype?
    29. 29. Conclusion: all Conclusion: all F1 plants are F1 plants are heterozygous! heterozygous!
    30. 30. The Test Cross • Allows to distinguish genotype of individual expressing dominant phenotype by crossing it with homozygous recessive individual
    31. 31. What other predictions can we test? • Mendel’s hypothesis predicts that F2 plants with the dominant phenotype can be homozygous or heterozygous • The heterozygous state (2/3) is twice as likely as the homozygous state (1/3) • HOW? Mendel used a selffertilization experiment to test the predictions of the hypothesis
    32. 32. F3 generation • Hypothesis confirmed: • 1/3 of plants were homozygous and breed true • 2/3 of heterozygous F2 plants generated a 3:1 ratio of dominant:recessive phenotype among their progeny
    33. 33. WHAT HAPPENS IF WE STUDY TWO TRAITS? Will the presence of one charastics affect the prescent of another?
    34. 34. Dihybrid-Cross Analysis of Two Genes • To study the simultaneous transmission of two traits, Mendel made dihybrid crosses between organisms that differed for two traits • He began each cross with pure-breeding lines (e.g., RRGG and rrgg) and produced F1 that were heterozygous for both traits (e.g., RrGg). • If assortment is random, four gametes should be equally likely in the F1 (e.g., RG, Rg, rG, rg)
    35. 35. 2.3 Dihybrid and Trihybrid Crosses • How can we calculate the crosses of two or more traits at the same time? • Dihybrid Punnet Square • Forked Diagram
    36. 36. An Aid to Prediction of Gamete Frequency • The forked-line diagram is used to determine gamete genotypes and frequencies
    37. 37. Let’s give it a try!  • Self Fertilization of a heterozygous yellow, round pea? • Round (R) is dominant to wrinkled (r) • Yellow (G) is dominant to green (g) F2 ? • What does the dihybrid Punnet square look like? • What does the forked diagram look like?
    38. 38. Independent Assortment of Alleles from the RrGg × RrGg Cross • Mendel predicted that alleles of each locus unite at random to produce the F2, generating • round, yellow R-G- (¾)(¾) = 9/16 • round, green R-gg (¾)(¼) = 3/16 • wrinkled, yellow rrG- (¾)(¼) = 3/16 • wrinkled, green rrgg (¼)(¼) = 1/16 9:3:3:1 ratio! 9:3:3:1 ratio! The dihybrid ratio: 9/16 both dominant traits, 3/16 each for two combinations of one dominant and one recessive, and 1/16 both recessives
    39. 39. Mendel’s Second Law • The 9:3:3:1 ratios generated in Mendel’s dihybrid crosses illustrate Mendel’s second law, also known as Mendel’s law of independent assortment • The law states that during gamete formation the segregation of alleles at one locus is independent of the segregation of alleles at another locus. • Within the 9:3:3:1 ratio, Mendel recognized two 3:1 ratios for each trait
    40. 40. Testing Independent Assortment by TestCross Analysis • Mendel wants to test his hypothesis about independent assortment. HOW? Test Cross! • He predicted that the F1 seeds were dihybrid, of genotype RrGr, and that crossing them to a plant of genotype rrgg would yield four offspring phenotypes with equal frequency
    41. 41. Testing Independent Assortment by Trihybrid-Cross Analysis • To test his hypothesis about independent assortment further, Mendel performed trihybrid-cross analysis • The trihybrid cross involved three traits: round vs. wrinkled peas, yellow vs. green peas, and purple vs. white flowers • The cross was: RRGGPP × rrggpp; the F1 were RrGgPp
    42. 42. How many possible combinations? • Double check yourself! Do you see all the possible combinations of phenotypes in your answer? • The number of possibilities can be expressed as 2 n, where n = number of genes • In a trihybrid cross, there are 8 possibilties 2 3 = 8!
    43. 43. Go try some problems!  • Chapter 2, problem 6
    44. 44. Questions?