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Mendelian Genetics

Mendelian Genetics. Vocabulary. dominant gene: F 1 generation: F 2 generation: gene: genetic trait: genotype: monohybrid cross:. Hybrid: phenotype: principle of independent assortment: principle of segregation: Punnett square: Recessive gene:. Study Questions.

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Mendelian Genetics

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  1. Mendelian Genetics KMarsh

  2. Vocabulary • dominant gene: • F1 generation: • F2 generation: • gene: • genetic trait: • genotype: • monohybrid cross: KMarsh

  3. Hybrid: • phenotype: • principle of independent assortment: • principle of segregation: • Punnett square: • Recessive gene: KMarsh

  4. Study Questions • 1. List and explain the four principles of genetics established by Mendel. • 2. Complete a monohybrid cross between a pure breeding tall plant (TT) and a pure breeding dwarf plant (tt). Carry the cross through to the second generation (F2 generation) by letting the plants of the first cross (Tt) self-fertilize. Describe the phenotypes and genotypes of both generations. KMarsh

  5. 3. Show how your knowledge of meiosis provides tangible evidence concerning Mendel’s Principles of Segregation and Independent Assortment. • 4. Complete a monohybrid cross between (Tt X tt). List all the potential types of gametes produced by each parent and then combine them into all possible combinations in the offspring. List the various phenotypes and ratios from this cross. • 5. List and discuss several of the approaches which made Mendel’s work successful. KMarsh

  6. Review Meiosis KMarsh

  7. Mendel’s Work with Pea Plants • First Steps to Genetics KMarsh

  8. Mendel’s Studies of the Pea Plant • A. Characteristics studied by Mendel (7 traits) KMarsh

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  10. Dominant vs Recessive • 1.Form of ripe seed 2.Color of seed coat Smooth Yellow Wrinkled Green • 3.Flower Color 4.Form of ripe pods • Purple Inflated • White Constricted • 5.Color of unripe pods 6.Position of flowers • Green Axial • Yellow Terminal • 7.Length of stem Tall & Dwarf KMarsh

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  13. Why Pea Plants? • 1- Many true breeding varieties available • 2- The flower is self-fertile • 3- Generation time is very short KMarsh

  14. Mendel’sProblems • 1- No concept of DNA or chromosomes • 2- No concept of meiosis KMarsh

  15. Some wise (educated guesses) decisions made by Mendel • 1- Selection of well-defined, contrasting traits • 2- Extensive groundwork completed prior to establishment of final experimental design • 3- Extensive replication of crosses KMarsh

  16. Mendel’s Test Evaluating the Inheritance of Seed Shape in Pea Plants • Recognized two different traits for the seed shape character in pea plants: round versus wrinkled seeds. KMarsh

  17. Established true-breeding varieties for each of these traits. RR and rr Offspring (F1 generation) are called “hybrids”. Rr Testing continued KMarsh

  18. 100% of these hybrids produced round seeds. Why?

  19. Crossed these F1 generation hybrids among each other (individual crosses being selected at random). Rr X Rr Results for their offspring (F2 generation): 5474 (74.7%) were plants that produced round seeds.1850 (25.3%) were plants that produced wrinkled seeds.Ratio of round:wrinkled = approx. 3:1 The Next Step KMarsh

  20. The Explaination • For every character (e.g., seed shape) an individual possess two instruction sets (alleles). • One of these alleles was originally derived from the individual’s mother, the other allele being originally derived from the individual’s father. KMarsh

  21. In true-breeding individuals, both alleles are the same. Homozygous RR or rr Hybrids, on the other hand, one of each kind of allele. Heterozygous Rr True vs Hybrid KMarsh

  22. Mendel’s Further Explanation • Mendel believed that only two alleles were possible for a given genetic character, and that one of the alleles (the dominant one) masked the expression of the other (the recessive one) in the hybrid. • When the hybrid formed gametes, only one of the two possible alleles would end up in a gamete. KMarsh

  23. When the hybrid formed gametes, only one of the two possible alleles would end up in a gamete. However, both alleles possessed an equal chance of appearing in a gamete. Law of Segregation KMarsh

  24. Law of Segregation con’t • In the formation of gametes, two members of a gene pair (alleles) segregate into different haploid gametes with equal probability. KMarsh

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  27. Dominant and Recessive • Defining alleles: • R = the round allele (dominant). • r = the wrinkled allele (recessive). KMarsh

  28. Phenotype & Genotype • Defining genotypes and their respective phenotypes: • RR genotype (homozygous dominant) = round phenotype. • rr genotype (homozygous recessive) = wrinkled phenotype. • Rr genotype (heterozygous) = round phenotype KMarsh

  29. Round Round RR Rr rr KMarsh

  30. P Generation and F1 Generation Parental (P) Cross • Round X Wrinkled RR rr • Gametes: R r F1 (first felial) Generation Hybrids 100% Round phenotype 100% Rr genotype KMarsh

  31. F1 and F2 Generations • Mendel’s Explanation for his Results • F1 Generation Cross Round X Round Rr RrGametes: R r R rF2 Generation OutcomeRound WrinkledRR Rr Rr rrNote: 3:1 ratio of round:wrinkled KMarsh

  32. Punnett Square Diagram KMarsh

  33. Mendel’s Genetics Experiments With Pea Plants continued… KMarsh

  34. Law of Segregation KMarsh

  35. Genotype Vs. Phenotype KMarsh

  36. Test Crosses When you cross an individual whose genetics you are not sure about with an individual who is homozygous recessive for the trait(s) in question.

  37. Test Crosses • F1 Hybrids (Rr) xF2 Recessives (rr) KMarsh

  38. Test Cross Pea Plants KMarsh

  39. Mendel’s Law of Independent Assortment • Whenever two or more pairs of contrasting characters are brought together in a hybrid, the alleles of the different pairs segregate independently of one another during gamete formation. KMarsh

  40. Dihybrid Crosses • Define Alleles and Associated Traits:Seed Shape CharacterR = round seed trait (dominant)r = wrinkle seed trait (recessive)Seed Color CharacterY = yellow seed trait (dominant)y = green seed trait (recessive KMarsh

  41. First Cross (Dihybrid) • Parental (P) cross: crossed true-breeding plants that produce round-yellow (RRYY) seeds with true-breeding plants that produce wrinkled-green seeds (rryy). • F1 Genereation: hybrids for both characters (Rr & Yy; or RrYy) were100% round-yellow. KMarsh

  42. Results • F1 Generation Individuals (RrYy) crossed among each other.F2 Generation Results:315 (56.7%) round-yellow108 (19.4%) round-green101 (18.2%) wrinkled-yellow32 (5.8%) wrinkled-green KMarsh

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  44. With Independent Assortment KMarsh

  45. Calculating Expected FrequenciesRound-Yellow • Expect 9/16 of the F2 generation offspring to be round-yellow.Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows:Frequency = (9/16)*556 = 312.75 KMarsh

  46. Calculating Expected Frequencies Round-Green • Expect 3/16 of the F2 generation offspring to be round-green.Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows:Frequency = (3/16)*556 = 104.25 KMarsh

  47. Calculating Expected FrequenciesWrinkled- Yellow • Expect 3/16 of the F2 generation offspring to be wrinkled-yellow.Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows:Frequency = (3/16)*556 = 104.25 KMarsh

  48. Calculating Expected FrequenciesWrinkled- Green • Expect 1/16 of the F2 generation offspring to be wrinkled-yellow.Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows:Frequency = (1/16)*556 = 34.75 KMarsh

  49. ObservedRound-Yellow315Round-Green108Wrinkled-Yellow101Wrinkled-Green32ObservedRound-Yellow315Round-Green108Wrinkled-Yellow101Wrinkled-Green32 Expected Round- Yellow312.75Round-Green104.25Wrinkled-Yellow104.25Wrinkled-Green34.75 Comparing Observed to Expected Results KMarsh

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