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Gregor Mendel

Mendelian Genetics. Gregor Mendel. Beliefs about Heredity. Fig. 1. De la propagation du genre humain, ou manuel indispensable pour ceux qui veulent avoir de beaux enfants de l’un ou l’autre sexe (Paris, Year VII). Image courtesy of the Bibliothèque Interuniversitaire de Médecine, Paris.

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Gregor Mendel

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  1. Mendelian Genetics Gregor Mendel

  2. Beliefs about Heredity • Fig. 1. De la propagation du genre humain, ou manuel indispensable pour ceux qui veulent avoir de beaux enfants de l’un ou l’autre sexe (Paris, Year VII). Image courtesy of the Bibliothèque Interuniversitaire de Médecine, Paris.

  3. Homunculus How is “heredity passed on: Spermist vs Ovists Spermist conception of a human sperm

  4. Homunculus Leeuwenhoek’s black male and white female rabbit experiments: spermist “proof”

  5. Darwin What he got right. What he got wrong. • Likes produce likes • Change can be permanent • There is no limit to cumulative change • Acquired characteristics Ex. Blind cave animals • Sex. Repro.- gemmules from all over body are packed in sperm and egg • Blended inheritance

  6. Mendel’s Three Principles • Dominance • Segregation • Independent Assortment (1822-1884) The foundation of “classical” science

  7. Dominance • Traits of both parents inherited, but one shows over the other • Traits are not blended

  8. Dominance Mechanism • Two alleles are carried for each trait • In true-breeding individuals, both alleles are the same. • Hybrids, on the other hand, have one of each kind of allele. • One trait is dominant, the other trait is recessive

  9. Segregation • Half the gametes (egg or sperm) will carry the traits of one parent and half the traits for the other parent Pairs of alleles are separated (=segregated) during meiosis

  10. Independent Assortment Two different parental characteristics will be inherited independently of one another during gamete formation. Example: flower color and leaf shape

  11. Mendel’s Genetics Experiments with Peas

  12. Pea Characters Investigated

  13. allele for purple flowers gene locus for flower color allele for white flowers Genetic Loci & Alleles

  14. Genetic Recombination at Fertilization

  15. Mendel’s Test for 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. • Established true-breeding varieties for each of these traits.

  16. Mendel’s Test for Evaluating the Inheritance of Seed Shape in Pea Plants • Crossed true-breeding round seed variety with the true breeding wrinkled seed variety (this represents the Parental, or P, cross). • Offspring (F1 generation) are called “hybrids”.

  17. Mendel’s Test for Evaluating the Inheritance of Seed Shape in Pea Plants • 100% of these hybrids produced round seeds. • Crossed these F1 generation hybrids among each other (individual crosses being selected at random).

  18. Mendel’s Test for Evaluating the Inheritance of Seed Shape in Pea Plants • Results for their offspring (F2 generation): • 5474 (74.7%) were plants that produced round seeds.

  19. Mendel’s Test for Evaluating the Inheritance of Seed Shape in Pea Plants • Results for their offspring (F2 generation): • 1850 (25.3%) were plants that produced wrinkled seeds.

  20. Mendel’s Test for Evaluating the Inheritance of Seed Shape in Pea Plants • Results for their offspring (F2 generation): • Ratio of round:wrinkled = approx. 3:1

  21. Mendel’s Explanation for his Results • 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.

  22. Mendel’s Explanation for his Results • In true-breeding individuals, both alleles are the same. • Hybrids, on the other hand, have one of each kind of allele.

  23. Mendel’s Explanation for his Results • 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.

  24. Mendel’s Explanation for his Results • 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.

  25. Mendel’s Explanation for his Results • Defining alleles: R = the round allele (dominant). r = the wrinkled allele (recessive).

  26. Mendel’s Explanation for his Results • Defining genotypes & their phenotypes: RR genotype (homozygous dominant) = round phenotype. rr genotype (homozygous recessive) = wrinkled phenotype. Rr genotype (heterozygous) = round phenotype

  27. Pea Shapes RR Rr rr

  28. Mendel’s Explanation for his Results Parental (P) Cross Round X Wrinkled RR rr Gametes: R r

  29. Mendel’s Explanation for his Results F1 Generation Hybrids 100% Round 100% Rr

  30. Mendel’s Explanation for his Results F1 Generation Cross Round X Round RrRr Gametes: R rRr

  31. Mendel’s Explanation for his Results F1 Gametes: R rRr F2 Generation Outcome Round Wrinkled RR RrRrrr Note: 3:1 ratio of round:wrinkled

  32. Rr gametes R r R RR Rr Rr r Rr rr Punnett Square Diagram

  33. Mendel’s Law of Segregation In the formation of gametes, two members of a gene pair (alleles) segregate into different haploid gametes with equal probability.

  34. A Testcross F1 Hybrids (Rr) x F2 Recessives (rr)

  35. Rr rr Punnett Square Diagram for Testcross rr Rr x rr gametes r R Rr r

  36. 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.

  37. Mendel’s Test Using the Seed Shape Character with the Seed Color Character Define Alleles and Associated Traits: Seed Shape Character R = round seed trait (dominant) r = wrinkle seed trait (recessive)

  38. Mendel’s Test Using the Seed Shape Character with the Seed Color Character Define Alleles and Associated Traits: Seed Color Character Y = yellow seed trait (dominant) y = green seed trait (recessive)

  39. Mendel’s Test Using the Seed Shape Character with the Seed Color Character • Parental (P) cross: crossed true-breeding plants that produce round-yellow (RRYY) seeds with true-breeding plants that produce wrinkled-green seeds (rryy).

  40. Mendel’s Test Using the Seed Shape Character with the Seed Color Character • F1 Generation: hybrids for both characters (Rr & Yy; or RrYy) were100% round-yellow. • F1 Generation Individuals (RrYy) crossed among each other.

  41. Mendel’s Test Using the Seed Shape Character with the Seed Color Character • F2 Generation Results: • 315 (56.7%) round-yellow • 108 (19.4%) round-green • 101 (18.2%) wrinkled-yellow • 32 (5.8%) wrinkled-green

  42. No Independent Assortment

  43. With Independent Assortment

  44. Calculating Expected Frequencies Round-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

  45. 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

  46. Calculating Expected Frequencies Wrinkled- 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

  47. Calculating Expected Frequencies Wrinkled- 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

  48. Observed Round-Yellow 315 Round-Green 108 Wrinkled-Yellow 101 Wrinkled-Green 32 Expected Round-Yellow 312.75 Round-Green 104.25 Wrinkled-Yellow 104.25 Wrinkled-Green 34.75 Comparing Observed to Expected Results

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