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BIO 105

BIO 105. Mendelian Genetics. Patterns of Inheritance. Classic thinking Constancy of species Direct transmission of traits Koelreuter (1760) actually started modern genetics Crossed different species of tobacco plants and offspring (hybrids) didn’t look like either parent

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BIO 105

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  1. BIO 105 Mendelian Genetics

  2. Patterns of Inheritance • Classic thinking • Constancy of species • Direct transmission of traits • Koelreuter (1760) actually started modern genetics • Crossed different species of tobacco plants and offspring (hybrids) didn’t look like either parent • Crossing hybrids produced plants some of which resembled the grandparents • Traits could be masked and then show up again

  3. Patterns (cont.) • Knight (1790) crossed two true breeding varieties of peas • One variety always produced purple flowers and the other always produced white flowers • He crossed them and the progeny all had purple flowers • He crossed the progeny and some had purple flowers and some had white flowers • Some trait was masked (but retained) to be expressed again in another generation.

  4. Key to the Genetic Revolution The Garden Pea Pisum sativum

  5. Man of the Genetic Revolution b. 1822

  6. Garden of the Genetic Revolution Austria

  7. Why did Mendel Choose the Pea Plant • Pea plants observed segregation of many traits to the offspring. • There were many true-breeding varieties….they kept expressing the same traits from one generation to the next. • Small and easy to grow w/ a short generation time • Male of the plant (anthers) could be easily removed to prevent self-fertilization and could easily do cross-fertilization.

  8. Cross-Fertilization

  9. Mendel’s Experimental Design • Made sure varieties were pure • Let them cross-fertilize for several generations • Checked that they always expressed the same trait over generations

  10. Mendel’s Experimental Design • Performed crosses between varieties for the same trait. • Flower color, seed color, pod color, plant height • Removed the male anthers from white flowering plants and fertilized white plants w/ purple flower pollen • Did reciprocal cross

  11. Traits

  12. Mendel’s Experimental Design • Allowed the hybrid offspring to self-fertilize • Unlike previous scientists, he counted the number of offspring with each trait.

  13. Mendel’s Data

  14. A Mendelian Trait

  15. Mendel’s Experiments • Looked at 7 different traits (characters) that had varients easily discernable from each other

  16. Mendel’s Experiments • Crossed 2 contrasting varieties – purple flowering peas w/ white flowering peas • The hybrids (offspring) did not express a color halfway between purple and white. • All offspring resembled one of the parents. • These offspring referred to as first filial or F1 generation (1st generation after parents) • When crossing pure purple with pure white, the F1offspring were all purple. • The dominant trait is the one expressed in F1. • The recessive trait is the one not expressed in F1.

  17. Mendel’s Experiments (cont.) • Allowed F1 offspring to mature and self-fertilize….all expressed purple flowers • Took seeds produced from self-fertilizing F1 plants and planted them. • Offspring from F1 plants are the second filial or F2 generation • Some F2 plants expressed white flowers!!! • Recessive trait (white flowers) was masked for one generation (F1) but was somehow retained and expressed in the next generation (F2).

  18. Mendel’s Experiments (cont.) • Mendel counted the number of purple and white flowering plants produced in the F2 generation. • 705 purple:224 white…75.9%:24.1% • ¾ had dominant trait, ¼ had recessive trait • He obtained the same ratio recessive with the other 6 characters. • Dominant:recessive ratio was 3:1 for all 7 characters.

  19. Mendel’s Experiments (cont.) • When white flowered F2 plants were allowed to self-fertilize, they produced all white flowered offspring. • BUT only 1/3 of the purple flowered F2 plants that self-fertilized produced all purple offspring. • The other 2/3 of the purple flowered F2 plants that self-fertilized produced purple:white flowering plants at a ratio of 3:1. • Thus the original 3:1 F2 ratio was really a 1:2:1 ratio • ¼ pure-breeding dominant, ½ not-pure-breeding dominant and ¼ pure-breeding recessive

  20. Mendel’s Model • Parents transmit discreet information about their traits….factors. • Each offspring receives 2 factors that may affect each trait….one maternal and one paternal. • The offspring is diploid (2n) for this trait. • When the offspring forms gametes (haploid, 1n), only one of the factors is included. • Which of the 2 factors that is incorporated into a gamete is randomly determined.

  21. Mendel’s Model (cont.) • Not all copies of a factor are identical. • Alternate forms of one character are called alleles. • If an organism has 2 copies of the factor that are the same, the organism is said to be homozygous. • When the alleles for a character are different from each other, the organism is said to be heterozygous. Only one of the alleles is expressed….the dominant one. • Alleles remain discrete….don’t blend or alter each other.

  22. Mendel’s Model (cont.) • The totality of alleles is considered the organism’s genotype. • The expression or physical appearance of that organism is its phenotype.

  23. My Vacation

  24. My Vacation (cont.)

  25. My Vacation (cont.)

  26. Mendel’s Interpretation

  27. Mendel’s Interpretation • Character is flower color. • Assign P to the dominant allele (purple) • Assign p to the recessive allele (white) • Cross a true breeding purple (PP) with a true breeding white (pp) • F1 generation plants were all purple

  28. Punnett Square gametes gametes

  29. Punnett Square gametes p p P P gametes

  30. Punnett Square gametes p p Pp P P gametes

  31. Punnett Square gametes p p P P Pp Pp Pp Pp gametes

  32. Results • A homozygous dominant (PP) purple crossed with a homozygous recessive (pp) white yields…………… • An F1 offspring that are all heterozygous and purple • F1genotype is Pp….all offspring are the same genotype • F1phenotype is purple

  33. F1 Self-Fertilization • We let the Pp fertilize themselves. • What would the F2 offspring be….both genotype and phenotype? • Let’s do another Punnett square!

  34. Punnett Square gametes gametes

  35. Punnett Square gametes P p P p gametes

  36. Results of F1 Cross • What are the genotypes of the F2 offspring and their ratios? • PP, Pp, pp 1:2:1 • What are the phenotypes of the F2 offspring and their ratios? • 3 purple:1white • Can you tell from the purple phenotype what the genotype is?

  37. Testcross • Purple phenotype could be one of two genotypes: PP or Pp • How could you distinguish? • What would you cross with a purple phenotype? • What would be the F1 results if the genotype was PP? • What would be the F1 results be if the genotype was Pp?

  38. Testcross

  39. Another Way? • Suppose you had a purple flower phenotype plant and nothing to cross it with except itself (self-fertilization). • What are the possible F1 outcomes? • PP X PP • Pp X Pp

  40. Mendel’s 1st Law of Genetics: Segregation • Alleles for a character segregate from each other in a heterozygous individual and remain distinct. • They are on 2 separate and homologous chromosomes.

  41. Chromosomal Theory ofInheritance • At the beginning of the 20th century, it was proposed (1902) that “alleles” of a character are on chromosomes. • Chromosomes were shown to segregate independently but so did many other organelles in the cell….mitochondria and chloroplasts.

  42. Mendel’s 2nd Law of Genetics: Independent Assortment • 1st Law – alleles of a character (purple and white flowers) can segregate independently. • But what about other characters? • Can they segregate independent of each other? • Can a color of seed (yellow or green) segregate independent from the shape of the seed (round or wrinkled)?

  43. Mendel’s 2nd Law of Genetics: Independent Assortment • Genes located on separate chromosomes segregate independently of each other. • An RrYy genotype can yield four distinct gametes.

  44. Independent AssortmentExperiment • Chose 2 characters • Seed shape (round, R and wrinkled, r) • Seed color (yellow, Y and green, y) • Made sure parents were RRYY and rryy • F1 generation results • All offspring were round and yellow • Does this prove independent assortment? • No…..all F1 offspring were RrYy • But, what about self-fertilization of F1 X F1 ?

  45. Dihybrid Cross • Dihybrid cross -- crossing individuals that are heterozygous for two characters (genes) • RrYy X RrYy • If alleles segregate independently (R from r and Y from y), can characters (seed shape and seed color) segregate independently? • If characters and alleles can all segregate independently, how many different gametes can be produced ? • Four • RY Ry rY ry

  46. Dihybrid Cross (cont.) • Do a Punnett square with self-fertilizing RrYy X RrYy. • List genotypes and phenotypes and numbers.

  47. Dihybrid Cross (cont.)

  48. Extensions of Mendelian Genetics • Continuous variation • Pleiotropic effects • Lack of complete dominance • Environmental effects • Multiple Alleles

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