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Mendel & Meiosis

Mendel & Meiosis. Chapter 6. Mendelian Genetics. What is this? Branch of genetics that deals with simple dominant/recessive traits based on the work of Gregor Mendel Ex: Height of a pea plant is tall (TT or Tt) or short (tt) Exceptions – We will get to this later….

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Mendel & Meiosis

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  1. Mendel & Meiosis Chapter 6

  2. Mendelian Genetics • What is this? • Branch of genetics that deals with simple dominant/recessive traits based on the work of Gregor Mendel • Ex: Height of a pea plant is tall (TT or Tt) or short (tt) • Exceptions – We will get to this later…

  3. Example Trait = Tongue Rolling R- a dominant allele that codes for muscles that help in tongue rolling r- is a recessive allele that does not code for that muscle This boy has at least one dominant allele in his 2 letter genotype. His phenotype is that he is a tongue roller

  4. GeneticsPre-Mendel Theory = BLENDING • Pre Mendel, theory of inheritance = qualities of the parents blended to form the qualities of the child • Ex: tall and short parent = medium height child Theory did NOT explain examples like: • two brown-eyed parents giving birth to a blue-eyed baby • Because of Mendel's work there became a consistent theory of heredity = GENETICS

  5. Pre-Mendel Theory – Humunculous

  6. The Origins of Genetics • Gregor Mendel (1822-1884) • “Father of Genetics” • Czechoslovakian • cross pollinated pea plants to see the outcomes • Noticed “atypical” characteristics • Figure 10.3 Gregor's life story

  7. Actual garden “sweet pea” flowers

  8. Cross Pollination Mendel mixed the pollen from a white flowered pea plant to a purple flowered pea plant pistil, and the results were….. Mendel’s Experiment

  9. Generation 1 • Purple x White = Purple (no white) (P1 Parents) (F1 offspring) What does this mean? Next, he mixed F1 plants with other F1 plants, and the results were…

  10. Generation 2 • F1 x F1 = Mostly purple, some white (P2 Parents) (F2) Out of 929 flowers, 705 = purple, 224 = white That’s a 3:1 RATIO!

  11. Mendel called these “new” plants as hybrids

  12. Genotype vs. PhenotypeWhat do these terms mean?

  13. Mendel’s Conclusion • “Each organism has 2 factors that control each of its traits” (now factors called alleles = types of a gene) • Dominant Trait • A trait that dominates (masks/covers up) a recessive one; written w/ Capital letter • Recessive Trait • Can’t be expressed (seen) if a dominant trait is present; written w/ lower case letter Ex: Purple Flower = P White Flower = p

  14. For every trait… • There are 2 alleles (types of genes) • One from female parent, one from male parent • Process of meiosis allows for this • Thus: • Purple flower = PP • HOMOZYGOUS DOMINANT = pure dominant • White flower = pp • HOMOZYGOUS RECESSIVE = pure recessive • What about Pp? • HETEROZYGOUS • “Hybrid”; carrier

  15. Mendel’s Laws (Principles) • The Principle of Unit Characters states that individuals pass information on as individual traits. • The Principle of Dominance states that some unit characters (genes) can mask the expression of others. • The Principle of Segregation states that each unit character (gene) separates into a different sex cell. • The Principle of Independent Assortment** states that genes segregate according to chance; different genes separate INDEPENDENTLY of each other. *Dihybrids

  16. Alleles = alternative versions of a gene

  17. Let’s go back and look at the genes (alleles) in Mendel’s Flowers • Parent Generation (P1) = PP x pp • Each parent donate only 1 gene to F1 • Thus, all offspring here must be… • Pp • Next (P2) he crossed F1 x F1 = Pp x Pp • In the F2 generation he received a ratio of 3:1 purple phenotype to white phenotype flowers • A punnett square predicts this ratio also

  18. Punnett Squares – using Mendel’s Work • Gamete possibilities from the parents are written on the outside • Each allele gets a separate box • This is a MONOHYBRID cross • It looks at only ONE trait

  19. Tt x Tt • Possible genotypes are placed inside the boxes by mixing alleles from the parents • All POSSIBLE combinations are shown here G ratio - ____:____:____ HD Het HR P ratio - _____ : _____ D R

  20. You Try! • A heterozygous purple flower pea plant is crossed with a white flower pea plant. • What are the genotypic and phenotypic ratios of the F1 generation?

  21. A Test Cross • Used to determine an unknown genotype of parents • (Works backwards) • ALWAYS CROSS UNKNOWN WITH RECESSIVE PHENOTYPE – Why?

  22. What Mendel Saw… • How did he know which were dominant & which were recessive?

  23. Dihybrid Crosses • The PROBABILITY of inheritance for 2 GENES for different traits can be calculated… • Ex: Mendel crossed a round, yellow pea plant (RrYy) with another round, yellow pea plant (RrYy). • How many of each gene can each parent give? • What are the possible outcomes?

  24. How to solve a dihybrid • 1. Draw a 16 box punnett square • 2. Write out ALL possible gamete combinations - FOIL • 3. Fill in the boxes • 4. Interpret the phenotypic ratio* • D/D: D/R: R/D: R/R

  25. Meiosis vs. Mitosis WS • Label the stages of Meiosis I and Meiosis II

  26. Meiosis • Making haploid (n) gamete cells for sexual reproduction  4 haploid gamete cells that are genetically different • Pre-Meiosis – 1 round of Interphase • what happens here? • Phases - 2 sets of PMAT-C • Meiosis I = PMAT-C I • Meiosis II = PMAT-C II

  27. Meiosis OverviewI-PMAT+ CI + PMAT+CII

  28. http://www.youtube.com/watch?v=eaf4j19_3Zg • http://www.youtube.com/watch?v=2aVnN4RePyI

  29. Figure 11-15 Meiosis Section 11-4 Meiosis I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell. Go to Section:

  30. Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Go to Section:

  31. Meiosis vs. Mitosis • Prophase I: • Homologous chromosomes form a tetrad • Tetrad synapsis can occur • Crossing over can occur

  32. Meiosis – Increasing genetic variation in offspring • Genetic Recombination Crossing over • Occurs: late Prophase 1 & Metaphase 1 • Provides new variation

  33. Meiosis vs. Mitosis • Metaphase I: • Homologous pairs line up on the equator • Crossing over can occur – increase variation Meiosis Mitosis

  34. Meiosis vs. Mitosis • Anaphase I: • Homologous pairs separate • Each with its 2 chromatids • No separation of centromeres/sisters

  35. Meiosis vs. Mitosis • Telophase I/Cytokinesis I: • Spindle breaks • RESULT: 2 genetically different cells with diploid # of chromosomes

  36. Figure 11-15 Meiosis Section 11-4 Meiosis I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell. Go to Section:

  37. Meiosis vs. Mitosis • PMAT-C II: • Equivalent of mitosis • RESULT: 4 genetically different cells with a haploid # of chromosomes

  38. Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Go to Section:

  39. Meiosis shows how Mendel’s Principle of Independent Assortment works 2 genes – 4 different combinations… 30,000 human genes x 2 parents… Means you are 1 in 43 trillion!

  40. Summary - Meiosis • Sexual reproduction • IPMAT+C I & PMAT + C II • Gametes (n) produced by both male and female organisms recombine in fertilization • Fertilization  2n zygote genetically DIFFERENT to parents • WHY must gametes be haploid (n) & not diploid like those produced in mitosis?

  41. Sperm Formation 4 sperm/round survive Occurs lifelong Egg Formation Only 1 survives All eggs formed at birth Egg contains organelles Spermatogenesis vs. Oogenesis Why the difference?

  42. Spermatogenesis vs. OogenesisWhere is the mistake?

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