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The Foundations of Genetics: Mendel’s Pioneering Work in Pea Plants

This chapter explores the fundamental principles of genetics established by Gregor Mendel, an Austrian monk whose research in the 1840s and 1850s focused on the heredity of traits in pea plants. Mendel’s experiments on plant height, flower color, and other characteristics led to the formulation of key concepts such as dominant and recessive traits, the Law of Segregation, and the Law of Independent Assortment. His statistical approach and ability to control pollination laid the groundwork for modern genetics, illustrating how traits are passed from parents to offspring.

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The Foundations of Genetics: Mendel’s Pioneering Work in Pea Plants

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  1. CHAPTER 9 FUNDAMENTALS OF GENETICS

  2. MENDELhttp://www.pbs.org/wgbh/nova/orchid/images/amat_mendel.jpgMENDELhttp://www.pbs.org/wgbh/nova/orchid/images/amat_mendel.jpg GENETICS- field of biology devoted to understanding how characteristics are transmitted from parents to offspring - founded with the work of Gregor Johann Mendel, an Austrian monk - studied pea plants (1840s, 1850s)

  3. - Mendel trained in the field of statistics - this training would help him in his research on HEREDITY- transmission of characteristics from parents to offspring - Mendel is most famous for his work on garden peas

  4. MENDEL’S PEAS 7 CHARACTERISTICS OF PEA PLANTS: 1. Plant height - long or short stems 2. Flower position along stem - axial or terminal 3. Pod color - green or yellow 4. Pod appearance - inflated or constricted

  5. 5. Seed texture - smooth or wrinkled 6. Seed color - yellow or green 7. Flower color - purple or white - Mendel collected seeds from the plants, and then planted those seeds the next year

  6. http://www.howe.k12.ok.us/~jimaskew/bio/pea.gif

  7. Mendel observed several variations: - white-flowering plants grew from seeds of purple-flowering plants - some short plants grew from seeds of tall growing plants

  8. CONTROLLED POLLINATION POLLINATION- occurs when pollen grains produced in the male reproductive parts of a flower (ANTHERS) are transferred to female reproductive parts of a flower (STIGMA) 2 TYPES: 1. Self-pollination- pollen is transferred from anthers of a flower to the stigma of the same flower or same plant

  9. 2. Cross-pollination- involves flowers of 2 separate plants - pea plants normally reproduce by self-pollination Mendel was able to control the pollination of his plants - he removed the anthers from one flower and transferred the anther of a flower from one plant to the stigma of a flower on another plant

  10. http://www.science.siu.edu/plant-biology/PLB117/JPEGs%20CD/0518.JPGhttp://www.science.siu.edu/plant-biology/PLB117/JPEGs%20CD/0518.JPG

  11. MENDEL’S EXPERIMENTS Mendel began by growing plants that were pure for each trait  always produce offspring with that trait STRAIN- plants pure for a specific trait - Mendel produced strains by allowing plants to self-pollinate for several generations - he obtained 14 strains, one for each trait - called each strain a parental or P1 generation

  12. Mendel then cross-pollinated these strains - he transferred pollen from the anthers of a plant pure for one trait to the stigma of another plant pure for the contrasting trait - he then recorded the number of each type of offspring - he called the offspring of the P1 generation the F1 generation (first filial)

  13. - he allowed flowers from F1 to self pollinate and called the offspring the F2 generation

  14. OBSERVATIONS Mendel hypothesized that something within the pea plants controlled the characteristics that he observed - he called these controls FACTORS - he hypothesized that each trait was inherited by means of a separate factor (a pair for each trait for the characteristics that he was studying)

  15. RECESSIVE AND DOMINANT TRAITS - when Mendel crossed strains, one of the P1 traits did not appear in the F1 plants - that trait would reappear in the F2 plants - hypothesized that the trait that DID appear in the F1 plants was controlled by a DOMINANT factor because it dominated the other factor - the trait that did NOT appear was controlled by a RECESSIVE factor

  16. - a trait controlled by a recessive factor had no effect on an organism’s appearance when paired with a trait controlled by a dominant factor LAW OF SEGREGATION Mendel concluded that paired factors separate during the formation of gametes - each gamete receives only one factor of each pair

  17. - this law states that a pair of factors is segregated, or separated, during the formation of gametes LAW OF INDEPENDENT ASSORTMENT - Mendel crossed plants that differed in two characteristics (ex: flower color and seed color) - the traits produced by dominant factors do not always appear together

  18. - this law states that factors for different characteristics are distributed to gametes independently MOLECULAR GENETICS- study of the structure and function of chromosomes and genes - most of Mendel’s findings agreed with modern molecular genetics

  19. RECALL: - a chromosome is a thread-like structure made of DNA - a gene is the segment of DNA on a chromosome that controls a trait - since chromosomes occur in pairs, genes occur in pairs - alternative forms of genes are called ALLELES

  20. Letters are used to represent alleles: - capital letters represent DOMINANT alleles - lowercase letters represent RECESSIVE alleles Ex: Green pod color is dominant, it would be represented by G - yellow pod color is recessive, it would be represented by g - it doesn’t matter what letter is selected to represent an allele

  21. RECALL: - during meiosis, gametes receive one chromosome from each homologous pair of chromosomes - when gametes combine, the offspring receives one allele for a given trait from each parent

  22. GENETIC CROSSES

  23. GENOTYPE- genetic makeup of an organism - consists of the alleles that the organism inherits from its parents Ex: White flowers = pp (white is recessive) Purple flowers = PP or Pp PHENOTYPE- appearance of an organism as a result of its genotype Ex: Phenotype of pp is white flowers Phenotype of PP or Pp is purple flowers

  24. HOMOZYGOUS- both alleles of a pair are alike - may be homozygous dominant or recessive - PP or pp HETEROZYGOUS- 2 alleles in the pair are different - Pp

  25. PROBABILITY PROBABILITY- likelihood that a specific event will occur - may be expressed as a decimal, percentage, or fraction Probability = number of times an event is expected to happen ---------------------------------------------------------------------------------------------- number of opportunities for an event to happen

  26. In Mendel’s experiments, the dominant trait of yellow seed color appeared 6,022 times - recessive trait of green seed color appeared 2,001 times - total individuals was 6,022 + 2,001 = 8,023 - to find the probability that the dominant trait would appear in a similar cross: 6,022 = 0.75 or 75% or ¾ ----------- 8,023

  27. - probability that the recessive trait will appear in the F2 generation: 2,001 = 0.25 or 25% or ¼ ------- 8,023 - fractions can also be expressed as ratios: ¼ = 1:4 - results predicted by probability are more likely to occur when there are many trials

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