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Mendelian Genetics and Meiosis

Mendelian Genetics and Meiosis. Chapters 10 and 12. Mendel’s Laws of Heredity. Gregor Mendel  19 th century Austrian monk. 1 st studies of heredity  genetics Traits  characteristics that are inherited. Studied pea plants. Contrasting traits were easily seen Short generation time

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Mendelian Genetics and Meiosis

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  1. Mendelian Genetics and Meiosis Chapters 10 and 12

  2. Mendel’s Laws of Heredity

  3. Gregor Mendel  19th century Austrian monk • 1st studies of heredity  genetics • Traits characteristics that are inherited

  4. Studied pea plants • Contrasting traits were easily seen • Short generation time • Many offspring per generation • Sexes on 1 flower  control of pollination • Mathematical analysis of data

  5. Phenotypes and Genotypes

  6. Phenotype • Physical appearance • Expressed in words  tall, short

  7. Genotype • Genetic makeup • Expressed in terms of alleles • Allele  form of a gene for a trait • Dominant allele  always expressed (T  tall) • Recessive allele  if present, may not be expressed (t  short)

  8. More Terminology • Homozygous  2 identical alleles (TT, tt) • Heterozygous (hybrid)  2 different alleles (Tt) • Different genotypes can have the same phenotype • Tall  TT or Tt

  9. Monohybrid Crosses A single trait

  10. Mendel crossed a tall plant with a short plant • Original parents  P generation • Offspring  F1 generation • All F1 were tall

  11. F1 were allowed to self-pollinate • F2 75% were tall, 25% short • 3:1 ratio

  12. Mendel’s conclusion • Each organism has 2 factors (alleles) that control each trait

  13. Rule of Dominance • When an individual is hybrid for a pair of contrasting traits, only the dominant trait can be seen • TT x tt  Tt • **recessive trait is masked** • Crossing 2 hybrids always results in 3:1

  14. Law of Segregation • Pairs of alleles for a trait are separated during the formation of gametes and are recombined during fertilization • Tt  T (egg) + t (sperm)  Tt (zygote) • Explains appearance of recessive traits in subsequent generations

  15. Dihybrid crosses  2 traits • Height and seed color

  16. Law of Independent Assortment • Genes for different traits are inherited independently of each other • Exception  genes on the same chromosome

  17. Punnett squares • Predict the ratio of all possible results for a certain genetic cross • Not what will happen, but what could happen • Exact ratios are not seen in nature due to chance

  18. Monohybrid cross  height (Tt x Tt)

  19. Dihybrid cross  height, color(TtYy x TtYy)

  20. Meiosis Type of cell division in which daughter cells receive only half the # of chromosomes of the parent cell

  21. Genes, chromosomes, and numbers

  22. Diploid vs. monoploid • Chromosomes occur in pairs • 1 allele is on each of the paired chromosomes

  23. Diploid Cell with 2 of each kind of chromosome (2n) Body cells (somatic)

  24. Monoploid Cell with 1 of each kind of chromosome (n) also called haploid Gametes  sperm and egg

  25. Homologous chromosomes paired chromosomes • Each of a pair has genes for the same traits • They may carry different alleles

  26. Why meiosis? • Associated with sexual reproduction  2 parents • Allows offspring to have the same number of chromosomes as parents • No doubling of chromosome number

  27. Phases Meiosis I and II 2 separate divisions

  28. Interphase • Replication of the chromosomes • Same as in mitosis

  29. Prophase I • Each pair of homologous chromosomes comes together to form a tetrad • This is known as synapsis

  30. Crossing-over may occur at this point • Exchange of genetic material between nonsisterchromatids • Results in genetic variation or mutation • Completely random and unpredictable

  31. Metaphase I • Tetrads line up at cell equator  metaphase plate

  32. Anaphase I • Homologous chromosomes separate and move to opposite poles  disjunction • Critical step  without disjunction, gametes would have abnormal numbers of chromosomes

  33. Telophase I • Cytokinesis forms 2 daughter cells • Each cell has only 1 chromosome from each homologous pair • Each chromosome is still doubled  another division is required

  34. Meiosis II • Identical to mitosis

  35. Results 4 monoploid daughter cells

  36. Let’s Review Meiosis! • Meiosis Overview

  37. Meiosis and genetic variation • Crossing-over results in genetic recombination  gene shuffling • Almost endless number of different possible chromosomes • You are not the exact blend of your parents • Explains Mendel’s results

  38. Mistakes in meiosis

  39. Nondisjunction • Failure of homologous chromosomes to separate • Both chromosomes move to the same pole • 1 cell has an extra chromosome • 1 cell is missing a chromosome

  40. Trisomy • A gamete with and extra chromosome fuses with a normal gamete • Zygote has 1 extra chromosome • 47 instead of 46 in humans • Trisomy 21  Down syndrome

  41. Monosomy • A gamete with a missing chromosome fuses with a normal gamete • Zygote has 1 missing chromosome • 45 instead of 46 in humans • Lethal most of the time • Turner syndrome  XO

  42. Polyploidy • Total lack of chromosomal separation • Lethal in animals • Frequent in plants • Larger and healthier fruits and flowers • Plant breeders induce polyploidy by using chemicals that cause nondisjunction

  43. Mendelian inheritance of human traits Chapter 12

  44. Pedigree • Graphic representation of genetic inheritance • A chart showing familial relationships and patterns of trait inheritance

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