Lecture Week 3

Genetics and Heredity

  1. Gregor Mendel (1822-1884) & the discovery of the laws of heredity
    1. Background
    2. Mendel's pea plants 1857-1865
      1. Controlled breeding & extensive records
      2. Tracked simple traits
      3. First bred pure plants
      4. Next cross-bred different varieties
      5. Traits disappeared in first generation
      6. Cross-bred those & traits reappeared
      7. 3:1 ratio in traits
    3. The laws of heredity
      1. Particulate inheritance
      2. Law of Segregation
      3. Law of Independent Assortment
      4. Dominant & Recessive
    4. Forgotten and rediscovered
      1. Paper published in 1866
      2. No-one understood it; was ignored
      3. Re-discovered in 1900 by 3 separate botanists; de Vries, Correns, & Tschermark
  2. Chromosomes, Genes and mutations
    1. First came discovery of cell, nucleus (1820's) & cell division (mitosis)
    2. Chromosomes found in 1870's
      1. Duplicate & divide to create new cells
      2. Each species has set number- 46 for humans
      3. Come in pairs; one of each pair from each parent- Homologous Chromosomes
      4. Sex cell division produces gametes with 1/2 number (meiosis)
        1. First homologous pair off then divide
        2. Then duplicate & divide to produce gametes
    3. Mutations
      1. Hugo deVries- 1890's
        1. Studied primroses looking for source of variation
        2. Discovered "new" traits- mutations
        3. Closing in on Mendel's Laws when discovered Mendel's work
        4. Called units of inheritance "pangens" later contracted to "genes"
    4. Genes- term coined in 1909
      1. By 1880's, people suspect that chromosomes made up of smaller bits that controlled for certain structures
          T. E. Morgan
          1. Professor at Columbia, fascinated by thought of mutations
          2. Decided to work with fruit flies
          3. First mutation discovered in 1910- White eyes
          4. Further work (& more mutations) proved that genes existed
        1. Important concepts
          1. Alleles- different types of the same gene
          2. Genotype- genetic constitution
          3. Phenotype- physical expression
          4. Homozygous- Homologous Pair with same allele
          5. Heterozygous- Homologous Pair with different alleles
    5. DNA - Deoxyribonucleic acid
      1. Studies during 1930's proved that chromosomes made up of DNA
      2. What it does is make proteins- strings of amino acids (20 different kinds of a.a.)
      3. Watson and Crick and the discovery of the shape of DNA- double helix 1950's
        1. Two backbones twisted around each other
        2. Base pairs connect as "steps" in center
        3. Four bases; A, G, C, & T
        4. A connects to T and G connects to C
      4. How DNA works
        1. Double helix allows "unzipping" of DNA
        2. During replication, new bases connect to each side of old; result 2 strands
        3. During protein synthesis, only part unzips
          1. messenger RNA copies gene
          2. goes out of nucleus where it connects with transfer RNA
          3. tRNA carries amino acids which connect to make a protein
          4. sequence of 3 bases equals a codon- codes for a particular amino acid
          5. 64 possible combos of base pairs; allows redundancy & 3 stop codes
        4. Mutation occurs when a base is changed (point mutation)
          1. Different base changes codon
          2. Since redundant, may not change amino acid
          3. If does, changes protein
    6. Population genetics and speciation
      1. Interbreeding populations of a species
        1. Where evolution occurs- changes in allele frequencies
        2. Total all genes in a population called the "gene pool"
      2. Changes in gene frequencies
        1. Mutations- introduces new traits into a population
        2. Gene flow- transfer of alleles between populations
        3. Genetic Drift- random events that change frequencies (including "founder effect")
        4. Speciation- when enough change has occurred that different populations are no longer the same species- often through reproductive isolation
      3. Gradualism
        1. When a slow accumulation of changes leads to speciation
        2. Adapting to conditions of particular enviro or to conditions that change slowly
      4. Punctuated Equilibrium
        1. Sudden changes giving rise to new species
        2. Mutations with large effects or rapidly changing environment
        3. Most of time Natural Selection "fine-tunes" to particular environment
    7. Example putting it all together- Sickle-Cell Trait
      1. Hemoglobin (in Red Blood Cells) has two pairs of proteins (2 alfa & 2 beta chains)
      2. Since genes are paired, one chain of each pair inherited from each parent
      3. Normal HbA (BetaABetaA)
      4. Beta chain 146 amino acids long
        1. Position 6- Glutamic Acid; codon CTC
        2. Mutation T>A; codon CAC, Valine
      5. Hemoglobin's job is to take up and release oxygen
      6. When both Beta chains have mutation (called HbS; BetaSBetaS)
        1. When oxygen released, cells "sickle"; become oddly shaped
        2. Valine causes Hb to stack up and make Red Blood Cells rigid
        3. Sickle cells then clog capillaries, get destroyed
        4. Many health effects from anemia; too few RBC's; die at an early age
      7. Yet Sickle-Cell trait very prevalent in certain populations
        1. More than mutation alone could account for Something must be selecting it- but not homozygous, heterozygous (BetaABetaS)
      8. Distribution HbS coincides with distribution of malarial parasite Plasmodium falciparum, a very nasty form of malaria
      9. Heterozygous, having one Beta chain mutated and one normal
        1. Survive malaria better than homozygous
        2. Malaria carried in RBC
        3. Reproduce till fill cell, then burst out and infect other RBC's
        4. If one Beta chain HbS,
          1. Sickles easier when parasite grows because parasite uses up oxygen
          2. Sickled cells get destroyed, removing parasite from system quicker
      10. In malarial areas, homozygous people either die from malaria or from anemia, heterozygous survive
      11. This is called a Balanced Polymorphism

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