2.4

• Variation


  Differences in organisms of the same species
• Types of variation


  • Genetic
  • Environmental
• Genetic variation


  Differences in the genotypes of organisms due to different alleles, creating differences in phenotypes
• Environmental variation


  Differences in phenotype acquired during lifespan due to environmental factors
• Some variation may be due to a combination of genetics and environment
• Types of variation


  • Continuous
  • Discontinuous
• Continuous variation


  Characteristics that do not fall into distinct categories, showing a continuous range
• Discontinuous variation


  Characteristics that fall into distinct categories
• Causes of genetic variation

  • Sexual reproduction
  • Spontaneous mutations
• Sexual reproduction


  Creates genetic variation through meiosis and fertilisation
• Asexual reproduction does not create genetic variation
• Mutation


  Random change to the base sequence in DNA resulting in genetic variants
• Effects of mutations


  • Neutral mutation (no change)
  • Minor change in phenotype
  • Completely change amino acid sequence (non-functional protein)
• Inherited disease


  Harmful alleles present in sex cells can be inherited by offspring
• Inherited disease


  • Cystic fibrosis
• Inheritance of heritable diseases can be investigated using family trees
• Gene therapy


  Techniques to counteract the effects of a defective allele
• Methods of gene therapy

  • Insertion of a functional allele
  • Turning off the faulty allele
• Ethical issues with gene therapy

  • Expensive
  • Religious objections
  • Health implications
• Natural selection


  Gradual change in inherited traits within a population over time
• Natural selection


  1. Genetic variation exists

  2. Competition for limited resources

  3. Beneficial mutation gives selective advantage

  4. Organism better adapted survives

  5. Organism reproduces, passing on beneficial alleles

  6. Frequency of advantageous alleles increases
• The theory of natural selection was proposed by Charles Darwin and Alfred Russel Wallace
• Darwin studied a variety of organisms and proposed the idea of 'survival of the fittest'
• Reasons for extinction


  • Organisms not adapted to environment
  • Organisms have not adapted rapidly enough to changing conditions
  • Outcompeted by better adapted species
• Modelling natural selection

  1. Take a large blue paper background

  2. Disperse equal number of blue and white paper squares (prey)

  3. Volunteer collects as many squares as possible in 30s (predator)

  4. Record number of blue and white squares remaining

  5. Repeat 3 times
• Limitations of modelling natural selection

  • Straws do not move
  • Extreme difference in straw colours
  • Paper is one colour
  • Potential bias by volunteer
  • Not affected by other factors
• Development of antibiotic resistance

  1. Genetic variation exists due to spontaneous mutations

  2. Mutation gives bacterium antibiotic resistance

  3. Resistant bacterium survives when antibiotic administered

  4. Resistant bacterium reproduces, passing on resistant variant

  5. Frequency of resistant allele increases
• Bacterial resistance to antibiotics is increasing due to overprescription and antibiotic misuse
• Human Genome Project


  Scientific research project that mapped the entire human genome
• Importance of Human Genome Project

  • Enables understanding of how lifestyle factors interact with genes
  • Allows rapid identification of disease-causing alleles
  • Enables prediction of individual's response to drugs
• Scientists now aim to identify the function of every gene in the human genome