genetic

• Mitosis


  • A type of nuclear division that gives rise to cells that are genetically identical and used for growth, repair of damaged tissues, replacement of cells and asexual reproduction
• Meiosis


  A type of nuclear division that gives rise to cells that are genetically different and used to produce gametes
• Sexual reproduction


  A process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other
• Gametes


  The sperm cells and egg cells of animals, the pollen cells and egg cells of flowering plants
• Fertilisation


  The fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring
• Asexual reproduction


  Does not involve sex cells or fertilisation, only one parent is required so there is no fusion of gametes and no mixing of genetic information, the offspring are genetically identical to the parent and to each other (clones)
• Meiosis


  1. Cells in reproductive organs divide by meiosis to form gametes (sex cells)

  2. The number of chromosomes must be halved when the gametes are formed

  3. Halving occurs during meiosis, a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells

  4. Starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell

  5. Cells divide twice so that only one copy of each chromosome passes to each gamete

  6. Produces four haploid cells
• Gametes


  • Haploid - having half the normal number of chromosomes
• Advantages of sexual reproduction

  • Increases genetic variation
  • The species can adapt to new environments due to variation, giving them a survival advantage
  • Disease is less likely to affect the population (due to variation)
• Rationality in classical economic theory is a flawed assumption as people usually don't act rationally
• Disadvantages of sexual reproduction

  • Takes time and energy to find mates
  • Difficult for isolated members of the species to reproduce
• Variation is defined as differences between individuals of the same species
• Advantages of asexual reproduction

  • The population can be increased rapidly when conditions are right
  • Can exploit suitable environments quickly
  • More time and energy-efficient
  • Reproduction is completed much faster than sexual reproduction
• Phenotypic variation


  The difference in features between individuals of the same species
• Causes of phenotypic variation

  • Genetic
  • Environmental
• Disadvantages of asexual reproduction

  • Limited genetic variation in population - offspring are genetically identical to their parents
  • The population is vulnerable to changes in conditions and may only be suited for one habitat
  • Disease is likely to affect the whole population as there is no genetic variation
• Genetic variation in humans

  • Blood group
  • Eye colour
  • Gender
  • Ability to roll tongue
  • Free or fixed earlobes
• Genetic variation


  Variation controlled entirely by genes
• Environmental variation


  Variation caused entirely by the environment in which the organism lives
• Some organisms reproduce by both sexual and asexual methods depending on the circumstances
• Some features vary because of a combination of genetic and environmental causes
• All genetic variants arise from mutations
• Organisms that reproduce both sexually and asexually

  • Malarial parasites
  • Fungi
  • Plants
• Mutations are random genetic changes that occur continuously
• Most mutations have no effect on the phenotype
• Rarely, mutations lead to the development of new alleles and so new phenotypes
• Genome


  The entire set of the genetic material of an organism
• Natural selection


  Individuals in a species with the best adaptive features are the ones most likely to survive and reproduce
• Biologists now know the entire human genome
• Natural selection


  1. Individuals show variation caused by differences in genes

  2. Organisms reproduce more offspring than the environment can support

  3. Competition for resources results in 'struggle for survival'

  4. Individuals with characteristics most suited to the environment have higher chance of survival and reproduction

  5. Alleles resulting in these characteristics are passed to offspring at a higher rate
• Charles Darwin's theory of natural selection became known as 'survival of the fittest'
• DNA


  The genetic material in the nucleus of a cell, a polymer made up of two strands forming a double helix
• If the environment does not change, selection does not change
• If the environment changes, or a chance mutation produces a new allele, selection might now favour individuals with different characteristics or with the new allele
• Evolution


  The change in adaptive features of a population over time as a result of natural selection
• Chromosomes


  Structures that contain DNA, located in the nucleus of cells
• Adaptation


  Over generations, features that are better adapted to the environment become more common
• If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species
• Genes


  Short lengths of DNA that code for a protein, found on chromosomes
• Selective breeding


  Selecting individuals with desirable characteristics and breeding them together