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  1. 1-biological-molecules

    1-1-biological-molecules-carbohydrates
    11 主题
  2. 1-2-biological-molecules-lipids
    3 主题
  3. 1-3-biological-molecules-proteins
    5 主题
  4. 1-4-proteins-enzymes
    12 主题
  5. 1-5-nucleic-acids-structure-and-dna-replication
    8 主题
  6. 1-6-atp-water-and-inorganic-ions
    4 主题
  7. 2-cell-structure
    2-1-cell-structure
    7 主题
  8. 2-2-the-microscope-in-cell-studies
    4 主题
  9. 2-3-cell-division-in-eukaryotic-and-prokaryotic-cells
    8 主题
  10. 2-4-cell-membranes-and-transport
    7 主题
  11. 2-5-cell-recognition-and-the-immune-system
    7 主题
  12. 2-6-vaccines-disease-and-monoclonal-antibodies
    6 主题
  13. 3-exchange-and-transport
    3-1-adaptations-for-gas-exchange
    6 主题
  14. 3-2-human-gas-exchange
    10 主题
  15. 3-3-digestion-and-absorption
    5 主题
  16. 3-4-mass-transport-in-animals
    6 主题
  17. 3-5-the-circulatory-system-in-animals
    8 主题
  18. 3-6-mass-transport-in-plants
    6 主题
  19. 4-genetics-variation-and-interdependence
    4-1-dna-genes-and-chromosomes
    7 主题
  20. 4-2-dna-and-protein-synthesis
    6 主题
  21. 4-3-genetic-diversity-mutations-and-meiosis
    7 主题
  22. 4-4-genetic-diversity-and-adaptation
    6 主题
  23. 4-5-species-and-taxonomy
    4 主题
  24. 4-6-biodiversity
    9 主题
  25. 5-energy-transfers-in-and-between-organisms-a-level-only
    5-1-photosynthesis-a-level-only
    5 主题
  26. 5-2-respiration-a-level-only
    7 主题
  27. 5-3-energy-and-ecosystems-a-level-only
    9 主题
  28. 5-4-nutrient-cycles-a-level-only
    4 主题
  29. 6-organisms-respond-to-changes-in-their-environments-a-level-only
    6-1-response-to-stimuli-a-level-only
    12 主题
  30. 6-2-nervous-coordination-a-level-only
    10 主题
  31. 6-3-skeletal-muscles-a-level-only
    6 主题
  32. 6-4-homeostasis-a-level-only
    11 主题
  33. 7-genetics-populations-evolution-and-ecosystems-a-level-only
    7-1-inheritance-a-level-only
    6 主题
  34. 7-2-populations-a-level-only
    3 主题
  35. 7-3-evolution-a-level-only
    5 主题
  36. 7-4-populations-in-ecosystems-a-level-only
    7 主题
  37. 8-the-control-of-gene-expression-a-level-only
    8-1-genetic-mutations-a-level-only
    2 主题
  38. 8-2-regulation-of-gene-expression-a-level-only
    11 主题
  39. 8-3-using-genome-projects-a-level-only
    4 主题
  40. 8-4-gene-technologies-a-level-only
    13 主题
  41. exam-guidance-and-skills
    essay-guidance
    3 主题
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Genetic variation

Phenotype variation

  • An organism’s phenotype is its observable characteristics.

  • Phenotypic variation refers to differences in these traits among individuals of the same species

  • This variation can arise from:

    • Genetic factors

      • e.g. human blood groups result from combinations of three alleles at the ABO gene

    • Environmental factors

      • e.g. genetically identical plants may grow to different heights in different conditions

    • Both genetic and environmental factors

      • e.g. the sickle cell allele is maintained in some populations because heterozygotes are resistant to malaria

Examiner Tips and Tricks

The phenotypic variation of the individuals in a population is determined by the genetic variation within the population and the interaction of the environment on the individuals:

Phenotypic variation = genetic variation + environmental influence

Genetic variation

  • Although members of the same species have similar genomes, small differences in their DNA base sequences create genetic variation

  • Genetic variation is transferred from one generation to the next, and it generates phenotypic variation within a species population

  • This variation arises mainly from mutations, which create new alleles

    • These alleles may be beneficial, harmful, or have no effect

    • Some may remain unexpressed for generations

  • Genetic variation also results from sexual reproduction, through:

    • Crossing over of non-sister chromatids during prophase I of meiosis

    • Independent assortment of chromosomes during metaphase I of meiosis

    • Random fertilisation of gametes during fertilisation

  • These processes generate new allele combinations in offspring, increasing diversity within a population

Process

Mechanism

Consequences

Independent assortment of homologous chromosomes during metaphase I

Random alignment of chromosomes results in different combinations of chromosomes and different allele combinations in each gamete

Genetic variation between gametes produced by an individual

Crossing over of non-sister chromatids during prophase I

Exchange of genetic material between non-sister chromatids leads to new combinations of alleles on chromosomes. It can also break the linkage between genes

Genetic variation between gametes produced by an individual

Random fertilisation of gametes during fertilisation

Any male gamete can fuse with any female gamete (random mating in a species population)

Genetic variation between zygotes and resulting individuals

Mutation

Random change in the DNA base sequence results in the generation of a new allele. A mutation must occur in gametes to be passed on

Genetic variation between individuals within a species population

Examiner Tips and Tricks

Some questions in the exam may ask you to explain why the variation in phenotype due to genetics is inherited, but the variation in phenotype due to environmental factors is not. This is because genetic variation directly affects the DNA of the gametes, but variation in phenotype caused by the environment does not.