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Biology_Alevel_Ocr

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  1. 4-1-communicable-diseases-disease-prevention-and-the-immune-system
    16 主题
  2. 4-2-biodiversity
    10 主题
  3. 4-3-classification-and-evolution
    15 主题
  4. 5-1-communication-and-homeostasis
    4 主题
  5. 5-2-excretion
    10 主题
  6. 5-3-neuronal-communication
    9 主题
  7. 5-4-hormonal-communication
    4 主题
  8. 5-5-plant-and-animal-responses
    16 主题
  9. 5-6-photosynthesis
    10 主题
  10. 5-7-respiration
    14 主题
  11. 6-1-cellular-control
    7 主题
  12. 6-2-patterns-of-inheritance
    13 主题
  13. 6-3-manipulating-genomes
    11 主题
  14. 6-4-cloning-and-biotechnology
    14 主题
  15. 6-5-ecosystems
    7 主题
  16. 6-6-populations-and-sustainability
    6 主题
  17. 1-1-practical-skills-written-assessment
    7 主题
  18. 1-2-practical-skills-endorsement-assessment
    16 主题
  19. 2-1-cell-structure
    9 主题
  20. 2-2-biological-molecules
    17 主题
  21. 2-3-nucleotides-and-nucleic-acids
    8 主题
  22. 2-4-enzymes
    9 主题
  23. 2-5-biological-membranes
    9 主题
  24. 2-6-cell-division-cell-diversity-and-cellular-organisation
    11 主题
  25. 3-1-exchange-surfaces
    7 主题
  26. 3-2-transport-in-animals
    12 主题
  27. 3-3-transport-in-plants
    11 主题
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Natural Clones in Animals

Asexual reproduction in animals

  • Asexual reproduction is much less common in animals than in plants

  • Some small animals reproduce asexually by parthenogenesis eg. aphids

  • The other naturally occurring incidence of cloning in animals is identical twins

Identical twins

  • An egg is fertilised by a sperm as in a singleton birth

  • This forms a zygote

  • The single zygote undergoes a few cell cycles (mitotic divisions) to become an embryo

    • This is why identical twins are referred to as monozygotic

  • At the embryo stage, the embryo splits in two; the exact causes of this kind of split are not well understood

  • Two embryos that form are identical, with the same genotype and develop in utero together

  • The result is the birth of identical offspring, always of the same gender, with identical phenotype

  • Because non-identical twins are formed from separate eggs and sperm, they are not considered clones

 

Examiner Tips and Tricks

Although identical (monozygotic) twins share the same genome at the moment when the embryo splits, identical twins are not clones in the true sense of the word. Because mutations occur with every cell cycle, Twin A will possess slightly different DNA base sequences to Twin B at the time of birth. The older the twins get, the more their genomes become dissimilar as mutations accumulate. They will still look very alike throughout their lives unless there are large differences in their environments as they grow up.