OCR A Level Biology

Revision Notes

6.4.5 Natural Clones in Animals

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


Exam Tip

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.


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