Explain why the offspring produced by cloning a mammal using somatic cell nuclear transfer are not completely identical to the donor animal that provided the diploid nucleus.

Pearson Edexcel International GCSE Biology (4BI1) — 5.4 Cloning · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Dolly the sheep was the first mammal to be cloned from an adult somatic cell. A diploid nucleus from the udder cell of one ewe was inserted into an enucleated egg cell from a different ewe before being implanted into a surrogate mother.

Model answer (4 marks)

The clone is not 100 % identical because the enucleated egg still contains its own mitochondria and mitochondrial DNA, which are inherited from the egg donor and differ from those of the somatic donor. In addition, the surrogate mother provides a different intra‑uterine environment that can influence gene expression and phenotype. Consequently the clone has a slightly different genetic make‑up and is not genetically identical to the nucleus donor.

Examiner tips

  • Use the word ‘because’ to link each reason to the conclusion.
  • Mention both nuclear and mitochondrial DNA, and the role of the surrogate environment.

Mark scheme (4 marks)

  1. The enucleated egg cell retains its own mitochondria / mitochondrial DNA
  2. The mitochondria / mitochondrial DNA in the egg cell cytoplasm come from a different individual (the egg donor)
  3. The surrogate mother / environment influences gene expression / phenotype of the clone
  4. Therefore the clone has a slightly different genetic make-up / is not 100% genetically identical to the nucleus donor

Key terms in this question

cloning · diploid nucleus · somatic cell nuclear transfer

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