Identical non-identical twins and non-identical identical twins

BMJ 2015; 351 doi: (Published 11 December 2015) Cite this as: BMJ 2015;351:h6589
  1. Nirmal Vadgama, PhD student, Institute of Neurology, University College London, London WC1N 3BG, UK,
  2. Niranjanan Nirmalananthan, consultant neurologist, Neurosciences, St George’s University Hospitals NHS Foundation Trust, London,
  3. Mustafa Sadiq, consultant obstetrician and gynaecologist, Warrington and Halton Hospitals NHS Foundation Trust, Warrington, UK,
  4. John Hardy, professor of neuroscience, Department of Molecular Neuroscience, University College London,
  5. Jamal Nasir, multiprofessional facilitator, Education Department, St George’s University Hospitals NHS Foundation Trust
  1. Correspondence to: N Vadgama nvadgama{at}

In recruiting monozygotic (MZ) twins for our ongoing research into de novo mutations in phenotypically discordant MZ twins, we have come across an increasing number of participants who have been told they are dizygotic (DZ) but have almost identical traits and physical features, or MZ but present more like fraternal siblings. Current practice guidelines for classifying twins as either MZ or DZ, which are still largely based on chorionicity (the notion of whether or not the twins …

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