We're always taught that twins can either be identical or non-identical. But what is the basis for this? And is there ever a situation where twin type is not quite so binary?

Sources for this episode:

Davis, N., the Guardian (2019), Scientists stunned by discovery of 'semi-identical' twins (online) [Accessed 07/02/2021].
Author unknown, BBC (2019), Semi-identical twins 'identified for only the second time' (online) [Accessed 07/02/2021].
Author unknown, Wikipedia (date unknown), Twin (online) [Accessed 08/02/2021].

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