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Since the initial
event in the mother's body (either splitting of a single egg or
two separate fertilizations) is not observed directly, inferential
means are employed for diagnosing a 
set of twins as fraternal or identical. The birth of fraternal
twins is frequently characterized by the passage of two separate
afterbirths. In many instances, identical twins are followed by
only a single afterbirth, but exceptions to this phenomenon are
so common that this is not a reliable method of diagnosis.
The most
trustworthy method for inferring twin type is based on the determination
of
genetic similarity. By selecting those traits that display the
least variation attributable to environmental influences (such
as eye colour and blood types), it is feasible, if enough separate
chromosome loci are considered, to make the diagnosis of twin
type with high confidence. HLA antigens, which, as stated above,
are very polymorphic, have become most useful in this regard.
Charles
Siebert
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