Zygosity determination means finding out whether twins, triplets
or more are identical (monozygotic - arising from one egg and one
sperm) or non-identical (dizygotic/fraternal - from two separate
fertilised eggs). This process is known as zygosity determination.
It is only natural for parents to want to learn all they can about
their babies, and with twins this includes their zygosity. Later,
the twins themselves are usually keen to know. The reasons given
by parents for wanting to know the zygosity of their twins include:
their own interest
- to avoid
embarrassment when asked the most common question, 'Are they identical?'
- to reinforce
their resolve to treat them as individuals if they are identical
- to assess
the risks of having twins again (there is an increased risk
for women who have non-identical twins).
if the twins wish to participate in any Twins Studies in the future,
knowledge of their zygosity will be essential.
In a third of cases, determination is straightforward, because the
children are of different sexes, therefore must be non-identical.
Amongst twins of the
same sex, by the time the children are around two years old, their
zygosity may be quite clear from their physical features. Colour
of hair and eyes, shape of ears, teeth eruption and formation, shape
of hands and feet, and the pattern of growth can all give a good
indication as to whether or not the twins are identical.
Examining the placenta
The placenta provides the answer in two-thirds of monozygotic twins.
If the babies have a single outer membrane, (the chorion) they must
be monozygotic. But one third of identical twins, those whose egg
has split early before the placenta started to form, have two chorions
with either a fused placenta, where the two placentas have grown
together, or two separate placentas. These placentae are indistinguishable
from those of dizygotic twins.
DNA testing using cells
from the inside of the cheek
The most accurate method of determining zygosity is by the DNA probe
method. The first stage in the analysis is to collect tiny amounts
of DNA from inside each twin's mouth, using a swab. Recently introduced
molecular techniques are used to 'amplify' small sections from a
number (currently 12) of specially chosen sites along the DNA.
The laboratory examine specific
markers (representing individual regions of genetic code) present
in the DNA of each twins. The regions have been chosen because most
individuals in the population carry slightly different versions
of DNA code at these sites. If only one or two of these markers
were compared by chance, some non-identical (dizygotic/fraternal)
twins, would give the same findings as identical (monozygotic) twins,
which is why twelve of these diagnostic targets are looked at. Although
dizygotic twins may share five marker patterns by chance, monozygotic
twins will have the same pattern for all twelve.
The laboratory can determine
if twins are identical with a reliability of 99.99%; in practice
actual probabilities can be calculated and provided for each individual