IDENTICAL AND NON-IDENTICAL TWINNING
Some introduction to the biology of twinning is important for the appreciation of multiples in school. There are two types of twins:
1) Identical, where the single fertilised egg splits sometime in the first 12 days after conception. If it splits in the first 3 4 days, the twins have separate placentas. In the most common situation, the split occurs 4 8 days after conception and the twins share the one placenta. A later split can lead to more complications and Siamese (conjoined) twins are those where the splitting process begins after 12 days.
2) Non-identical, who stem from the fertilisation of two eggs and are no more alike genetically than ordinary brothers and sisters. The release of two or more eggs every month runs in families to some extent and also changes with age, as well as differing between ethnic groups. Such twins have separate placentas although sometimes these may be fused so closely they appear as one until there is detailed examination. The Dixon Quads are non-idential siblings, a result of fraternal twinning. Initially in the pregnancy, Liz Dixon had conceived five separate sacs, five unique sets of DNA.
Identical twins are only identical genetically and there are many physical and behavioral characteristics less determined by heredity where they may differ considerably and where non-identical twins may be very similar.
Higher multiples (i.e. triplets, quads, quints, etc.) can be any combination of monozygotic (MZ) and dizygotic (DZ) eggs. For example, two eggs may be fertilized (making DZ twins) and then one developing embryo splits to form a set of MZ twins. It would be possible to have identical triplets where one early embryo splits and then one of these splits again.
There certainly can be much confusion in the early years, as DZ twins may look and behave as similarly as MZ twins. But by the age of 3-4, then questionnaires such as this one can help. The 'gold standard" is of course genetic testing, done in the past by bloodtyping but increasingly by directly examining the DNA.
Previously a blood sample was needed and many medical staff were unwilling to take blood from young children, just to resolve uncertainty over zygosity. Now DNA can usually be obtained successfully from cells gathered by brushing inside the cheek and more and more laboratories are offering DNA zygosity testing. It is still expensive and for many families, completing a questionnaire will resolve things in their minds.
In the humorous words of one mother of multiples, "Of course my twins are not identical – their socks are always different colors."
