Zygosity twin testing is used to determine whether multiple children from the same birth (twins, triplets, quadruplets, etc.) are genetically identical or not. If siblings are identical, they are known as monozygotic. This means that they were formed from a single fertilised egg, which went on to split into separate embryos. It also means that their DNA is exactly the same.
Non identical siblings from the same birth are known as dizygotic (i.e., having two zygotes). Triplets are trizygotic (three zygotes), quadruplets are quadzygotic (four zygotes), and polyzygotic is the umbrella term for multiple zygotes. This means that these siblings are the result of separate fertilised eggs and that they share DNA in common (as in the case of siblings from different births).
Siblings that are of different sexes are non identical. Twins of the same sex can start to have different physical features as they grow. Growth patterns (such as hair and eye colour, ear shape, teeth formation, hand and feet appearance) can indicate whether or not the children are identical. By around 2-years-old, their zygosity may be quite clear from their physical features.
Reasons why zygosity twin determination would be required:
It can be interesting to know whether multiple siblings are identical
It can be useful to know whether multiple siblings are identical
In addition, some people buy a twin DNA test as a unique gift idea for multiple birth parents or their children and the DNA certificate can be used as a special keepsake or even wall art.