Identical twins are formed when a single fertilized egg cell, called a zygote, starts to divide but splits into two groups of cells and each group of cells goes on to form a separate child. It only happens early in pregnancy when the embryo consists of only a small number of cells.
Each of the twins originates from the same fertilized egg, hence are described as monozygotic, they have identical DNA and will have a very similar physical appearance.
If we conduct a DNA test on monozygotic twins we expect to obtain identical results. STR tests are used which provide strong support for the view that they are indeed identical when matching results are obtained.
Non-identical twins are formed if the mother produces two separate eggs during ovulation and each of these eggs is fertilized separately to form two different fertilized eggs or zygotes. Twins formed in this way, sometimes called dizygotic twins, are non-identical and are genetically no different from regular brothers and sisters. STR tests conducted on dizygotic twins will usually provide 100% conclusive proof that the twins are not identical.