Most people assume that twins means that two people have similar physical appearance. However, the type of twins is determined long before the twins were born and commonly used terms such as identical or non-identical refer to the way the gametes were fertilized at conception. Monozygotic twins, which means ‘a single zygote’, are genetically identical and are formed when a fertilized egg is divided into two genetically identical cells. Since the physical appearance is also influenced by environmental factors, and not only genetic, monozygotic twins may actually have different physical characteristics, but are always of the same sex. Dizygotic means ‘two zygotes’ and refers to twins which result when two separate eggs are fertilized and implanted in the uterus. Dizygotic twins may or may not resemble each other.
The only way we can determine with certainty the tpe of twin zygosity is through DNA testing. The twin zygosity DNA test is a fairly simple procedure, performed prenatally of after birth, and establishes the type of twin zygosity with >99,995 probability.
For this test we use 16 internationally recommended and validated polymorphic STR markers (plus the non-polymorphic marker AMEL for gender determination), thus achieving a very high probability (likelihood ratio).