Twin, either of two young who are simultaneously born from one mother. Twinning, common in many animals, is of two biological kinds: the one-egg (monozygotic), or identical, type and the two-egg (dizygotic), or fraternal, type.
There are different types of twins, identical and fraternal being the most common. Identical, or monozygotic, twins develop from the same fertilized egg that splits into two. These twins are...
Identical twins, also known as monozygotic twins, share the same egg at conception and are always either both boys or both girls. Fraternal twins, also known as dizygotic twins, come from two separate eggs fertilized at the same time. They could be the same sex or different.
What are the most common types of twins? Dizygotic (fraternal) are the most common type, accounting for two-thirds of all twin births. These multiples share about 50 percent of their DNA, the same as non-twin siblings. They can be any combination of gender: boy-boy, girl-girl, or girl-boy.