Paternal twins — more commonly called identical twins — are monozygotic. They form when an egg is fertilized by a single sperm, creating a zygote. After conception, the zygote splits, creating two individual embryos. Because they come from a single fertilized egg, paternal twins share the same genetic origins and DNA.
However, their genetics may still differ. Certain mechanisms happen after the separation of the zygote, which can result in minor external differences in twins. After birth, environmental factors also influence genetic expressions, so even identical twins will still have individual personalities, behaviors, and physical traits.
Paternal twins (monozygotic twins) occur in three to four per 1,000 births worldwide. According to scientific research, most cases of monozygotic twinning don’t result from strictly genetic factors. However, some families do have higher rates of monozygotic twins, so genetics may play a role. In any case, no single cause of monozygotic twinning has been identified.
What we do know is that twin pregnancies are primarily a matter of chance. Monozygotic twins come from all areas of the world, but their presence varies by geographic location. For example, there are 1.3 cases of identical twins per thousand births in Japan but 50 cases per thousand births in Nigeria. On average, these twins happen in 13 of every thousand births.