The main difference between identical twins and fraternal twins is the way they are formed:
- Identical twins: Identical twins, also known as monozygotic twins, are formed when a single fertilized egg (zygote) splits into two identical halves early in development. This results in two embryos with the same genetic material, meaning they are always of the same sex and have identical physical characteristics.
- Fraternal twins: Fraternal twins, also known as dizygotic twins, are formed when two separate eggs are fertilized by two separate sperm at the same time. This results in two embryos with different genetic material, meaning they can be of the same or opposite sex and may or may not share physical characteristics.
Other differences between identical twins and fraternal twins include:
- Probability of occurrence: Identical twins occur randomly and account for about 1 in every 250 pregnancies, while fraternal twins are more common and occur in about 1 in every 90 pregnancies.
- Genetic similarity: Identical twins have 100% genetic similarity, while fraternal twins have, on average, 50% genetic similarity, which is the same as any other siblings.
- Placenta: Identical twins usually share a placenta, while fraternal twins each have their own placenta.
- Amniotic sac: Identical twins may or may not share the same amniotic sac, while fraternal twins each have their own amniotic sac.
- Physical differences: Identical twins often have very similar physical characteristics, while fraternal twins may look quite different from each other, just like any other siblings.
