Final answer:
Dizygotic twins, also known as fraternal twins, occur when two eggs are released and fertilized simultaneously. They are genetically as similar as siblings born at different times. In rarer cases, monozygotic twins, or identical twins, occur when a single fertilized egg divides early in development.
Step-by-step explanation:
When two eggs are released and fertilized at the same time, the resulting twins are known as dizygotic twins, or fraternal twins. These twins form when two separate eggs are fertilized by two separate sperm, leading to the development of two zygotes that implant and develop independently. As a result, dizygotic twins are genetically distinct individuals and are as genetically similar as non-twin siblings born at different times.
Much less commonly, when a single fertilized egg divides during early development, it can lead to monozygotic twins, or identical twins. The timing of when this division occurs will affect whether the twins share a placenta or have separate placentas.