What are identical twins the result of?

Identical twins, also known as monozygotic twins, originate from the division of one fertilized egg into two distinct embryos. This process occurs after the egg is fertilized by a single sperm, resulting in a zygote. Shortly after fertilization, the zygote undergoes a series of cell divisions. At some point during these divisions, the developing structure can split, leading to the formation of two embryos that share the same genetic material. This genetic similarity is the reason identical twins look alike and share the same DNA.

The other options describe different scenarios of twinning or fertilization. For example, having two separate fertilized eggs or two eggs fertilized simultaneously results in fraternal twins, which are genetically different, while combining two embryos does not naturally occur in human reproduction processes. Understanding the specific mechanisms that lead to identical twins helps clarify the unique genetic and developmental characteristics that distinguish them from other types of twins.