Answer:
When sperm and egg unite, they create a zygote through fertilization. Over time, the zygote will divide into millions upon millions of cells. Each cell consists of 46 chromosomes; these cells, as a whole, will become the baby. However, the process is not the same for every situation and some things can go differently.
Monozygotic twins, also known as identical twins, come from a single zygote. During the process stated above, the zygote for whatever reason splits into two separate entities and eventually they will become the babies. The babies will be identical in nature (same sex and same physical features for the most part). This occurs because they each contain the same set of 46 chromosomes deriving from the single zygote.