Final answer:
Identical twins result from a single fertilized egg that splits into two embryos, not from two separate eggs. Fraternal twins, however, are from two different eggs fertilized by two different sperm. Identical twins share all of their genes, while fraternal twins share about half, like typical siblings.
Step-by-step explanation:
No, identical twins are not the result of two separate eggs being fertilized at the same time. Instead, identical twins develop when a single fertilized egg (zygote) divides into two separate embryos during early development. In contrast, fraternal twins occur when two different eggs are fertilized by two separate sperm cells, resulting in twins that are genetically similar to siblings born at different times. Identical twins will share all of their genes because they come from the same zygote, while fraternal twins will share only about half of their genes, similar to any other pair of siblings.
When considering the development of identical twins, there are two scenarios based on when the zygote divides. A division at the two-cell stage will lead to twins with individual placentas, while a division at the blastocyst stage, with roughly 70-100 cells, results in twins sharing the same placenta and chorionic cavity.