How do monozygotic twins arise?

Monozygotic twins arise from the fertilization of a single egg that splits into two embryos. This process occurs when a fertilized egg, or zygote, undergoes early cell division and then divides into two separate masses of cells, each developing into an individual. Since they originate from the same zygote, monozygotic twins share the same genetic material and are genetically identical.

The other options describe different scenarios of twinning or reproduction. The first option refers to dizygotic twins, where two separate eggs are fertilized by two different sperm, leading to siblings who are genetically similar but not identical. The third option also relates to dizygotic twins, emphasizing the independent fertilization of separate eggs by different sperm. The last choice, relating to asexual reproduction, is unrelated to twinning, as asexual reproduction involves a single organism producing offspring without the genetic contribution of another organism.