Monozygotic twins, also known as identical twins, are formed from a single fertilized egg that splits into two embryos. This means they share the same genetic material, making them genetically identical. They typically have the same sex and look very similar, although they may still have slight differences in appearance due to environmental factors. These twins usually share the same placenta during pregnancy, which can lead to unique health considerations. Because they come from the same zygote, monozygotic twins can provide valuable insights into the roles of genetics and environment in human development and behavior.