What defines monozygotic twins?

Monozygotic twins, often referred to as identical twins, are defined by their origin from a single zygote that divides into two embryos. Because they originate from the same fertilized egg, these twins share nearly identical genetic material, which is why they exhibit strikingly similar physical characteristics and genetic traits. This unique formation process distinguishes them from other types of twins, especially in terms of genetic similarity.

The other choices highlight different aspects of twin formation: one mentions genetic differences, which applies more to fraternal twins rather than the genetically identical monozygotic twins. Another option describes twins originating from two separate zygotes, a situation typical of fraternal twins, while the last choice refers to fraternal twins that may share a placenta, which can occur in some cases yet does not pertain to the definition of monozygotic twins. Each of these scenarios contrasts with the unified genetic identity that characterizes monozygotic twins.