Which statement describes the origin of monozygotic (identical) twins?

• A. Fertilization of a single ovum that splits
• B. Fertilization of two ova
• C. Mutation during embryo development
• D. Conjoined twins

Answer: (A)

Monozygotic twins arise when a single fertilized egg splits into two embryos. Because both babies come from the same zygote, they have identical genetic material. The timing of the split influences how the placental and amniotic sacs form, but the fundamental origin is one fertilized ovum dividing into two developing organisms.

Fertilization of two ovaries would produce two separate zygotes, resulting in dizygotic (fraternal) twins with typically different genetics. A mutation during embryo development could alter some cells within one embryo (mosaicism) but does not create two genetically identical individuals. Conjoined twins are the result of incomplete splitting of a single zygote, a late split, and are still monozygotic, but their physical joining stems from that late separation rather than a normal split pattern.