Which term refers to twins that originate from one zygote that splits apart very early in development and share 100% of their genes?

• A. Monozygotic (MZ) twins
• B. Dizygotic twins
• C. Fraternal twins
• D. Conjoined twins

Answer: (A)

Monozygotic twins arise when a single fertilized egg splits very early, producing two genetically identical individuals. Because both twins come from the same zygote, they inherit essentially the same DNA, so they share nearly 100% of their genes. In practice, tiny differences can arise from rare post-zygotic mutations or epigenetic changes, but the fundamental point is that a single zygote provides all the genetic material for both twins. The other terms refer to different patterns of fertilization: two separate eggs fertilized by two sperm give dizygotic or fraternal twins, who are genetically like ordinary siblings and share about half their genes; conjoined twins result from incomplete splitting of the same zygote later in development.