Which statement correctly distinguishes monozygotic and dizygotic twins?

Which statement correctly distinguishes monozygotic and dizygotic twins?

• A. Dizygotic twins are genetically identical; monozygotic are fraternal.
• B. Monozygotic twins are genetically identical; dizygotic are fraternal.
• C. Both types are genetically identical.
• D. Both types are genetically different.

Answer: (B)

When twins come from the same fertilized egg that splits, they receive nearly identical genetic material, making monozygotic twins genetically identical. In contrast, dizygotic twins arise from two separate fertilized eggs, so they are like regular siblings who happen to share the womb, and they share about half of their genes. Therefore, the statement that monozygotic twins are genetically identical and dizygotic twins are fraternal best captures the true distinction. Dizygotic twins are not genetically identical, and monozygotic twins are not fundamentally different in origin from regular siblings in that they are not simply “genetically different” from each other. Minor differences can occur due to mutations or epigenetic factors, but the core distinction remains.

When twins come from the same fertilized egg that splits, they receive nearly identical genetic material, making monozygotic twins genetically identical. In contrast, dizygotic twins arise from two separate fertilized eggs, so they are like regular siblings who happen to share the womb, and they share about half of their genes. Therefore, the statement that monozygotic twins are genetically identical and dizygotic twins are fraternal best captures the true distinction. Dizygotic twins are not genetically identical, and monozygotic twins are not fundamentally different in origin from regular siblings in that they are not simply “genetically different” from each other. Minor differences can occur due to mutations or epigenetic factors, but the core distinction remains.