Identical twins, and how does it happen?

• A. Twins who develop from separate fertilized eggs
• B. Twins who develop from a single fertilized egg; one cell splits after fertilization
• C. A clone created in a lab
• D. Two eggs fertilized by two sperm at same time

Answer: (B)

Identical twins form when a single fertilized egg splits into two embryos early in development. Because they come from the same zygote, they share the same DNA and are usually the same sex. The timing of the split can affect whether they have separate placentas or share a placenta and membranes. Other scenarios describe two separate eggs being fertilized or artificial cloning in a lab, which produce twins with different DNA or are not how natural twins occur.