Which description best defines dizygotic (fraternal) twins?

• A. Fertilization of two ova by two different sperm cells
• B. Splitting of a single fertilized ovum
• C. Mutation during embryo development
• D. Conjoined twins

Answer: (A)

Dizygotic twins occur when two separate eggs are released and fertilized by two separate sperm during the same pregnancy. Each fertilization forms its own zygote, so the twins are essentially two siblings in one pregnancy. They share about half of their genes on average and can be either same or different sexes. Typically they each develop their own placenta and amniotic sac (a dichorionic diamniotic arrangement), though placentas can occasionally appear fused.

This is different from identical twins, which come from splitting a single fertilized egg, creating two individuals with nearly identical DNA. Conjoined twins arise from incomplete separation of a single fertilized egg. Mutations during embryo development are not what defines dizygotic twinning.