Final answer:
The opposite of dizygotic (fraternal) twins is monozygotic (identical) twins, which develop from a single fertilized egg that splits during early development. Unlike dizygotic twins which are genetically unique, monozygotic twins share the same genetics.
Step-by-step explanation:
The opposite of dizygotic twins, also known as fraternal twins, are monozygotic twins, commonly referred to as identical twins. Dizygotic twins result from the fertilization of two separate eggs by two separate sperm, making them genetically unique and no more identical than siblings born at different times. Conversely, monozygotic twins develop from a single fertilized egg that splits into two embryos during early development. If this split occurs at the two-cell stage, each twin will have its own placenta. However, if the split occurs during the blastocyst stage, with roughly 70-100 cells, the twins will likely share a placenta and chorionic cavity. Both types of twins can provide insight into studies on nature versus nurture, as identical twins share the same genetic makeup, while dizygotic twins do not.