Rhonda J. Pomerantz, MD is a practicing Dermatologist (Skin Specialist) in New York, NY. Dr. Pomerantz graduated from New York University School of Medicine in 1989 and has been in practice for 28 years. She completed a residency at Suny Health Science Center. Dr. Pomerantz also specializes in Internal Medicine.
Identical twins form when one egg has been fertilised by one sperm and the zygote splits into two. This happens at the very earliest stage of development, when the zygote is no more than a cluster of a few cells.
This is why the DNA of fraternal twins is different. In fact, fhe DNA of fraternal twins is no more similar than the DNA any other sibling pair. Usually, a woman only releases a single egg at a time. Fraternal twins can only happen if a mother releases two eggs in one cycle. This is called hyperovulation. Unlike embryo splitting, ovulation is a normal biological process that is controlled by our genes.