Sometime after fertilization, the egg splits into two genetically identical halves. Fraternal twins are formed when two eggs are released by the mother, and each egg is fertilized by a different sperm.
Fraternal twins can be of the same or opposite sex.
It is also possible for fraternals to be of different races or parentage, or to be conceived at different times, resulting in a large weight difference at birth. This occurs when the mother's egg splits before fertilization and then each half is then fertilized by a sperm. Conjoined twins form exactly like identical twins, but at some point during the stage where the single egg splits, the process stops, and the twins develop attached to one another. In the rare cases of identical quads, both halves would divide to form 4 identical babies. The Dionne quints of Canada were identical quintuplets, the only set in the world.