13, Fraternal twinning is genetically predisposed. Identical twinning is random. Fraternal twinning is the result of a woman releasing multiple eggs at the same time, and is largely the result of a genetic predisposition to release this extras. Identical twins, however, are the result of a random split of a single egg, something which cannot be genetically predisposed. In recent years, the number of fraternal twins has risen in response to fertility treatments, while the number of identical twin births has stayed the same.