Monozygotic Twins

Humans are polyclonal beings since each one of us is born of a unique fertilization event. Rarely, however, two babies or more can develop from a single embryo producing monoclonal siblings. Referred to as monozygotic (MZ), identical twins are true reproductive clones; like Dolly they share a genome but breathe a sovereign existence. Each minute a pair of MZ twins is born, making it more prevalent than any genetic disease (e.g., cystic fibrosis, Down syndrome) yet its cause has remained enigmatic. Several animals have mastered this mode of reproduction and a handful of places have reported unusual levels of MZ twinning.