ZYGOSITY TESTING - CURRENT STATUS AND EVOLVING ISSUES

This paper reviews the current status and evolving issues in the field of zygosity testing. Monozygotic twins are seldom absolutely identica l. Traditional methods of zygosity testing, mathematical ones and asse ssment of physical characteristics are reviewed. The proper delineatio n of zygosity based on placentation and chorionicity is outlined. Gene tic and environmental discordance patterns are described, with a discu ssion of why ''identical'' twins are never identical preceding specifi c guidelines for clinical practice.