X-CHROMOSOME INACTIVATION IS MOSTLY RANDOM IN PLACENTAL TISSUES OF FEMALE MONOZYGOTIC TWINS AND TRIPLETS

Patterns of X-chromosome inactivation in chorion, amnion, and cord fro m 79 pairs of twins were examined, Seven sets of triplets were include d in the analysis, both as twin pairs and triplets, Twins were stratif ied as dizygotic (DZ), monozygotic (MZ), monochorionic, and dichorioni c and were selected for birth weight discordance, discordance for cong enital anomalies, twin-twin transfusion syndrome, and various patterns of vascular anastomosis. X-inactivation was predominantly symmetric. Chorion was the most likely tissue to show asymmetric X-inactivation a nd was found most frequently in MZ dichorionic twins. There was no cor relation of X-inactivation pattern with the selected clinical criteria . This study does not confirm that asymmetric X-inactivation in embryo nic tissues is a common phenomenon in female twins, including monozygo tic twins. (C) 1996 Wiley-Liss, Inc.