Author:

Keywords:

Adolescent, Adult, Child, Child, Preschool, Chorion, DNA, Female, Fetal Blood, Humans, Infant, Mouth Mucosa, Pregnancy, Research Support, Non-U.S. Gov't, Restriction Mapping, Twins, Dizygotic, Twins, Monozygotic, X Chromosome, Science & Technology, Life Sciences & Biomedicine, Genetics & Heredity, SEVERE COMBINED IMMUNODEFICIENCY, FEMALE MONOZYGOTIC TWINS, CHROMOSOME-INACTIVATION, MUSCULAR-DYSTROPHY, PATTERNS, STABILIZATION, 06 Biological Sciences, 11 Medical and Health Sciences, 31 Biological sciences, 32 Biomedical and clinical sciences, 42 Health sciences

Abstract:

To gain insight into the timing of twinning, we have examined a closely related event, X-chromosome inactivation, in female MZ twin pairs. X-inactivation patterns in peripheral blood and buccal mucosa were compared between monochorionic MZ (MC-MZ) and dichorionic MZ (DC-MZ) twins. Overall, the MC-MZ twins displayed highly similar X-inactivation patterns, whereas DC-MZ twins frequently differed in their X-inactivation patterns, when both tissues were tested. Previous experimental data suggest that commitment to X inactivation occurs when there are 10-20 cells in the embryo. Simulation of embryo splitting after commitment to X inactivation suggests that MC-MZ twinning occurs three or four rounds of replication after X inactivation, whereas a DC-MZ twinning event occurs earlier, before or around the time of X inactivation. Finally, the overall degree of skewing in the MZ twins was not significantly different from that observed in singletons. This indicates that X inactivation does not play a direct role in the twinning process, and it further suggests that extreme unequal splitting is not a common mechanism of twin formation.