Figure 1.

Schematic possible patterns of X chromosome inactivation

Prior to XCI, both the maternal and the paternal X chromosomes are present (purple cells). The blue cell represents the maternal X is inactivated, and the red cell represents the paternal X is inactivated. If XCI occurs very early in development and propagates to all daughter cells, we would observe extreme skewing, where all placentas show the same X inactivated. In this case, the same X chromosome is inactivated in both sites sampled from this study. If XCI occurs at an intermediate stage, XCI is random but is present with large patches of daughter cells with only the maternal X or paternal X. In this case, three possible scenarios can happen: (1) both of the sites sampled in this study happen to come from two patches with the same X inactivated, (2) each site sampled in this study comes from two patches with different X inactivation, and (3) one of the sites sampled in this study comes from the boundary of two patches. If XCI occurs very late in development, there are either small or no patches, creating a mosaic pattern of XCI.