Complex sentence: Preferential inactivation of the paternal X-chromosome occurs in both marsupials and in cell lineages that form the membranes surrounding the embryo, whereas in mammals either the maternally or the paternally derived X-chromosome may be inactivated in different cell lines.
Split into simpler sentences: Preferential inactivation of the paternal X-chromosome occurs in both marsupials and in cell lineages that form the membranes surrounding the embryo. Whereas in mammals either the maternally or the paternally derived X-chromosome may be inactivated in different cell lines.