Female mammals have two X chromosomes, but males only have one. Early in development, the female embryo must shut down one X chromosome in every cell, or an overdose of the genes on those chromosomes will kill her.
Scientists haven't worked out all the details of X inactivation, and a new study prolongs the puzzlement. A gene that scientists have found to regulate X chromosome activity in mice doesn't work in people, report researchers from the Johns Hopkins Medical Institutions in Baltimore.
In a female embryo, the choice of which X chromosome to inactivate–either that from the mother or that from the father–is random in each cell. Thus, women have patches of cells expressing X chromosome genes derived from Mom and others expressing genes derived from Dad. In various tissues of mice and some other mammals, the maternal X chromosome is always on and the paternal X chromosome remains silent. This specification is called imprinting.