Mammals are thought to have a type of dosage compensation not so far known in any other animal group: however many X chromosomes are present, only one remains genetically active in somatic cells. Considerable evidence for this idea exists, in spite of criticism; the greatest difficulty is presented by the abnormalities in human individuals with X chromosome aberrations. Possible explanations for these abnormalities include: wrong X chromosome dosage in early development before X inactivation, reversal of inactivation, partial inactivation of both X chromosomes, activity of the X while in the condensed inactive state, and the presence of a homologous non-inactivated region of the human X and Y. In female germ cells X inactivation apparently does not occur, but the situation in male germ cells is less clear. The Y chromosome is probably also inactive in somatic cells of adults, but again its function in germ cells is not yet clear. Some species have a presumed doubly inactive X chromosome region, as well as the singly active one. The origins and functions of this region are unknown; it may have a role in female germ cells.