active X chromosome. Overview The Xi marks the inactive, Xa the active X chromosome. XP denotes the paternal, and XM to denotes the maternal X chromosome. When the egg (carrying XM), is fertilized by a sperm (carrying a Y or an XP) a diploid zygote forms. From zygote, through adult stage, to the next generation of eggs, the X chromosome undergoes the following changes: XiP XiM zygote → undergoing zygotic genome activation, leading to: XaP XaM → undergoing imprinted (paternal) X-inactivation, leading to: XiP XaM → undergoing X-activation in the early blastocyst stage, leading to: XaP XaM → undergoing random X-inactivation in the embryonic lineage (inner cell mass) in the blastocyst stage, leading to: XiP XaM OR XaP XiM → undergoing X-reactivation in primordial germ cells before meiosis, leading to: XaM XaP diploid germ cells in meiotic arrest. As the meiosis I only completes with ovulation, human germ cells exist in this stage from the first weeks of development until puberty. The completion of meiosis leads to: XaM AND XaP haploid germ cells (eggs). The X activation cycle has been best studied in mice, but there are multiple studies in humans. As most of the evidence is coming from mice, the above scheme represents the events in mice. The completion of the meiosis is simplified here for clarity. Steps 1–4 can be studied in in vitro fertilized embryos, and in differentiating stem cells; X-reactivation happens in the developing embryo, and subsequent (6–7) steps inside the female body, therefore much harder to study. Timing The timing of each process depends on the species, and in many cases the precise time is actively debated. [The whole part of the human timing of X-inactivation in this table is highly questionable and should be removed until properly substantiated by empirical data] Inheritance of inactivation status across cell generations The descendants of each cell which inactivated a particular X chromosome will also inactivate that same chromosome. This phenomenon, which can be observed in the coloration of tortoiseshell cats when females are heterozygous for the X-linked gene, should not be confused with mosaicism, which is a term that specifically refers to differences in the genotype of various cell populations in the same individual; X-inactivation, which is an epigenetic change that results in a different phenotype, is not a change at the genotypic level. For an individual cell or lineage the inactivation is therefore skewed or 'non-random',