X and Y chromosomes hove evolved from a common proto-chromosome, 300 million years ago. They shore 29 genes, scottered in two homologous regions (PAR1 and PAR2). Ten per cent of humon genetic diseases result from mutations of X-linked genes. Disorders with mental handicap, muscle involvement or infertility ore overrepresented. X chromosome inactivation is a complex epigenetic process. This mechanism keeps a single transcriptionally active chromosome per cell. At fecondotion, in XX embryo, the active X is always the maternal one, with the paternal X is kept inactive. At the time of implantation of a XX egg, inactivation is erased and reinstalled at random in the embryo, except in the germinal cell line, where both X remain inactive, and in placenta, where only the maternal X is expressed. Skewed inactivation of a X in a woman may indicate the presence of a mutated gene on the inactive X. Contrasting with outosomal disorders, the difference between recessivity and dominance is often relative, as heterozygous women con disclose a phenotype comparable to affected males, as a result of random X inactivation.