MEDICAL DICTIONARY
X chromosome: A sex chromosome found in both females and males. Normal females usually have two X chromosomes. Normal males usually have one X along with a Y chromosome . The complete chromosome complement consisting of 46 chromosomes is conventionally written as 46,XX for females and 46,XY for males.
The X chromosome carries hundreds of genes . Only a minority of the genes on the X have anything to do with sex. This is consistent with the concept that the X and Y chromosomes evolved from an ancient pair of autosomal (nonsex) chromosomes and that the X chromosome retains much of its old autosomal information.
The inheritance of genes on the X follows special rules because males have only a single X chromosome, almost no gene on the X has a counterpart on the Y, and genes on the X are expressed in males. Genes inherited in this manner are said to be sex-linked or, more precisely, X-linked .
Examples of the genes on the X chromosome include those for the enzyme steroid sulfatase, episodic muscle weakness, ocular albinism , Charcot-Marie-Tooth disease (one kind of this neurologic disease), retinoschisis (retinal splitting due to degeneration), infantile spasm syndrome, agammaglobulinemia (a form of immune deficiency), Aicardi syndrome , Duchenne and Becker muscular dystrophy , chronic granulomatous disease, hemophilia A , hemophilia B , and red-green colorblindness .
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