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The sex chromosome associated with female characteristics, occurring paired in the female and singly in the male sex-chromosome pair.
The sex chromosome that in female mammals is paired with another X-chromosome and in males is paired with a Y-chromosome. Very few genes on the X-chromosome have counterparts on the Y-chromosome, and since males have only one X-chromosome, any gene present on it (even if the gene is recessive in females) is expressed in males. In females, one of the two X-chromosomes in each cell is deactivated. See more at sex chromosome. See note at sex.