DAVID NICHOLLS / SCIENCE PHOTO LIBRARY DAVID NICHOLLS / SCIENCE PHOTO LIBRARY
Human X chromosome. Computer illustration of a human X chromosome, on a background of symbols representing the female sex. The X chromosome is one of the two sex chromosomes. The other chromosome is the Y chromosome. Each human cell has 46 chromosomes of which 22 pairs are identical in females and males. The last pair is composed of two X chromosomes for a female, whereas a male has one X and one Y chromosome. In any cell of a normal female one of the X chromosomes is inactivated early in embryonic life and remains inactive ever after. The genes responsible for this inactivation are situated in the lower half of the chromosome.
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