ALFRED PASIEKA / SCIENCE PHOTO LIBRARY ALFRED PASIEKA / SCIENCE PHOTO LIBRARY
X chromosome. Computer graphic based on a light micrograph of a human X chromosome, one of the two sex chromosomes. 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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