DAVID NICHOLLS / SCIENCE PHOTO LIBRARY DAVID NICHOLLS / SCIENCE PHOTO LIBRARY
Human X and Y chromosomes. Computer artwork of an X (left) and a Y (right) chromosome against a background of symbols representing the male sex. Every human cell has 46 chromosomes, of which 22 pairs are identical in both males and females. However, in males the last pair is composed of and X and Y chromosome, while females have two X chromosomes.
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