BIOPHOTO ASSOCIATES / SCIENCE PHOTO LIBRARY BIOPHOTO ASSOCIATES / SCIENCE PHOTO LIBRARY
False-colour scanning electron micrograph (SEM) of the X and Y chromosomes in a human male. Known as the sex chromosomes. Every human cell has 46 chromosomes of which 22 pairs are identical in females and males. The last pair is composed of two X chromosomes in a female, whereas a male has one X (at top) and one Y chromosome (at bottom). Chromosomes carry genetic information in the form of genes. Sex chromosomes determine gender of male or female. In a female, one X chromosome is inactivated early in embryonic life while the other is active; on the shorter arm of the male Y chromosome (lower left) lie the genes responsible for male genital development.
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