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Male sex chromosomes. Coloured light micrograph of the X and Y chromosomes of a human male. 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 an X (left) and a much smaller Y chromosome (right), while females have two X chromosomes. Chromosomes carry genetic information in the form of genes and the sex chromosomes carry the genes which control sexual development. (KEYSTONE/SCIENCE PHOTO LIBRARY/DEPT. OF CLINICAL CYTOGENETICS, ADDENBROOKES HOSPITAL)