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Female sex chromosomes. Coloured light micrograph of the two X chromosomes of a human female. Every human cell has 46 chromosomes, of which 22 pairs are identical in both males and females. However, in females the last pair comprises two X chromosomes, while males have one X and a much smaller Y chromosome. 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)