BSIP, DUCLOUX / SCIENCE PHOTO LIBRARY BSIP, DUCLOUX / SCIENCE PHOTO LIBRARY
Human female chromosomes. Coloured light micro- graph of 23 chromosomes from a normal human female. This constitutes half the karyotype, that is half the total number of chromosomes found in each cell of the human body. Each cell contains 46 chromosomes in 22 matched pairs and one sex- determining pair. Male and female sets of chromosomes differ only in the sex chromosome; a male would be labelled XY and a female XX. A normal female karyotype is written: 46,XX. The chromosomes are made up mainly of DNA, some proteins (principally histones), and small amounts of RNA. The chromosomes carry genes that determine the characteristics of the individual.
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