GWEN SHOCKEY / SCIENCE PHOTO LIBRARY GWEN SHOCKEY / SCIENCE PHOTO LIBRARY
Illustration of a Y chromosome. Each chromosome consists of two identical strands (chromatids), aligned parallel to each other and joined at an area called a centromere. An individual's sex is determined by the presence or absence of the Y chromosome. Women have two X chromosomes, while men have an X and a Y. Chromosomes consist of very condensed deoxyribonucleic acid (DNA). DNA contains sections, called genes, which encode the body's genetic information. The Y chromosome only contains genes for 'maleness', for example a gene that triggers the development of the testes, and is much smaller than the X chromosome.
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