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Your body is made up of billions of cells, which are too small to see without a strong microscope. Inside most of those cells are chromosomes, which are thread-like strands that contain hundreds, or even thousands, of genes. Genes determine physical traits, such as the color of your eyes. Most people have 23 pairs of chromosomes, and you received half your chromosomes from your mother and the other half from your father. Even after you're born, your 46 chromosomes continue to guide the way your body grows and develops.