Genes are best known for determining physical and some other traits of an organism, such as blue, brown or hazel eyes in people. Genes are an organism's chemical code book.

They order cells exactly what to do, how and when to do it, and when to stop doing it. Genes in the nucleus, or central body, of a cell are lined up as part of a long, complicated molecule, a twisting strand called DNA, or deoxyribonucleic acid.

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