Each person has the same set of genes - about 20,000 in all. The differences between people come from slight variations in these genes. For example, a person with red hair doesn't have the "red hair gene" while a person with brown hair has the "brown hair gene." Instead, all people have genes for hair color, and different versions of these genes dictate whether someone will be a redhead or a brunette.
Your body contains 50 trillion tiny cells, and almost every one of them contains the complete set of instructions for making you. These instructions are encoded in your DNA. DNA is a long, ladder-shaped molecule. Each rung on the ladder is made up of a pair of interlocking units, called bases, that are designated by the four letters in the DNA alphabet - A, T, G and C. 'A' always pairs with 'T', and 'G' always pairs with 'C'.
The long molecules of DNA in your cells are organized into pieces called chromosomes. Humans have 23 pairs of chromosomes. Other organisms have different numbers of pairs - for example, chimpanzees have 24 pairs. The number of chromosomes doesn't determine how complex an organism is - bananas have 11 pairs of chromosomes, while fruit flies have only 4.
Chromosomes are further organized into short segments of DNA called genes. If you imagine your DNA as a cookbook, then your genes are the recipes. Written in the DNA alphabet - A, T, C, and G - the recipes tell your cells how to function and what traits to express. For example, if you have curly hair, it is because the genes you inherited from your parents are instructing your hair follicle cells to make curly strands.
Cells use the recipes written in your genes to make proteins - just like you use recipes from a cookbook to make dinner. Proteins do much of the work in your cells and your body as a whole. Some proteins give cells their shape and structure. Others help cells carry out biological processes like digesting food or carrying oxygen in the blood. Using different combinations of the As, Cs, Ts and Gs, DNA creates the different proteins - just as you use different combinations of the same ingredients to make different meals.
Cells come in a dizzying array of types; there are brain cells and blood cells, skin cells and liver cells and bone cells. But every cell contains the same instructions in the form of DNA. So how do cells know whether to make an eye or a foot? The answer lies in intricate systems of genetic switches. Master genes turn other genes on and off, making sure that the right proteins are made at the right time in the right cells.