 Genes, DNA, and chromosomes are what make you who you are. They are the set of instructions that are given to you by your father and mother. These instructions are in your cells, and all living organisms are composed of cells. There are many different types, like nerve cells and hair cells and skin cells. They all have different shapes and forms, but every cell has the same basic parts. The cell has an outer border called the membrane, which contains a liquid material called cytoplasm. In the cytoplasm is the nucleus, and inside the nucleus are chromosomes. In humans, each cell normally contains 23 pairs of chromosomes for a total of 46. 22 of these pairs, called autosomes, look the same in both males and females. The 23rd pair, the sex chromosomes, differ between males and females. Females have two copies of the X chromosome, while males have one X and one Y chromosome. The chromosomes are really long strings of DNA, deoxyribonucleic acid. DNA is shaped like a ladder that's been twisted. This shape is called a double helix. The steps of the ladder are made of four bases. Addonine is A, thymine is T, guanine is G, and cytosine is C. A stretch of the DNA is called a gene. Your body reads the genes and the letters like a recipe and builds protein. The order of the bases in the DNA, along with the length and sequence of the gene, determines the size and shape of the protein it builds. The size and shape of the protein determines the function it will have in your body. Proteins make up cells, cells make up tissue, and tissue makes up organs, like your eyes and your skin. So the genes determine what you are, a cow, an apple, or a human, and what you will look like, the color of your hair, your skin, your eyes, and so on.