Genes are the fundamental units of heredity. In everyday terms, we often think of genes as being instructions for a particular trait, such as hair or eye color. Genes are made of a chemical called deoxyribonucleic acid (DNA). Each gene has the chemical instructions for making a specific protein, and each protein has a particular function in the body.
For example, a gene may have the instructions to make a protein called an enzyme, which is necessary for an important biochemical process in the body.
Individual genes are packaged into structures called chromosomes. Humans have a total of 46 chromosomes which are located in the center of every cell of the body. We receive 23 chromosomes (half our genetic information) from our mothers and 23 chromosomes from our fathers. Of those 23 pairs, 22 of them are identical in men and women and are called autosomes. The last pair, the sex chromosomes, differs in men and women. Men have one X and one Y chromosome, and women have two X chromosomes.