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, or DNA. Each gene has the chemical instructions for making a specific protein, and each protein has a particular function in the body, such as manufacturing enzymes for biochemical processes.
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 half of these chromosomes from our mothers and half from our fathers. Twenty-two pairs of chromosomes are identical in men and women; these are called autosomes. The last pair, the sex chromosomes, differ in men and women. Men have one X and one Y chromosome, and women have two X chromosomes.
Genetic disorders arise when one or both copies of a specific gene have undergone an alteration known as a mutation. When an alteration in just one copy of a gene pair causes a genetic disorder, the disorder is referred to as dominant. When alterations in both copies of a gene are required for the disorder to be present, the disorder is considered recessive.