A mutation is a change in the structure of a gene, the unit of heredity. Genes are made of deoxyribonucleic acid (DNA), a long molecule composed of building blocks called nucleotides. Each nucleotide is built around one of four different subunits called bases. These bases are known as guanine, cytosine, adenine, and thymine. A gene carries information in the sequence of its nucleotides, just as a sentence carries information in the sequence of its letters.

One type of mutation is a change in a base. This is called a point mutation and it is like changing one letter in a word. Most genes carry instructions for making proteins. When a base is changed in a gene, different results are possible, depending on which base is changed and what it is changed into. The gene may produce an altered protein, it may produce no protein, or it may produce the usual protein. Most mutations are not harmful, but some can be. A harmful mutation can result in a genetic disorder or even cancer.

Another kind of mutation is a chromosomal mutation. Chromosomes, located in the cell nucleus, are tiny threadlike structures that carry genes. A chromosome consists of a molecule of DNA together with proteins. Sometimes, a long segment of DNA is inserted into a chromosome, deleted from a chromosome, flipped around within a chromosome, duplicated, or moved from one chromosome to another. Such changes are usually very harmful.

One example of a chromosomal mutation is a condition called Down syndrome. In each cell, humans normally have forty-six chromosomes, consisting of two copies of the twenty-three kinds of chromosomes. Down syndrome usually results from the presence of one extra copy of a particular chromosome, or an extra portion of that chromosome. The presence of that extra chromosome leads to problems with certain organs of the body, such as the heart. It can also lead to leukemia—a cancer of the blood-forming cells—and produce mental disabilities. Many people with Down syndrome also have distinct facial features.

Mutations can be inherited or acquired during a person's lifetime. Mutations that an individual inherits from their parents are called hereditary mutations. They are present in all body cells and can be passed down to new generations. Acquired mutations occur during an individual’s life. If an acquired mutation occurs in an egg or sperm cell, it can be passed down to the individual’s offspring. Once an acquired mutation is passed down, it is a hereditary mutation. Acquired mutations are not passed down if they occur in the somatic cells, meaning body cells other than sperm cells and egg cells. Some acquired mutations occur spontaneously and randomly in genes. Other mutations are caused by environmental factors, such as exposure to certain chemicals or radiation.

Mutations occur throughout the natural world. Some mutations are beneficial and increase the possibility that an organism will thrive and pass on its genes to the next generation. When mutations improve survival or reproduction, the process of natural selection will cause the mutation to become more common over time. When mutations are harmful, they become less common over time. Therefore, mutation is a force that helps drive evolution.