A sound body depends on the continuous interplay of thousands of proteins, acting together in just the right amounts and in just the right places - and each properly functioning protein is the product of an intact gene. Genes can be altered (mutated) in many ways. The most common gene mistake involves a single changed base in the DNA - a misspelling. Other alterations include the loss or gain of a base. Sometimes long segments of DNA are multiplied or disappear.
Some mutations are silent; they affect neither the structure of the encoded protein nor its function. Other mutations result in an altered protein. In some instances, the protein is normal enough to function, but not well; this is the case of the flawed hemoglobin, the oxygen-carrying protein in the blood that causes sickle-cell anemia. In other instances, the protein can be totally disabled. The outcome of a particular mutation depends not only on how it alters a protein's function but also on how vital that particular protein is to survival.