Mutations occur whenever something causes permanent changes in the DNA sequences that make up a person's genes. Mutations vary in size from single DNA building blocks to large sections of chromosomes.
Gene mutations are either inherited or acquired. Parent-to-child mutations are called hereditary or germline mutations; they happen in the egg and sperm cells, also called germ cells. These types of mutations affect all body cells and last a lifetime.
Mutations that occur just after fertilization, or only in egg or sperm cells, are called new (de novo) mutations and may explain how a genetic abnormality can affect all the body cells of a child with no family history of the disorder.
Acquired (or somatic) mutations of individual DNA cells occur during a person’s life and are caused by such environmental factors as the sun’s ultraviolet radiation, or can occur when the DNA imperfectly copies itself during cell division. Acquired mutations in somatic cells (not sperm and egg cells) can’t be inherited.
When mutations occur in single cells of early embryos, as all the cells divide during growth and development, there are some mutated cells and some cells without the genetic change. This is called mosaicism.
Some genetic changes are very rare, but other are common. Changes in more than one percent of the population are called polymorphisms, which are normal variations that are responsible for different eye colors, hair colors and blood types. Many polymorphisms don’t affect health, but other variations may make a person prone to developing certain disorders.