Genetic testing examines the genetic information contained inside a person's cells to determine if that person has or will develop a certain disease, or could pass a disease to his or her offspring.

Some families or ethnic groups have a higher incidence of a certain disease than does the population as a whole. Before having a child, a couple from such a family or ethnic group may want to know if their child would be at risk of having that disease. Early in pregnancy, the fetus's cells can be studied for certain defects that could result in physical abnormalities or mental retardation. This testing is most common when the mother is over the age of 35, or there is a family history of physical or mental abnormalities.

A genetic disease may be apparent when the child is born, or may appear later as...