Some diseases are caused by genetic, or hereditary, defects in which there is an abnormal number of chromosomes or the chromosomes have an abnormal structure. Down syndrome, for example, is usually caused by the presence of an extra chromosome in the nucleus.

If a parent has a dominant gene for a disease, each child has a 50 per cent chance of receiving the defective gene. Examples of dominant-gene disorders are Huntington's disease (a progressive degeneration of the nervous system), achondroplasia (a form of dwarfism), and hypercholesterolemia (high levels of blood cholesterol). If both parents are carriers of a recessive trait, each child has a 25 per cent chance of inheriting a genetic disease. Examples of recessive-gene diseases are cystic fibrosis (a malfunction of the mucus and sweat glands), phenylketonuria (a deficiency of an essential liver enzyme), sickle-cell anemia (a blood disorder primarily affecting blacks), and Tay-Sachs disease (a fatal brain disorder affecting children of Eastern European Jewish ancestry).

In genetic screening, the presence of a genetic disorder is determined by analyzing a person's chromosomes and DNA. The material to be analyzed is isolated from cells that are usually obtained from blood, bone marrow, skin, testes, or placental tissue. Microscopic examinations can reveal flaws in the chromosomes. A chemical test called restriction fragment length polymorphism, or RFLP, analysis is commonly used to detect segments of DNA associated with specific diseases.

Genetic screening helps to determine the probability that individuals or their existing or future offspring will have health problems caused by genetic defects. It is most commonly used by persons with a family history of genetic disorders. If a gene for a disorder is present, certain steps (such as dietary changes) can sometimes be taken to lessen the chance that the disorder will develop; other steps (such as frequent checkups) can ensure that the disorder, should it develop, will be detected in an early, treatable stage. Also, a person with a known genetic defect may decide not to have children. When it has been determined that a pregnant woman's unborn child will probably have a genetic disorder (and when abortion is not an option), counseling can help prepare the parents to accept the child and provide appropriate care.