Genetic testing allows the genetic diagnosis of vulnerabilities to inherited diseases, and can also be used to determine a person's ancestry. Every person carries two copies of every gene, one inherited from their mother, one inherited from their father. The human genome is believed to contain about 25,000 genes. In addition to studying chromosomes to the level of individual genes, genetic testing in a broader sense includes biochemical tests for the presence or absence of key proteins that signal aberrant genes.

Genetic testing usually involves examining a person's DNA - taken from cells in a sample of blood or, occasionally, from other body fluids or tissues - for some anomaly that flags a disease or disorder. The DNA change can be relatively large: a missing or added piece of a chromosome - even an entire chromosome - that is visible under a microscope. Or it can be extremely small, as little as one extra, missing, or altered chemical base. Genes can be overexpressed (too many copies), inactivated, or lost altogether. Sometimes, pieces of chromosomes become switched, or transposed, so that a gene ends up in a location where it is permanently and inappropriately turned on or off.

Since genetic testing may open up ethical or psychological problems, genetic testing is often accompanied by genetic counseling. Some possible future ethical problems of genetic testing were dramatised in the science fiction film GATTACA.

See also: DNA fingerprinting, Genetic counseling