The genetic material that determines a person's inherited features (such as sex, eye color, and blood type) is called DNA (deoxyribonucleic acid). DNA is a long molecule that is made up of building blocks called nucleotides. The order of the nucleotides in the DNA is extremely important. The nucleotides are grouped together to form genes. Each person's DNA contains 30,000 to 60,000 genes. One or more genes determines each inherited feature. Every person's order of nucleotides in the DNA is unique.

DNA makes up the thin strands (called chromosomes) found in the center, or nucleus, of all body cells except red blood cells and platelets. Genetic testing can be done using almost any cell or tissue from the body. It is frequently done to evaluate a amniocentesis or chorionic villus sampling (CVS). Genetic testing can also be done on the white blood cells in a blood sample.

Genetic testing detects diseases caused by an error in a person's DNA sequence. These genetic errors may be inherited from a parent or may occur during the development of a fetus. Some diseases caused by genetic defects may be apparent at birth (such as Down syndrome). Others do not appear until later in life (such as Huntington's disease). Genetic testing also can be done to determine whether a person is a carrier of a disorder caused by a genetic defect.

Genetic disorders can be caused by:

• Defects of the chromosomes, which can be broken, split, separated, or duplicated in abnormal ways. Down syndrome is an example of this type of disorder.
• Errors in one or more genes within the chromosomes. These disorders (such as cystic fibrosis) are clearly inherited along family lines and some (such as hemophilia or Duchenne muscular dystrophy) are caused by errors in the genes on the chromosomes that also determine sex.
• Factors that make a person with a certain gene more likely to develop the disorder. For example, almost all the people who develop a condition called ankylosing spondylitis, which damages the joints of the spine, have a certain gene called HLA-B27. But many healthy people also have HLA-B27. Other conditions (that are not completely understood) must be present for ankylosing spondylitis to develop.

Genetic testing can be done by:

• Examining the chromosomes under a microscope. For more information, see the medical test Karyotyping.
• Examining the DNA from a cell. For DNA analysis, the DNA is removed from a sample of blood or tissue. Copies of the DNA are made using a chemical process. The DNA is then broken up into fragments that are separated using an electrical current. The pattern of the DNA fragments is examined for abnormalities. For more information, see the medical test DNA Fingerprinting.
• Examining markers, or antigens, that are found on the surface of body cells and tissues. These markers help the body's immune system know what is part of the body and what is foreign tissue. Each person has a particular set of antigens. Half of these come from the person's mother and half from the father. Except for identical twins, every person's antigen pattern is different. A person's antigen pattern is called his or her tissue type. For more information, see the medical test Tissue Typing.
• Examining the proteins or a certain type of genetic material in a cell called messenger RNA (mRNA). Messenger RNA is a type of genetic material that carries information from the DNA of a cell's chromosomes to the parts of the cell that produce proteins. Because certain genes on the DNA direct the cells to produce specific mRNA and proteins, the presence or absence of a protein or type of mRNA can provide information about whether a person has a gene that is functioning normally.

Genetic Testing
Results

The results of genetic testing depend on the type of test done.

• Some types of tests can provide a definite diagnosis of an inherited disease.
• Other types of tests can provide only a probability of having or developing a disorder in the future.
• Some tests can tell whether a person has the disease or is a carrier of the disease.
• Sometimes the severity of an inherited disease can be predicted using genetic testing.

Genetic tests can produce false-positive and false-negative results. It is important to clearly understand what the results mean. Genetic counseling can help make test results understandable.