Gene Testing

What is gene testing? How does it work?
Gene tests (also called DNA-based tests), the newest and most sophisticated of the techniques used to test for genetic disorders, involve direct examination of the DNA molecule itself. Other genetic tests include biochemical tests for such gene products as enzymes and other proteins and for microscopic examination of stained or fluorescent chromosomes. Genetic tests are used for several reasons, including:

• carrier screening, which involves identifying unaffected individuals who carry one copy of a gene for a disease that requires two copies for the disease to be expressed
  
  
• preimplantation genetic diagnosis (see the side bar, Screening Embryos for Disease)
  
  
• prenatal diagnostic testing
  
  
• newborn screening
  
  
• presymptomatic testing for predicting adult-onset disorders such as Huntington's disease
  
  
• presymptomatic testing for estimating the risk of developing adult-onset cancers and Alzheimer's disease
  
  
• confirmational diagnosis of a symptomatic individual
  
  
• forensic/identity testing 

In gene tests, scientists scan a patient's DNA sample for mutated sequences. A DNA sample can be obtained from any tissue, including blood. For some types of gene tests, researchers design short pieces of DNA called probes, whose sequences are complementary to the mutated sequences. These probes will seek their complement among the three billion base pairs of an individual's genome. If the mutated sequence is present in the patient's genome, the probe will bind to it and flag the mutation. Another type of DNA testing involves comparing the sequence of DNA bases in a patient's gene to a normal version of the gene. Cost of testing can range from hundreds to thousands of dollars, depending on the sizes of the genes and the numbers of mutations tested.