Genetic tests can be used to look for possible
predisposition to disease as well as to confirm a suspected mutation in an
individual or family.
The most widespread type of genetic testing is
newborn screening. Each year in the United States, four million
newborn infants have blood samples tested for abnormal or missing gene
products. Some tests look for abnormal arrangements of the chemical bases
in the gene itself, while other tests detect
inborn errors of metabolism (for example,
phenylketonuria) by verifying the absence of a protein that the cell
needs to function normally.
Carrier testing can be used to help couples to learn if they carry -
and thus risk passing to their children - a recessive allele for inherited
disorders such as cystic fibrosis, sickle-cell anemia, or
Tay-Sachs disease (a lethal disorder of lipid metabolism). Genetic
tests - biochemical, chromosomal, and DNA-based - also are widely
available for the
prenatal diagnosis of conditions such as Down syndrome.
In clinical research programs, doctors make use of genetic tests to
identify telltale DNA changes in cancer or precancer cells. Such tests can
be helpful in several areas: early detection (familial
adenomatous polyposis genes prompt close surveillance for colon
cancer); diagnosis (different types of
leukemia can be distinguished); prognosis (the product of a mutated
p53
tumor-suppressor gene flags cancers that are likely to grow
aggressively); and treatment (antibodies block a gene product that
promotes the growth of breast cancer).
Much of the current excitement in gene testing, however, centers on
predictive gene testing: tests that identify people who are at risk of
getting a disease, before any symptoms appear. Tests are already available
in research programs for some two dozen such diseases, and as more disease
genes are discovered, more gene tests can be expected.
