Biomedical Engineering Reference
In-Depth Information
Tests for Genetic Conditions
Tests for inherited conditions that are apparent at birth have been
used for many years to help physicians and families choose appro-
priate treatments. Other inherited conditions do not show up
early in life, but DNA-sequence differences allow detection before
symptoms appear. The question of using the tests usually comes
up when a family member is diagnosed with one of these condi-
tions. The
genetic tests
may allow family members, particularly the
children of those diagnosed, to know if they are at high risk. Genetic
tests are available for some families that have a history of breast
and ovarian cancer, for a neurologic condition called Huntington
disease; and for a disease of the colon called
familial adenomatous
polyposis
, in which many small growths that can turn into cancer
develop in the colon. The use of these tests is complicated, both
medically and psychologically.
What Is PCR?
DNA tests rely on two basics facts, 1) the distance a molecule of DNA moves in a thin slap
or small column of gelatin-like material when an electric current is passed through it
depends on the length of the piece of DNA, a process called electrophoresis and 2) single
strands of DNA will bind to each other if they have complementary base sequences: An
A will pair with a T, a T with an A, a G with a C, and a C with a G.
PCR, or polymerase chain reaction, is used to make many copies of one or more
important stretches of the DNA extracted from the sample. To see if the PCR product
contains a specific, inherited sequence for medical or forensic purposes, the PCR
product is heated to separate the two strands of DNA and treated with a probe, a short
piece of DNA complementary to the target sequence and tagged with a dye. If the spot
changes color, then the targeted sequence is present. The target may be a mutant form
of a breast cancer susceptibility gene, a tissue transplant compatibility gene, or a gene
for a blood enzyme known to be different in different people.
PCR is also used with specific probes to detect the presence of a virus or other infec-
tion in the blood or tissue and to determine how many virus particles are present.
Many of these tests are automated so that a technician need only place the extracted
DNA in a small tube, instruct the machine which tests to run, push a button, and wait
for the printout.
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