Encyclopedia of Health and Medicine

130 Genetics and Molecular Medicine
ease, and type 2 DIABETES. An individual’s GENOTYPE
establishes genetic vulnerability through mechanisms
researchers do not fully understand.
These genetic elements influence the effects of
environmental factors such as cigarette smoking,
physical activity and inactivity, exposure to chemical
toxins, ALCOHOL consumption, and nutrition
(lack of certain NUTRIENTS or excesses of other
nutrients) in the development of disease
processes. Once the disease becomes established,
its genetic underpinnings may allow more rapid
progression of damage or severity of symptoms.
Unlike GENETIC DISORDERS that encode for specific
disturbances of structures and functions that
inevitably produce a disease state (such as DOWN
SYNDROME), genetic predisposition for a condition
does not make that condition certain. Many doctors
and researchers believe knowing of genetic predispositions
gives an individual the opportunity to
engage in lifestyle modifications to prevent health
problems from developing.
See also LIFESTYLE AND HEALTH.
genetic screening Procedures that indicate
whether an individual has the potential to have a
genetic disorder. Among the most commonly performed
genetic screening procedures in the United
States are prenatal ULTRASOUND and maternal BLOOD
levels of multiple biomarkers, such as ALPHA FETO-
PROTEIN (AFP), during PREGNANCY. These procedures
may present suspicious findings though are not
precise enough to allow diagnosis. Other genetic
screening procedures are those that test for conditions
that occur in the general population and
have significant consequences when undetected
and untreated. For example, hospitals in the
United States conduct routine newborn testing for
PHENYLKETONURIA (PKU), an inherited metabolic disorder
that results in severe intellectual impairment
without treatment at the time of birth.
The findings of genetic screening, positive or
negative, can have a margin of error for falsenegative
as well as false-positive results. However,
doctors use genetic screening when factors of
increased risk for GENETIC DISORDERS, such as maternal
age in pregnancy or family history, exist. The
doctor may conduct further GENETIC TESTING and
diagnostic testing when the overall health picture
points to an increased risk for genetic disorders,
even when the findings of genetic screening procedures
appear normal. Genetic screening procedures
are minimally invasive and typically present
no risk to the mother or the fetus in prenatal
screening or to the individual in screening conducted
following birth or in adults.
See also AMNIOCENTESIS; AUTOSOMAL TRISOMY;
CHORIONIC VILLI SAMPLING (CVS); CHROMOSOMAL DISOR-
DERS; ETHICS IN GENETICS AND MOLECULAR MEDICINE;
FAMILY PLANNING; FOLIC ACID SUPPLEMENTATION;
GENETIC TESTING; NEURAL TUBE DEFECTS.
genetic testing Methods and procedures to
determine the presence of a genetic disorder. The
KARYOTYPE, which uses microphotographs to examine
and depict an individual’s chromosomes, is
one of the more common methods of genetic testing.
Other methods include cytogenic analysis,
AMNIOCENTESIS, and CHORIONIC VILLI SAMPLING (CVS).
Some genetic testing methods are highly sophisticated
and require specialized equipment and
knowledge available only in research centers.
Other methods, such as CVS, have become fairly
commonplace.
Diagnostic genetic testing can identify the cause
of symptoms resulting from GENE mutations and
CHROMOSOMAL DISORDERS. This knowledge can be
helpful when there are treatments and treatment
choices for the resulting conditions, and in FAMILY
PLANNING decisions. The matter of genetic testing to
screen for the presence of GENETIC DISORDERS, particularly
in people who do not have symptoms or
apparent increased risk for conditions of genetic
origin, remains an issue of intense ethical debate.
Some such practices, such as testing for PHENYLKE-
TONURIA (PKU) in newborns, have become standard
in the United States. Others, such as those for the
so-called CANCER genes (BRCA-1, BRCA-2, CA-125, and
others), often raise more questions than answers
because the consequence of having such genes
remains uncertain.
Even when the outcome is certain, the knowledge
of the genetic disorder may have little therapeutic
value yet create distress for the individual.
This is currently a significant issue with genetic
testing for HUNTINGTON’S DISEASE, for example, a
fatal neurodegenerative disorder for which there
is no treatment or cure. People who carry the
gene MUTATION for Huntington’s disease are certain