ease, and type 2 DIABETES. An individual’s GENOTYPE
establishes genetic vulnerability through mecha-
nisms researchers do not fully understand.
These genetic elements influence the effects of
environmental factors such as cigarette smoking,
physical activity and inactivity, exposure to chemi-
cal toxins, ALCOHOLconsumption, 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 DISORDERSthat 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 predis-
positions 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 per-
formed genetic screening procedures in the United
States are prenatal ULTRASOUNDand 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 condi-
tions 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 dis-
order that results in severe intellectual impair-
ment without treatment at the time of birth.
The findings of genetic screening, positive or
negative, can have a margin of error for false-
negative as well as false-positive results. However,
doctors use genetic screening when factors of
increased risk for GENETIC DISORDERS, such as mater-
nal age in pregnancy or family history, exist. The
doctor may conduct furtherGENETIC TESTING and
diagnostic testing when the overall health picture
points to an increased risk for genetic disorders,
even when the findings of genetic screening pro-
cedures appear normal. Genetic screening proce-
dures are minimally invasive and typically present
no risk to the mother or the fetus in prenatal
screening or to the individual in screening con-
ducted 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 exam-
ine and depict an individual’s chromosomes, is
one of the more common methods of genetic test-
ing. Other methods include cytogenic analysis,
AMNIOCENTESIS, andCHORIONIC VILLI SAMPLING(CVS).
Some genetic testing methods are highly sophisti-
cated 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 GENEmutations and
CHROMOSOMAL DISORDERS. This knowledge can be
helpful when there are treatments and treatment
choices for the resulting conditions, and in FAMILY
PLANNINGdecisions. The matter of genetic testing to
screen for the presence of GENETIC DISORDERS, par-
ticularly 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 CANCERgenes (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 knowl-
edge of the genetic disorder may have little thera-
peutic 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 MUTATIONfor Huntington’s disease are certain130 Genetics and Molecular Medicine