See also ETHICAL ISSUES IN GENETICS AND MOLECU-
LAR MEDICINE; MOLECULARLY TARGETED THERAPIES;
RECOMBINANT DNA; SOMATIC CELL.
genetic carrier An individual whose GENOTYPE
contains a recessive GENE MUTATIONcapable of caus-
ing a genetic disorder though the individual does
not have or show symptoms of the disorder the
mutation causes. Typically a genetic CARRIERhas
one “good” gene and one mutated gene. A genetic
carrier may pass on the mutated gene to his or her
biological children, though typically two mutated
genes are necessary for the child to acquire the
genetic disorder.
See also CELL FUNCTION AND STRUCTURE; GENETIC
DISORDERS; GENETIC SCREENING; GENETIC TESTING;
INHERITANCE PATTERNS.
genetic code The organizations of nucleotides
(DNAsequences) within messenger RNAinto triplet
structures called trinucleotides or codons. The
codons convey the order of amino acids for the
structure of the protein for which a particular GENE
encodes. The process of protein synthesis takes
place in the ribosomes in the cell cytoplasm; the
messenger RNA carries the encoding to the ribo-
somes.
See also CELL STRUCTURE AND FUNCTION; CHROMO-
SOME; GENOME; GENOTYPE; NUCLEOTIDE; PHENOTYPE.
genetic counseling A multidisciplinary approach
to evaluating the risk for specific genetic diseases
or CHROMOSOMAL DISORDERS. Doctors often recom-
mend genetic counseling for people who have
strong family history for GENETIC DISORDERSsuch as
TAY-SACHS DISEASEor HUNTINGTON’S DISEASEand older
women who are or who are planning to become
pregnant. Obstetricians also will recommend
genetic counseling for couples who receive posi-
tive results from prenatal genetic tests so they may
make informed decisions and conduct appropriate
planning for pregnancies in which there are
genetic or chromosomal abnormalities.
A genetic counseling team may include a clinical
geneticist (physician specializing in genetics and
molecular medicine), genetic psychologist, and a
social worker. The intent of genetic counseling is to
evaluate family history, results of genetic tests, and
current health circumstances to provide individu-
als, couples, or families with as much information
as possible about whatever genetic risks or situa-
tions they are facing and the options for addressing
them.
The role of the genetic counseling team is to
answer questions and provide support for the deci-
sions individuals and couples make. Major health-
care centers and high-risk obstetrical practice
groups generally have genetic counseling practi-
tioners and services available.
See also ETHICAL ISSUES IN GENETICS AND MOLECU-
LAR MEDICINE; FAMILY MEDICAL PEDIGREE; GENETIC
SCREENING; GENETIC TESTING; PREGNANCY.genetic disorders A collective classification for
syndromes, diseases, and congenital anomalies
that result from alterations of the genes and chro-
mosomes. There are four general categories of
genetic disorders, each relating to the way in
which the alterations manifest.Chromosomal Disorders
The normal human GENOMEcontains 23 paired
chromosomes. CHROMOSOMAL DISORDERS occur
when there are disruptions in these pairings in
which there is an extra CHROMOSOME(trisomy) or a
missing chromosome (monosomy). Chromosomal
disorders also occur when large segments of a
chromosome are damaged or missing (deletion
syndromes). Less often, a broken segment of a
chromosome attaches itself to another chromo-
some (TRANSLOCATION). Common chromosomal dis-
orders include DOWN SYNDROME, EDWARDS
SYNDROME, PATAU’S SYNDROME, TURNER SYNDROME,
and KLINEFELTER’S SYNDROME.Single-Gene Disorders
Each GENEencodes, or directs, a specific action
within the body. Single-gene disorders, in which a
MUTATIONof a gene or set of genes causes malfunc-
tions of the proteins that carry out the gene’s
instructions, cause conditions of faulty encoding,
either because the gene’s protein messenger is
missing or incomplete. The disorders that result
often become more severe over time as the mal-
function continues to repeat itself. CYSTIC FIBROSIS,
SICKLE CELL DISEASE, Duchenne’sMUSCULAR DYSTRO-
PHY, HUNTINGTON’S DISEASE, and MARFAN SYNDROME
are single-gene disorders.128 Genetics and Molecular Medicine