Encyclopedia of Health and Medicine

118 Genetics and Molecular Medicine
that alter its configuration. In TRANSLOCATION, fragments
of a chromosome break away and reattach
to other chromosomes or are lost, potentially
changing several chromosomes with unpredictable
and random results. Inversions, rings, duplications,
and deletions are other disorders of structure
involving fragments of the chromosome that
are fairly uncommon though tend to produce
symptoms when they occur. The types of symptoms
depend on the involved chromosome.
Inversions In a chromosomal inversion the
chromosome breaks in two or more locations,
then the segments rejoin with one or more segments
inverted (upside-down). Some genetic
material may be lost in the process, and the genes
are out of position. Inversions may or may not
cause symptoms, depending on the involved chromosome
and the degree of inversion.
Rings Chromosomal rings occur when the ends
of the chromosome are missing and the remaining
chromosome reshapes itself into a ring. The extent
and nature of symptoms depends on the involved
chromosome and the amount of missing genetic
material. A ring of chromosome 15, for example,
tends to produce symptoms such as facial anomalies
and growth deficiency.
Duplications and deletions In duplications and
deletions, the chromosome acquires (duplication)
or loses (deletion) fragments of its structure. The
severity of the consequences depends on the chromosome
involved and the extent of the altered
genetic material.
Symptoms and Diagnostic Path
The symptoms of chromosomal disorders vary
with the chromosome involved and the extent of
damage present. Because chromosomal disorders
tend to affect large segments of genetic material,
the resulting symptoms and syndromes are often
complex and affect multiple organs, structures,
functions, and systems. The diagnostic path may
include imaging procedures such as ULTRASOUND,
COMPUTED TOMOGRAPHY (CT) SCAN, and MAGNETIC RES-
ONANCE IMAGING (MRI) to evaluate structural anomalies
of internal organs. A KARYOTYPE (picture of
the chromosomes a in a cell) reveals overt chromosomal
problems, and molecular studies may be
necessary to unravel the circumstances of less
obvious chromosomal disruptions.
Treatment Options and Outlook
For nearly all chromosomal disorders, treatment
focuses on improving physical anomalies and
maintaining function to the extent possible. Children
born with chromosomal disorders often
require ongoing medical care and other kinds of
support. Outlook and QUALITY OF LIFE vary widely
even within the same syndrome.
Risk Factors and Preventive Measures
Most chromosomal disorders are random events
for which there are no preventive measures.
Parental age and exposure to teratogenic substances
(chemicals, drugs, or other materials that
disrupt embryonic or fetal development) are risk
factors for certain chromosomal disorders. Doctors
recommend all women of childbearing age who
could become pregnant, whether or not they
are planning PREGNANCY, take folic acid supplementation,
which appears to reduce the risk for
numerous congenital anomalies and perhaps
chromosomal damage.
See also CONGENITAL ANOMALY; GENETIC DISORDERS;
INHERITANCE PATTERNS.
chromosome A coiled DNA molecule within the
cell’s nucleus that carries an individual’s GENETIC
CODE. Most of the time the chromosome’s structure
is loose and indistinguishable. Only in the
stage of cell division immediately before the cell
divides (the metaphase) does the chromosome
draw itself into a compact, rodlike structure the
geneticist can see under a microscope after applying
a special dye to the cell that the chromosomes
absorb. It is this ability to absorb a colored dye
that gives the chromosome its name, which means
“colored body.”
Chromosome Complements
The nucleus of every diploid cell, also called a
SOMATIC CELL, contains the full complement of 46
chromosomes arranged in 23 pairs. One pair contains
the sex chromosomes that establish gender,
paired either as XX (female) or XY (male). The
other 22 pairs are autosomes. The haploid cells,
the gametes (spermatozoa and OVA), contain one
half the chromosome complement. When gametes
merge in CONCEPTION the diploid cell they form, the
ZYGOTE, acquires the full chromosomal comple-