Encyclopedia of Health and Medicine

variation 143
at highest risk for Tay-Sachs disease are those of
Ashkenazi Jewish heritage. A blood test became
available in 1985 to detect carriers of Tay-Sachs
disease, who do not themselves have the disease
but who have lower than normal amounts of hex-
A in their blood. GENETIC COUNSELING can help couples
who are carriers make informed decisions
about whether to have children. Assisted reproductive
technologies (ARTs) such as in vitro fertilization
allow genetic testing before implantation
so the couple knows the conceived child does not
carry the mutated genes.
See also ASSISTED REPRODUCTIVE TECHNOLOGY (ART);
ETHICAL ISSUES IN GENETICS AND MOLECULAR MEDICINE;
GENETIC CARRIER; GENETIC DISORDERS; INHERITANCE PAT-
TERNS.
telomere A structure of noncoding DNA (DNA
that does not convey genetic instruction) at each
end of a CHROMOSOME. Telomeres are essential for
chromosome duplication during cell division. They
function as handles to pull the chromatids (dividing
chromosomes) apart as the mother cell divides
into the two new daughter cells. The process of
cell division permanently destroys a tiny fragment
of the telomere, however. Eventually the telomere
becomes too short to participate in chromosome
duplication, and the cell stops dividing.
Researchers believe the shortening of telomeres is
key to APOPTOSIS, the apparently programmed
death of cells. In cancer cells the telomeres regenerate
after cell division, which researchers believe
is one of the factors that allows cancer cells to
grow uninhibited.
For further discussion of telomeres within the
context of the structures and functions of genetics,
please see the overview section “Genetics and
Molecular Medicine.”
See also CELL STRUCTURE AND FUNCTION; CEN-
TROMERE; CHROMATID; SENESCENCE.
translocation A chromosomal disorder in which
a fragment of a CHROMOSOME breaks from its original
chromosome and attaches itself to a different
chromosome. The fragment may exchange with
another fragment, may add itself to another chromosome,
or may become lost. Some translocations
are random and others occur in predictable patterns.
Translocations can be reciprocal, in which
chromosome fragments trade places with one
another. Such balanced translocations are common
and usually do not produce symptoms
because all the normal genetic material remains
within the GENOME.
A Robertsonian translocation occurs when the
long arms of two acrocentric chromosomes, in
which the CENTROMERE (waistlike indentation) is so
high on the chromosome that the upper arms
appear nonexistent and the upper arms contain
almost no genetic material. Robertsonian translocations
occur only among the five acrocentric
chromosomes, which are chromosomes 13, 14,
15, 21 and 22. Like reciprocal translocations,
Robertsonian translocations generally do not produce
harmful consequences because the genetic
material remains unadulterated despite the
translocation. Robertsonian translocations are
fairly common.
One reciprocal translocation that tends to produce
harmful health effects is the Philadelphia
chromosome, in which a segment of chromosome
9 and a segment of chromosome 22 exchange
places. Geneticists commonly find this translocation
in people who have chronic myeloid LEUKEMIA
(CML).
See also CELL STRUCTURE AND FUNCTION; CHROMO-
SOMAL DISORDERS; DNA.
trisomy 13
trisomy 18
trisomy 21
See PATAU’S SYNDROME.
See EDWARDS SYNDROME.
See DOWN SYNDROME.
variation The genetic differences among individuals.
There are trillions of possible GENE combinations
within the human GENOME. Except for
identical twins, no two people share exactly the
same GENOTYPE (genetic constitution). Though any
two individuals may have 99.9 percent of the
same DNA sequences and gene pairings, the 0.1
percent of pairings that differ accounts for the
endless details that make each individual unique.
The same genotype can have multiple expressions
(phenotypes) among individuals. The genotype
for EYE color, for example, can express itself
as blue eyes in one person and brown eyes in
another. Such variability exists for every gene