GENE MAPPING
used as bases for
topical
skin treatments,
particularly those that are used on the
face and scalp.
gemfibrozil
A drug that lowers the level of fats in
the blood. Gemfibrozil is usually given
to
people
with
hyperlipidaemia
after
dietary measures have failed to reduce
blood fat levels. It may cause nausea and
diarrhoea, and should not be taken by
people with liver disease.
gender identity
An individual’s inner feeling of male-
ness or femaleness. Gender identity is
not necessarily the same as biological
sex. It is fixed w ithin the first two to
three years of life and is reinforced dur-
ing puberty; once established, it cannot
usually be changed.
A m inority of people have persistent
feelings of discomfort about their sex-
ual identity. This
condition is called
gender dysphoria; in the most severe
cases, a person may feel that he or she is
the wrong sex (see
transsexualism
) .
gene
A particular area of
DNA
,
the material
within cells that governs the physical
characteristics, development, and func-
tioning of an individual. DNA is a very
long, chain-like structure that exists in
the
nucleus
of a cell. Each strand has
about 30,000 pairs of genes, arranged
on 23 pairs of
chromosomes
.
All body
cells (except egg and sperm cells) con-
tain identical sets of genes because they
are all derived, by a process of division,
from a single fertilized egg.
During growth and cell repair, cells
divide to form two identical new cells,
each containing a full set of genes (see
mitosis
).
Gametes
(cells involved in repro-
duction), however, undergo a different
form of division, called
meiosis
;
as a
result, they contain just one gene from
each pair, so that each parent con-
tributes half of the genetic material
used to form an offspring.
GENE FUNCTION
Each gene controls or influences a spe-
cific feature or process in the body.
Genes act by directing the manufacture
of specific
proteins
(see
What genes are and
what they do
,
overleaf). Many proteins are
involved in forming body structures, or
in regulating particular chemical pro-
cesses
(see
enzyme
) .
Certain proteins,
however, are made solely to influence
other genes by switching them “on” or
“off” .The genes that make these regula-
tory proteins are called control genes.
The whole complex process of devel-
opment and growth is regulated by the
sequential switching “on” and “ off” of
particular genes. The activities of control
genes also determine the specialization
of cells. W ithin any cell, some genes are
active and others are idle, according to
the cell’s particular function; for exam-
ple, nerve cells and liver cells w ill have
different sets of active and idle genes. If
the control genes are disrupted, however,
cells lose their specialist abilities and
multiply
out of control; this is the
probable mechanism by which cancers
develop (see
carcinogenesis
;
oncogenes
) .
GENE MUTATION
Occasionally, when new cells are formed,
a fault occurs in the copying process,
leading to a
mutation
(change). The m u-
tant gene is then passed on each time
the cell subsequently divides. Disorders
that result from such mutant genes are
known as
genetic disorders
.
ALLELES
The gene at any particular location on a
chromosome can exist in any of various
forms, called
alleles
.
If the effects of a
specific allele mask those of the allele
at the same location on its partner
chromosome, that allele is described as
dominant
;
the masked allele is
recessive
.
(See also
genetic code
;
inheritance
. )
gene mapping
A process by which the location of par-
ticular
genes
on each
chromosome
is
determined. In some cases, gene loca-
tion gene can be found by certain tests.
G
WHERE DO YOUR GENES COME FROM?
A person’s genes are inherited from
his or her parents. Half come from the
mother and half from the father via
the egg and sperm cells. Each parent
provides a different selection, or
“mix”, of his or her genes to each
child; this accounts for the marked
differences in appearance, health, and
personality among most brothers and
sisters. Everyone holds a copy of his
or her genes within each body cell.
Gene transmission
In this diagram, only eight genes are
shown - in reality, each cell in the
body contains about
30,000
genes.
Half of them come from the mother
and half from the father - thus a
quarter of the genes originate from
each of the four grandparents.
333
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