GENETICALLY MODIFIED FOODS
overrides a normal one. In these cases,
the abnormal gene must be removed
and is then replaced by a normal copy.
The copies of the normal gene can be
introduced into the body in various
ways. In one technique, a person’s white
blood cells are grown in a
containing the normal gene
and then reintroduced into the body,
method, currently being tried to treat
cystic fibrosis, involves packaging nor-
mal DNA in tiny liposomes, w hich are
added to an aerosol spray. The spray is
inhaled and the liposomes bind to the
walls of cells in the lung tissue and the
normal gene is activated.
Research is also being carried out to
assess the potential of gene therapy in
treating cancer. In certain forms of can-
cer, a gene called p53 (which controls
programmed cell death) is faulty, allow-
ing cells to multiply uncontrollably to
form a tumour. The introduction of a
normal p5 3 gene may prompt the death
of these abnormal cells.
genetically modified foods
material has been deliberately altered in
order to modify certain characteristics.
In genetic modification,
from one organism and inserted into
the DNA of another (the recipient).
The procedure is intended to be ben-
eficial; for example, transferred DNA
may enable recipient plants to be more
resistant to pests, w hich is likely to
increase crop yields. There may also be
disadvantages, however. Ecological and
health concerns about GM foods are the
subjects of ongoing debate.
The inherited instructions, contained in
that specify the activities of cells
and thereby the development and func-
tioning of the body. Each gene in a
contains the coded instruc-
tions for a cell to make a
a specific function in the body.
The DNA that makes up genes con-
sists of two long, intertwined strands,
each comprising a sequence made up of
four chemicals called nucleotide bases
These four bases are
adenine, thymine, cytosine, and guanine
(often abbreviated to A, T, C, and G).
They are joined in pairs (base-pairs),
thus linking the two strands of the DNA
molecule. The sequence of these bases
along the DNA strands makes up the
genetic code. During
(ribonucleic acid) is used to help
read this code and create the protein.
Medical guidance offered to people who
have a known risk of having a child with
such as cystic fibrosis,
or who are at increased risk of develop-
ing a genetic disorder themselves. The
counsellor w ill examine individual and
family medical histories and, in some
cases, arrange for tests such as
Genetic counselling enables people
to make informed decisions about their
future, particularly parenthood. If there
is a significant risk of a couple having
an affected child, doctors may be able to
offer pre-implantation genetic testing
in vitro fertilization
or antenatal diagnosis, to optim-
ize the chances of their having a healthy
child. If parents already have an affected
child, genetic counselling w ill provide
information on the outlook for that child.
Any disorder caused, wholly or partly,
by one or more faults in a person’s
(present at birth) or may become appar-
ent later in life. Many of them are
(shared by various people in the same
family). A child may, however, be born
with a genetic disorder when there is no
previous family history.
A genetic disorder can occur in two
different ways: one or both parents have
a defect in their own genetic material
that is then inherited by the child, or a
occurs during the formation of
The disorders fall into three broad cat-
gene defects, and multifactorial defects.
Chromosomal abnormalities involve a
child having an abnormal number of
or extra or missing bits of
chromosomes. Single gene defects are
rare, and are caused by one abnormal
gene or pair of genes. Multifactorial dis-
orders are thought to be due to the
effects of several genes combined with
SINGLE GENE DEFECTS
There are two main forms of single gene
defective gene is carried on one of the
sex chromosomes - almost always the X
chromosome. In autosomal disorders,
the defective gene is carried on one of
the other 44 chromosomes. These disor-
dominant and autosomal recessive dis-
Disorders due to single gene
(previous page) for examples.
Another, rare group of single gene
defects involves the
w hich exists outside the nuclei.
X-linked recessive disorders The
common type of sex-linked disorder, X-
linked recessive disorders are caused by
a defective gene on an X chromosome.
and colour blindness (see
colour vision deficiency
are this type.
Women have two X chromosomes;
men have only one, inherited from their
mothers. When a woman inherits one
defective gene, its effect is masked by
the normal gene on her other X chro-
mosome and she has no abnormality.
She is, however, capable of passing the
gene on to her children, and is called a
carrier. On average, carriers transmit the
defective gene to half their sons, who
are affected, and to half their daughters,
who become carriers in turn. When a
male inherits the defective gene from
his mother, he has no normal gene on a
second X chromosome to mask it, so he
displays the abnormality. Affected males
females. The males pass the defective
gene to none of their sons but to all of
their daughters, who become carriers.
autosomal dominant disorders
conditions, the defective gene is
in relation to the equivalent normal
gene, so only one copy needs to be pre-
sent in order to cause an abnormality.
People who have an autosomal dominant
disorder carry one normal copy and one
defective copy of the affected gene, and
have a 50 per cent chance of passing the
defective gene on to their children.
autosomal recessive disorders The defec-
recessive disorder is
to the normal gene, so two faulty copies
of the gene are required to cause an
abnormality. People who have the disor-
der carry two identical defective copies
of the gene, and are called homozy-
gotes. In most cases, both parents of an
affected individual are heterozygotes:
they carry one copy of the defective
gene and one copy of the normal gene.
In rare cases, a defect in a specific area of