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
culture
with a
retrovirus
containing the normal gene
and then reintroduced into the body,
where
they
multiply.
An
alternative
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
Food
or
food
constituents
that
are
derived
from
plants
whose
genetic
material has been deliberately altered in
order to modify certain characteristics.
In genetic modification,
DNA
is isolated
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.
genetic code
The inherited instructions, contained in
genes
,
that specify the activities of cells
and thereby the development and func-
tioning of the body. Each gene in a
chromosome
contains the coded instruc-
tions for a cell to make a
protein
that has
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
(see
nucleic acids
) .
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
protein synthesis,
RNA
(ribonucleic acid) is used to help
read this code and create the protein.
genetic counselling
Medical guidance offered to people who
have a known risk of having a child with
a
genetic disorder,
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
chromo-
some analysis
and
genetic probes.
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
with
in vitro fertilization
(see
embryo diag-
nosis),
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.
genetic disorders
Any disorder caused, wholly or partly,
by one or more faults in a person’s
DNA.
Genetic
disorders
may
be
congenital
(present at birth) or may become appar-
ent later in life. Many of them are
familial
(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
mutation
occurs during the formation of
the
ovum
or
sperm
cell.
The disorders fall into three broad cat-
egories:
chromosomal abnormalities,
single
gene defects, and multifactorial defects.
Chromosomal abnormalities involve a
child having an abnormal number of
whole
chromosomes
(as in
Down’s syn-
drome),
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
environmental influences.
SINGLE GENE DEFECTS
There are two main forms of single gene
defect.
In
sex-linked
disorders,
the
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-
ders
are
subdivided
into
autosomal
dominant and autosomal recessive dis-
orders. See
Disorders due to single gene
defects
(previous page) for examples.
Another, rare group of single gene
defects involves the
mitochondrial DNA
,
w hich exists outside the nuclei.
X-linked recessive disorders The
most
common type of sex-linked disorder, X-
linked recessive disorders are caused by
a defective gene on an X chromosome.
Haemophilia
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
therefore
greatly
outnumber
affected
females. The males pass the defective
gene to none of their sons but to all of
their daughters, who become carriers.
autosomal dominant disorders
In these
conditions, the defective gene is
domi-
nant
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
are termed
heterozygotes
.
Affected people
have a 50 per cent chance of passing the
defective gene on to their children.
autosomal recessive disorders The defec-
tive
gene
that
causes
an
autosomal
recessive disorder is
recessive
in relation
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.
MITOCHONDRIAL DISORDERS
In rare cases, a defect in a specific area of
mitochondrial
DNA
causes
diseases.
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