GENE THERAPY
DISORDERS DUE TO SINGLE GENE DEFECTS
Autosomal dominant
In these disorders, only one
copy ofthe defective gene
needsto be present in order
to cause an abnormality. Each
child of an affected person
usually has a 1 in 2 chance of
inheriting the defective gene
and being affected, and a 1 in
2 chance of being unaffected.
Examples
• Achondroplasia
• Familial polyposis
• Hereditary spherocytosis
• Huntington’s disease
• Marfan syndrome
• Neurofibromatosis
• Polycystickidneydisease
(adult type)
• Tuberous sclerosis
Key
Defective gene
Normal gene
Autosomal recessive
In this group, the disorder occurs
when a pair of abnormal genes is
inherited. Usually, both parents
of an affected person are
unaffected carriers ofthe
defective gene. Each oftheir
children has a 1 in 4 chance of
being affected and a 2 in 4
chance of being a carrier.
Key
• Defective gene
Examples
• Albinism (oculocutaneous)
• Cystic fibrosis
• Friedreich’s ataxia
• Galactosaemia
• Hurler’s syndrome
• Phenylketonuria
• Sickle cell anaemia
• Tay-Sachs disease
Normal gene
U n a f f e c t e d p a r e n t ( c a r r i e r )
U n a f f e c t e d
U n a f f e c t e d
c h i l d
c h i l d ( c a r r ie r )
• •
• •
1 in 4 c h a n c e
1 in 4 c h a n c e
U n a f f e c t e d p a r e n t ( c a r r ie r )
U n a f f e c t e d
A f f e c t e d
c h i l d ( c a r r ie r )
c h i l d
• •
• •
1 in 4 c h a n c e
1 in 4 c h a n c e
G
X-linked recessive
These conditions are caused by
defects on the X chromosome. They
usually cause abnormalities only in
males, in whom the defect cannot
be masked bythe equivalent gene
on a second, normalXchromo-
some. Women may be carriers of
the defect. There isa 1 in 2 chance
oftheir sons having the disorder
and a 1 in 2 chance thattheir
daughters will be carriers.
Examples
• Christmasdisease
• Colour blindness(mosttypes)
• Fragile Xsyndrome
• G6PD deficiency
• Haemophilia
• Muscular dystrophy(Duchenne)
C a r r i e r m o t h e r
U n a f f e c t e d f a t h e r
X
X
©
©
U n a f f e c t e d
A ffe c t e d
U n a f f e c t e d
U n a f f e c t e d
b o y
b o y
g ir l
g ir l ( c a r r ie r )
X
Y
X ©
<X
Y
X ©
1 in 4 c h a n c e
1 in 4 c h a n c e
1 in 4 c h a n c e
1 in 4 c h a n c e
Key
o
Defective X chromosome
© Normal X chromosome
© Y chromosome
Alternatively, the positions of genes that
are close to one another on one chromo-
some
(and so are usually passed on
together) may be found by a process
called linkage analysis. The information is
shown as a diagram of a chromosome
with the gene locations marked on it.
general anaesthetic
See
anaesthesia, general.
generalized anxiety disorder
A psychiatric illness characterized by
chronic
and
persistent
apprehension
and tension that has no particular focus.
There may also be various physical
symptoms such as trembling, sweating,
lightheadedness, and irritability.
The condition can be treated with
psychotherapy, antidepressants,
or with
drugs such as
beta blockers
or
sedatives
that relieve symptoms but do not treat
the underlying condition. (See
anxiety
;
anxiety disorders.)
general paralysis ofthe insane
An outdated term describing the mental
and physical deterioration of untreated
or unsuccessfully treated
syphilis
.
general practice
The term used in the UK for the provi-
sion of personal medical care outside a
hospital setting. Doctors who provide it
are called general practitioners (GPs).
This form of health-care is now more
commonly known as
primarycare
.
generic drug
A
medicinal
drug
that
is
marketed
under its
official medical name
(its
generic
name)
rather
than
under
a
patented name (its brand name). Most
drugs are now prescribed under their
generic names.
gene therapy
A treatment in w hich copies of a normal
gene
are inserted into the
DNA
of a per-
son’s cells to help fight cancer or to
counter the effects of a faulty gene.
Gene therapy is most likely to be suc-
cessful in the treatment of autosomal
recessive diseases (see
Disorders due to
single gene defects
) ,
such as cystic fibro-
sis, w hich are caused by a pair of
defective genes. When a normal copy of
the gene is introduced into the body,
it
overrides the abnormal ones and
corrects the condition. It is less success-
ful,
however,
in
treating
autosomal
dominant disorders, such as Hunting-
ton’s disease, in w hich a faulty gene
335
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