DOMINANT
In the tropics, walking barefoot on soil
that is contaminated with dog faeces
can lead to dog
hookworm infestation
.
Bites from dog
fleas
are an occasional
nuisance.
Ticks
and
mites
from dogs,
including a canine version of the
sca-
bies
mite, are other common problems.
The fungi that cause
tinea
infections in
D
n
be caught by humans.
Some people become allergic to ani-
mal
dander
(tiny scales from fur or
skin). They may, for example, suffer
from attacks of
asthma
or
urticaria
(net-
tle rash) when a dog is in the house.
(See also
zoonoses
. )
dominant
A term used in
genetics
to describe the
characteristics of a
gene
.
Many charac-
teristics are determined by a single
pair of genes, one of each pair being
inherited from each parent. The dom i-
nant gene of the pair overrides its
equivalent
recessive
gene, allowing it to
be expressed.
For example, the gene for brown eye
colour is dominant, so that if a child
inherits the gene for brown eyes from
one parent and the gene for blue eyes
from the other parent, he or she w ill
have brown eyes.
Some genetic disorders are deter-
mined by a dominant abnormal gene
that overrides the effect of an equiva-
lent
normal
gene
that
is
recessive.
Examples
of such
disorders include
Marfan syndrome
and
Huntington’s dis-
ease
.
In this case, a child w ill have the
disease if he or she inherits the gene
from one or both parents.
dominant characteristic
An inherited characteristic (external or
internal physical feature) that appears
in offspring even if they have inherited
the
gene
for it from only one of their
parents. See
dominant
.
dominant gene
See
dominant
.
dominant trait
An inherited
trait
(a characteristic or
condition)
that appears when just a
single copy of the
gene
for that trait
is present, as opposed to a
recessive
trait, w hich only appears when two
copies
of the
appropriate
gene
are
present.
Dominant
genes always mask
the presence of recessive genes, and
dominant traits can be inherited from
one parent alone.
domperidone
An
antiemetic drug
used to relieve nau-
sea and vomiting associated with some
gastrointestinal
disorders
or
during
treatment with certain drugs or
radio-
therapy.
Adverse
effects may include
breast
enlargement and secretion of
m ilk from the breast.
donepezil
An
acetylcholinesterase inhibitor
that is
used to treat m ild to moderate
dementia
due to
Alzheimer’s disease.
donor
A person who provides blood for
trans-
fusion
;
body
tissues
or
organs
for
transplantation
; or eggs or semen for
artificial insemination.The
organs that are
most frequently donated are kidneys,
corneas, heart, lungs, liver, and pan-
creas. Certain organs can be donated
during a person’s lifetime; some are
only used following
brain death
.
All donors should be free of cancer,
serious infection (such as hepatitis B),
and should not carry
HIV.
Organs for
transplantation must be removed within
a few hours of brain death, and before
or immediately after the heartbeat has
stopped. In some kidney transplants, the
kidney is provided by a living donor,
usually a relative whose tissues match
well on the basis of
tissue-typing.
Tests
are performed to ensure that both kid-
neys are healthy before one is removed.
Suitable related donors may also pro-
vide bone marrow for transplantation
and sometimes skin for grafting. (See
also
blood donation
;
bone marrow trans-
plant; organ donation; transplant surgery.)
Doose syndrome
A rare, inherited form of
epilepsy.
The
condition is typified by muscular jerk-
ing, especially symmetrical jerking of
the arms and shoulders and, in severe
cases, twitching of the facial muscles.
In addition, the affected person may
fall to the ground and briefly lose con-
sciousness.
Doose
syndrome
can be
treated with
anticonvulsant drugs.
dopa
An amino acid that the body uses to
form
dopamine,
an important neuro-
transmitter. Dopa is also formed as an
intermediate
stage
when
the
body
makes
epinephrine, norepinephrine,
and
melanin. Levodopa,
a form of dopa, is
used to correct dopamine deficiency in
people who have
Parkinson’s disease.
dopa-decarboxylase inhibitors
Drugs used in the treatment of the
movement disorder
Parkinson's disease
.
The two principal dopa-decarboxylase
inhibitors, co-beneldopa and co-carel-
dopa, are a combination of
levodopa
and benserazide and levodopa and car-
bidopa, respectively. These drugs stop
levodopa from being activated except
w ithin the brain, w hich reduces the
incidence of common side effects such
as nausea and vomiting.
dopamine
A
neurotransmitter
(chemical released
from nerve endings) found in the brain
and around some blood vessels. It helps
to control body movements: a defi-
ciency of dopamine in the
basal ganglia
(groups of nerve
cells deep
in the
brain) causes
Parkinson’s disease
.
Synthetic dopamine is injected as an
emergency treatment for shock caused
by a
myocardial infarction
(heart attack)
or
septicaemia
(blood infection) and as
a treatment for severe
heart failure
.
Doppler echocardiography
An investigation in w hich blood flow
through the
heart
is assessed by the use
of
ultrasound
(high-frequency sound
waves). An ultrasound transducer is
moved over the chest in the area of the
heart. The waves emitted by the trans-
ducer are reflected by blood, forming
echoes that are recorded to give a pic-
ture of blood flow. This allows the
direction and speed of the blood flow
to be measured.
Doppler
echocardiography
can
be
used to investigate the presence of some
conditions of the heart and circulation,
including
abnormalities
of the heart
valves
(see
valvular heart disease
) .
The
procedure is painless and takes between
15 and 30 minutes to perform.
Doppler effect
A change in the frequency with w hich
sound waves from a given source reach
an observer when the source is in rapid
motion with respect to the observer.
Approaching sounds seem higher in
pitch (frequency) than sounds that are
moving away. This is due to the fact that
the wavelengths of sounds from an
approaching source are progressively
foreshortened, whereas those from a
receding source are stretched.
The Doppler effect is used in various
medical
ultrasound scanning
techniques
(see
Doppler ultrasonography
) .
242
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