groin. It can be direct, in which there is
a localized weakness in the abdominal
wall, or, in men, indirect, in which the
intestine protrudes through the inguinal
canal, the passage through which the
testes descend into the scrotum.
inhalationThe act of taking in breath
(see breathing). An inhalation is also a
substance, in the form of a gas, vapour,
powder, or aerosol, to be breathed in.
inhalerA device used for administering
a drug in powder or vapour form, used
mainly in the treatment of various res-
piratory disorders, including asthmaand
chronic bronchitis. Metered-dose inhalers
deliver a precise dose when the inhaler is
pressed. Drugs taken by inhalation in-
clude bronchodilatorsand corticosteroids.
inheritanceThe transmission of char-
acteristics and disorders from parents to
their children through the influence of
genes. Genes are the units of DNA(de-
oxyribonucleic acid) that are contained
in a person’s cells; DNA controls all
growth and functioning of the body.
Half of a person’s genes come from the
mother, half from the father.
Genes are organized into chromosomes
in the cell nucleus. Genes controlling
most characteristics come in pairs, 1 from
the father, the other from the mother.
Everyone has 22 pairs of chromosomes
(called autosomes) bearing these paired
genes, in addition to 2 sex chromosomes.
Females have 2 X chromosomes; males
have an X and a Y chromosome.
Most physical characteristics, many
disorders, and some mental abilities
and aspects of personality are inherited.
The inheritance of normal traits and
disorders can be divided into those
controlled by a single pair of genes on
the autosomal chromosomes (unifactorial
inheritance, such as eye colour); those
controlled by genes on the sex chromo-
somes (sex-linked inheritance, such as
haemophilia); and those controlled by
the combination of many genes(multi-
factorial inheritance, such as height).
Either of the pair of genes controlling
a trait may take any of several forms,
known as alleles. For example, the genes
controlling eye colour exist as 2 main
alleles, coding for blue and brown eye
colour. The brown allele is dominant over
blue in that it “masks” the blue allele,
which is called recessive to the brown
allele. Only 1 of the pair of genes con-
trolling a trait is passed to a child from
each parent. For example, someone with
the brown/blue combination for eye col-
our has a 50 per cent chance of passing
on the blue gene, and a 50 per cent
chance of passing on the brown gene, to
any child. This factor is combined with
the gene coming from the other parent,
according to dominant or recessive rela-
tionships, to determine the child’s eye
colour. Certain genetic disorders are also
inherited in a unifactorial manner (for ex-
ample, cystic fibrosisand achondroplasia).
Sex-linked inheritance depends on the
2 sex chromosomes, X and Y. The most
obvious example is gender. Male gender
is determined by genes on the Y chro-
mosome, which is present only in males.
Any faults in a male’s genes on the X
chromosome tend to be expressed out-
wardly because such a fault cannot be
masked by the presence of a normal gene
on a 2nd X chromosome (as it can in
females). Faults in the genes of the X
chromosome include those responsible
for colour vision deficiency, haemophilia,
and other sex-linked inherited disorders,
which almost exclusively affect males.
Multifactorial inheritance, along with
the effects of environment, may play a
part in causing certain disorders, such as
diabetes mellitusand neural tube defects.
inhibitionThe process of preventing any
mental or physical activity. Inhibition in
the brain and spinal cord is carried outINHALATION INHIBITION
I
INHALERMetered-dose
inhaler