chemicals called nucleotide bases.
These 4 bases are adenine, thymine,
cytosine, and guanine (often abbreviat-
ed to A, T, C, and G). The sequence of
these bases along the DNA strands
makes up the genetic code.
genetic counsellingMedical guidance
offered to prospective parents based on
an assessment of the probability that a
future child would be affected by a genet-
ic disorder. This assessment is made
from individualand family medical his-
tories and, in some cases, the results of
tests such as chromosome analysisand
genetic probes. Genetic counselling ena-
bles people to make informeddecisions
about parenthood, which may involve
the use of techniques such as selective
terminationof pregnancy or in vitro fer-
tilization to optimize the chances of
having a healthy child.
genetic disordersAny disorder caused,
wholly or partly, by one or more faults
in a person’s DNA. Genetic disorders
may be congenitalor may become ap-
parent later in life. Many of them are
familial. However, a child may be born
with a genetic disorder when there is no
previous family history.
A genetic disorder can occur in 2 ways:
one or both parents have a defect in
their own genetic material which is then
inherited, or a mutationoccurs during
the formation of the eggor spermcell.
Genetic disorders fall into three broad
categories: chromosomal abnormalities,
unifactorial defects, and multifactorial
defects. In the first, a child is born with
an abnormal number of whole chromo-
somes(as in Down’s syndrome), or extra
or missing bits of chromosomes.
Unifactorial disorders are rare, and are
caused by a single defective gene or pair
of genes. They may be sex-linked (with
the defective gene carried on one of the
sex chromosomes) or autosomal (with
the defective gene carried on one of the
other 44 chromosomes). X-linked reces-
sive disorders are the most common
type of sex-linked disorder. In these
conditions, the defective gene is on the
X chromosome. Women have 2 X chro-
mosomes in their cells; men have only
1, inherited from their mothers. When a
woman inherits 1 defective gene, itseffect is masked by the normal gene on
her other X chromosome and she has no
outward abnormality. She is still capa-
ble of passing the gene onto her
children, and is called a carrier. Carrier
females transmit the defective gene on
average 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, there is no normal gene on
a second X chromosome to mask it, and
he displays the abnormality. Affected
malestherefore far outnumber affected
females. They pass the defective gene to
none of their sons but to all of their
daughters, who become carriers. Haemo-
philiais a disorder of this type.
Multifactorial disorders such as asthma
are caused by the additive effects of sev-
eral genes, along with environmental
factors; the pattern of inheritance is
less straightforward.
genetic engineeringA branch of gen-
etics concerned with the alteration of
the genetic material of an organism to
produce a desired change in the organ-
ism’s characteristics. In practice, genetic
engineering has mainly been used to
mass-produce a variety of substances
that are useful in medicine. A gene
responsible for making a useful protein
is identified and insertedinto another
cell (most often a bacterium or a yeast)
that reproduces rapidly to form a colony
of cells containing the gene. This colony
produces the protein in large amounts.
Some human hormones (notably insulin
and growth hormone) and proteins such
as factor VIII (used to treat haemophilia)
are made in this way.
genetic fingerprintingA technique
that can be used to demonstrate rela-
tionships between people (for example
in paternity testing) or in forensic inves-
tigations to identify a criminal suspect.
DNA contains a genetic codethat is
unique to each individual (except for
identical twins). DNA can be extracted
from a sample of a person’s body fluids,
such as blood and semen, and analysed
to reveal differences in the code: the
“genetic fingerprint”.
genetic probeA specific fragment of
DNAthat is used in laboratory tests toGENETIC COUNSELLING GENETIC PROBEG