Cambridge International A Level Biology Revision Guide

Chapter 19: Genetic technology
abdominal skin surface. Generally, this position is away
from the fetus, umbilical cord and placenta.
Chorionic villus sampling can be carried out between
10 and 13 weeks of pregnancy, so it allows parents to get an
earlier warning of any genetic abnormalities in the fetus
than is possible with amniocentesis.
A small sample of part of the placenta called the
chorion is removed by a needle. The needle is narrow (less
than 0.8 mm in diameter). The procedure is monitored by
ultrasound scanning (Figure 19.17).
Like amniocentesis, chorionic villus sampling has a
small increased risk of miscarriage. It has been estimated
that miscarriage rate is increased by about 1–2%. (The
typical miscarriage rate for all women is about 2–3% at
10 to 12 weeks of pregnancy.) This is a slightly greater risk
than for amniocentesis, but, before 15 weeks, chorionic
villus sampling is probably less risky than amniocentesis.
Thalassaemia is a blood disease similar to sickle cell
anaemia. It used to be a common genetic disease in
countries around the Mediterranean: Cyprus, Greece and
southern Italy. The incidence of the disease has decreased
significantly over recent years as a result of genetic
screening and giving advice to couples who are identified
as carriers of the mutant allele. Testing during pregnancy
was also carried out. When a fetus was identified as having
inherited the disorder, couples received advice about the
possibility of terminating the pregnancy. Terminating
pregnancies for a medical reason, rather than for any
other, is known as therapeutic abortion.
Huntington’s disease is a late-onset disease – symptoms
do not usually appear until middle age, by which time
people have usually already had children. There is no
cure for this disease and the treatments available can
only alleviate the symptoms. People in families with
Huntington’s face a dilemma: should they have the genetic
test to find out whether or not they have the dominant
allele for the disease? This also poses ethical dilemmas:
would you rather be told that you are at high risk of
developing this disease, even though nothing can be done
about it, or live with the uncertainty of not knowing? Is
it a good idea to have this information before you start
a family? Decisions are made even more difficult by the
possibility that a person with the dominant allele for
Huntington’s may live their whole life completely free of
the disorder as it sometimes does not develop.
477
QUESTION
19.9 Explain the advantages of genetic screening.
Figure 19.17 In chorionic villus sampling, an ultrasound
scanner is used to guide the needle to the placenta to remove
a small sample of the fetal chorionic villi which are embedded
in the placenta. A small sample of the fetal blood is removed
for analysis.
Gene therapy
Gene technology and our rapidly increasing knowledge
of the positions of particular genes on our chromosomes
have given us the opportunity to identify many genes
that are responsible for genetic disorders such as sickle
cell anaemia (page 388) and cystic fibrosis. When genetic
engineering really began to get going in the 1990s, it was
envisaged that it would not be long before gene technology
could cure these disorders by inserting ‘normal’ alleles
of these genes into the cells. This process is called gene
therapy. But gene therapy has proved to be far more
difficult than was originally thought. The problems lie in
getting normal alleles of the genes into a person’s cells and
then making them work properly when they get there.