1104
UNIT 5
Continuity
29
Advances in diagnosing human genetic diseases have been fol-
lowed quickly by new applications of “gene therapy” to allevi-
ate or even cure the disorders, particularly in cases traced to
a single defective gene or protein. Indeed, genetic engineering
has the potential to replace a defective gene with a normal
version.
One approach (used successfully in treatment of severe com-
bined immunodeficiency disorders) is to “infect” the defective
cells with a virus into which a functional gene has been inserted.
Another is to inject the “corrected” DNA directly into the pa-
tient’s cells. Such therapies have had mixed results in treating
cystic fibrosis and muscular dystrophy. However, genetic en-
gineering processes are prohibitively expensive and raise some
thorny ethical, religious, and societal questions: Who will pay?
Who determines who will be treated with the new therapies?
Are we playing God?
can be done almost immediately on the rapidly dividing chori-
onic cells, much earlier than in amniocentesis.
Both of these procedures are invasive, and they carry with
them an inherent risk to both fetus and mother. (For example,
increased fetal risk of finger and toe defects is linked to CVS.)
Tese tests are routinely ordered for pregnant women over 35
(because of the higher risk of Down syndrome), but they are
performed on younger women when the probability of finding
a severe fetal disorder is greater than the probability of doing
harm during the procedure. If a serious genetic or congenital
defect is detected in the developing fetus, the parents must de-
cide whether or not to continue the pregnancy.
Human Gene Therapy
As indicated earlier, the Human Genome Project has opened
an exciting world to scientists investigating gene expression.
(a)
Amniocentesis
Centrifugation
Cervix
Fluid
Fetal
cells
Fetal
cells
Placenta
Chorionic villi
Suction tube
inserted through
cervix
Fetus
Several
weeks
Several
hours
Biochemical
tests
Uterus
Placenta
Fetus
Amniotic
fluid
withdrawn
(b)
Chorionic villus sampling (CVS)
Karyotyping
of
chromosomes
of cultured cells
1
A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
2
Biochemical tests (tests
for genetic disorders) can
be performed immediately
on the amniotic fluid or
later on the cultured cells.
3
Fetal cells must be
cultured for several weeks
to obtain sufficient
numbers for karyotyping
(testing for chromosomal
disorders).
2
Karyotyping and biochemical
tests can be performed on the
fetal cells immediately, providing
results within a day or so.
1
A sample of
chorionic villus tissue
can be taken as early as
the 8th to 10th week of
pregnancy.
Figure 29.7
Fetal testing—amniocentesis and chorionic villus sampling.
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