148 ■ CHAPTER 08 Chromosomes and Human GeneticsGENETICS
Prenatal Genetic Screening
H
ow is the baby? This is one of the first questions we
ask after a child is born. Usually everything is fine,
but sometimes, as with Felix and Zoe, the answer can be
devastating. Today, some parents choose to have prenatal
genetic screening tests performed to check their baby’s
health before birth.
In amniocentesis, a needle is inserted through the
abdomen into the uterus to extract a small amount of
amniotic fluid from the pregnancy sac that surrounds the
fetus. This fluid contains fetal cells (often sloughed-off
skin cells) that can be tested for genetic disorders. Another
method is chorionic (kohr-ee-AH-nik) villus sampling (CVS),
in which a physician uses ultrasound to guide a narrow,
flexible tube through a woman’s vagina and into her uterus,
where the tip of the tube is placed next to the villi, a cluster
of cells that attaches the pregnancy sac to the wall of the
uterus. Cells are removed from the villi by gentle suction
and then tested for genetic disorders.
Risks associated with amniocentesis and CVS—including
vaginal cramping, miscarriage, and premature birth—have
declined dramatically in recent years because of advances
in technology and more extensive training. Recent studies
suggest that the risk of miscarriage after CVS and
amniocentesis is essentially the same: about 0.06 percent.
The tests are widely used by parents who know they face
an increased chance of giving birth to a baby with a genetic
disorder. Older parents, for example, might want to test for
Down syndrome, since the risk of that condition increases
with the age of the mother and perhaps the father too. A
couple in which one parent carries an allele for a dominant
genetic disorder (such as Huntington disease), or both
parents are carriers for a recessive genetic disorder (such
as cystic fibrosis), might also choose prenatal genetic
screening.
Couples who elect to have such tests performed have
only two choices if their fears are confirmed: they can abortthe fetus, or they can give birth to a child with a genetic
disorder. Prior to conception, however, couples at risk
of having a child with a genetic disorder have options to
minimize that risk.● (^) If they are willing and can afford the procedure, a couple
can choose to have a child by in vitro fertilization (IVF), in
which an egg is fertilized by a sperm in a petri dish, after
which one or more embryos are implanted into the moth-
er’s uterus.
● (^) During preimplantation genetic diagnosis (PGD), one or
two cells are removed from the developing embryo in
the dish, usually 3 days after fertilization. The cell or
cells removed from the embryo are tested for genetic
disorders. Finally, one or more embryos that are free of
disorders are implanted into the mother’s uterus, and
the rest of the embryos, including those with genetic
disorders, are frozen. Typically, parents who opt for
PGD either have a serious genetic disorder or carry
alleles for one.
Like all other genetic screening methods, the use of
PGD raises ethical issues. People who support PGD feel
that amniocentesis and CVS provide parents with a bleak
set of moral choices: if the fetus has a serious genetic
disorder, the parents can either abort the fetus or allow
the birth of a child who will live a life that may be short and
full of suffering. In their view, discarding an embryo at the
4- to 12-cell stage is morally preferable to aborting a well-
developed fetus, or to giving birth to a child that will suffer
the devastating effects of a serious genetic disorder. Those
opposed to PGD agree that the moral choices are bleak, but
they argue that once fertilization has occurred, a new life
has formed and it is immoral to end that life, even at the
4- to 12-cell stage. What do you think?
gene and thus could not produce the important
WAS protein. These mice had some of the same
symptoms that Felix had, including a reduction
in the number of blood and immune system
cells. Then the researchers used a virus to insert
a healthy copy of the WA S gene into the mice’s
blood cells, where they hoped it would produce
the WAS protein. Months later, Klein was
thrilled to find that the mouse cells were express-
ing healthy WAS protein, and that mature blood
and stem cells were being produced in normal
quantities.
that they might be able to treat WAS using gene
therapy, a technique for correcting defective
genes responsible for disease development. Gene
therapy is a type of genetic engineering, the
permanent introduction of one or more genes
into a cell, tissue, or organism. Klein hoped that
by correcting the defective WA S gene in young
affected boys, he could offer a short-term treat-
ment, and potentially even a permanent cure,
for the disorder.
First, Klein and his team tested their plan in
mice. They bred mice that lacked the entire WA S