Unit A Reproduction

4.6
Prenatal Procedures
(a)
(b)
Figure 1 An ultrasound involves using
sound waves to produce an image of
the fetus (a). The image is called a
sonogram (b).
LEARNING TIP
The photographs and diagrams in this
section will help you understand
prenatal procedures. First, read the
explanation of each procedure. Then
look carefully at the related figure.
4A
Investigation
The Human Karyotype
To perform this investigation,
turn to page 139.
In this investigation, you will create a
karyotype of human chromosomes.
Certain diseases are caused by mutated genes. For some diseases, scientists have
identified which gene has the mutation. For example, scientists have identified
the genes that, when mutated, cause cystic fibrosis and Huntington’s disease.
The main symptom of cystic fibrosis, a buildup of mucus in the lungs,
develops in childhood. Breathing is difficult, and most people do not live
beyond their twenties. The symptoms of Huntington’s disease usually do not
develop until people reach their mid-thirties or forties. Symptoms include
personality changes and progressive loss of memory and muscle control.
Scientists have been able to diagnose genetic disorders since the 1950s.
Many disorders, such as Down syndrome and Turner syndrome, can be
detected before a baby is born. Prenatal (before birth) tests most often
confirm that fetuses are normal.
Testing for genetic disorders prior to birth raises some ethical issues.
Parents could decide to terminate the pregnancy. Knowing the gender of the
fetus early on in the pregnancy may also be used as a reason to terminate the
pregnancy. In most cases, however, the information helps parents make
decisions that will better prepare them to raise their offspring.
Ultrasound
The technology that is most commonly used to detect abnormalities in the
developing fetus is ultrasound. Ultrasound (Figure 1(a)) uses high-frequency
sound waves to create a 3-D image, called a sonogram (Figure 1(b)), of the
developing fetus. It is used to view the developmental progress of the fetus
and to detect any physical abnormalities. It can also be used to determine the
gender of the fetus.
Amniocentesis
In a procedure called amniocentesis (Figure 2), the doctor uses a syringe to
remove amniotic fluid from the sac that surrounds the fetus. The amniotic
fluid contains cells from the fetus, and these cells contain the chromosomes
of the developing fetus. These cells are treated, allowed to reproduce, and
then stained and placed on a microscope slide. A photo of the genetic material
is used to produce a karyotype of the fetus. The test is done around the
20th week of pregnancy. 4A Investigation
Since the 1970s, amniocentesis has been a common procedure used to view
the chromosomes of fetuses in high-risk pregnancies. A pregnancy is considered
to be high-risk when the mother is over the age of 35, the mother has already
had a child with Down syndrome or a child with a defective spinal cord or brain,
the mother or father has a genetic disease in the family, or the mother has had
previous miscarriages. A miscarriage occurs when a fetus is involuntarily
expelled from the womb before it has developed enough to survive on its own.
Miscarriages can be caused by birth defects in the fetus, accidents, or illnesses.
130 Unit A Reproduction
NEL