Activity 4: Genetic Testing
Objective
Students will investigate the impact of carrier screening and prenatal
testing.
Background Information
Genetic disorders affect large numbers of people from all racial and
cultural groups. Preventive approaches to these disorders are becoming more
feasible as scientists gain an understanding of genes and of the causes
of genetic disorders. In some cases, the risk that a particular genetic
disorder will occur can be assessed before a child is conceived or in the
early stages of pregnancy. In other cases, newborns can be screened for
a disorder so that appropriate treatment can be started before the condition
becomes fatal.
One key to assessing genetic risk for having a child with a particular
disorder is carrier testing. This type of testing determines whether the
prospective parents are carriers of the gene for the trait. If both the
parents are identified as carriers, then there is a 25% chance with each
pregnancy that the child will inherit the disease. Carrier testing can also
be conducted for X-linked disorders.
One form of prenatal testing involves collecting a sample of fetal cells
from the amniotic fluid surrounding the fetus, and testing the fetal DNA
to determine if there are chromosomal or special genetic abnormalities.
Some types of prenatal diagnosis techniques can be performed as early as
nine weeks into the pregnancy. A newer test, known as preimplantation diagnosis,
can be performed before in-vitro fertilized cells are implanted in the womb.
Prenatal tests are available for hundreds of conditions ranging from mental
retardation to late-onset disorders that cause serious mental and physical
deterioration in middle age. In about one percent of the cases, prenatal
testing may cause complications such as maternal bleeding or fetal death.
Some genetic diseases can be detected at birth by biochemical tests.
This type of testing is referred to as newborn screening. Newborn tests
are particularly important if the disorder can be treated with interventional
therapy.
Materials
For teacher preparation:
- Small head of red cabbage
- Cooking pot
- Distilled water
- Strainer
- 2 Bowls
- Red & blue food dyes
- White paper coffee filters
- Scissors
For each pair of students
Preparation
- Before class, prepare red cabbage indicator strips. Tear or cut the
cabbage leaves into small pieces and place them in a cooking pot filled
with distilled water. Boil for five minutes, then allow to cool to room
temperature. Pour the cooled cabbage extract through the strainer into
a bowl. Soak coffee filters in the solution, and allow them to dry. Cut
the dry filters into 1 x 1/2-inch strips and store them a closed container.
- Also prepare control indicator strips. Mix red and blue food coloring
with distilled water to produce a dye the same color as the red cabbage
solution. Soak coffee filters in the solution and allow them to dry. Cut
the filters into 1 x 1/2 inch strips and store in a closed container.
Instructions
- Have students read the information on cystic
fibrosis, then discuss the information with the class. Ask students
to think about how they might feel if they knew there was a chance that
they could have a child with the disorder.
- Divide the class into groups of two. Explain that each group represents
a pair of potential parents. Explain to students that they will have the
opportunity to test to see if they "carry" the gene for cystic
fibrosis. Have each "parental" unit discuss how they feel about
being tested. How would they react if they knew they were carriers for
cystic fibrosis? Would they want to have children? Why or why not? Ask
students to write a brief description of their position.
- Mix the red-cabbage indicator strips and control indicator strips together,
then hand out one indicator strip to each student. Also have students place
a small amount (a tablespoon or so) of vinegar in a paper cup. Explain
that the strip represents a blood sample taken from the student. If the
student is a carrier for the cystic fibrosis gene, the strip will turn
pink when dipped in the vinegar. If the student is not a carrier for the
gene, the strip will not change color.
- Have students put a drop of vinegar onto their indicator strips and
record the results. Then ask each "couple" to determine the chances
of having a child with cystic fibrosis. Have students discuss how they
feel about the results of the test.
- Next, ask students in each pair to imagine that they are a couple expecting
their first baby. This time provide each pair with a single strip of indicator
paper. Explain that this strip represents the genetic makeup of the couple's
"fetus." If the strip turns pink, the fetus has cystic fibrosis.
Once again have each couple write a brief description about how they feel
about the testing and the results.
Discussion Questions
- Do you think carrier testing is a good idea? Why or why not? Do you
think prenatal testing is a good idea? Why or why not?
- Should genetic screening for lethal diseases be mandatory? Why or why
not?
- How would you feel if you knew there were genetic tests for traits,
such as stature, physical appearance, or mental ability? Would you take
such a test? Why or why not? Who should determine which traits are "desirable"
and which are "undesirable?"
- What is the relationship between genetic testing for a disorder and
curing the disorder? Would the availability of a cure for a disorder affect
your desire to be tested? Explain your answer.
- Suppose that scientists knew the function of every human gene and were
able to test for each trait. For what genetic characteristics, if any,
would you want to test an embryo? Which characteristics, if any, might
make you decide not to have children or to terminate a pregnancy? Why?
Extensions
- Divide the class into groups and supply each group with the Genetic Disorder Profiles and Organizations page. Ask
the groups to read the profiles. Then ask them to make a list showing the
disorders for which they would consider being tested and the disorders
for which they would consider testing a fetus or newborn. Have groups present
their lists and discuss the reasons for their choices.
- Divide the class into groups and have each group choose one of the
disorders listed in the Genetic Disorder Profiles and Organizations. Then
have them research news articles and other sources for further information
about the disorder. (You may want them to contact one of the organizations
listed for more information.) Ask the groups to write two editorials--one
presenting the pros of testing for the disorder and another presenting
the cons. Have students share their editorials with the class, and discuss
their positions.
- Have teams of students explore the potential implications of genetic
testing by having them research different tests currently used to detect
latent or hidden conditions. Ask students to make a brief oral presentation
of their findings.
Link
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Resource Book Index: Promises and Perils
