Activity 3: Predicting Genetic Combinations
Objective
Students will develop pedigrees to trace the inheritance pattern of a
genetic disease.
Background Information
A pedigree is a chart showing the genetic history of a particular trait
within a family. Pedigrees can show the inheritance patterns of cosmetic
traits such as hair texture and eye shape or of potentially life-threatening
traits such as genetic disorders. These charts are also useful in predicting
possible outcomes from future gene combinations.
A pedigree showing the occurrence of a recessive trait in three generations
of a family.
By convention, circles in a pedigree represent females and squares represent
males. A horizontal line between a circle and a square indicates a marriage
or partnership; vertical lines indicate the children from the marriage or
partnership. In this activity, a filled-in circle or square shows that the
individual has both alleles for the trait. A half-filled-in circle or square
indicates that the individual has one recessive allele
for the trait.
A person who is heterozygous for a recessive
genetic disorder is called a carrier. A carrier does not usually manifest
symptoms of the disease, but may pass on the defective copy to his or her
child. If a child of two carriers receives a recessive allele for the trait
from each parent, he or she will express the disorder.
Geneticists can use pedigrees to help predict the probability of whether
a child will express a given trait. One method of illustrating the possible
gene combinations that may occur when two people reproduce is a chart called
a Punnett square. In a Punnett square, the alleles from one parent are written
across the top of the square, while the alleles from the other parent are
written down the side of the square. All the different possible combinations
of alleles are shown within the square.
 A Punnett
Square that shows the possible
crosses between two parents with the alleles Pp. Here, white indicates the
recessive trait.
Materials
For teacher preparation:
- Colored markers or crayons, in three colors
- 2 Popsicle sticks, tongue depressors, or strips of cardboard per student
For each pair of students:
Preparation
Duplicate the Genetic Profile Worksheet and
distribute to students. The Popsicle sticks or tongue depressors will represent
chromosomes. Indicate three "genes"
on each "chromosome" by drawing lines with the markers or crayons.
Use a different color marker or crayon for each type of gene, and use thick
lines for dominant alleles and thin lines for recessive alleles. Vary the
frequency of dominant and recessive alleles for each gene. For example,
for the first gene, mark 80% of the sticks with a thin line and 20% of the
sticks with a thick line. For the second gene, use a 50/50 ratio, and for
the third gene, use a 20/80 ratio. Be sure to put the lines for each gene
at the same location on each stick to help convey the idea that they represent
alleles on a chromosome. Finally, assign the color that will represent the
gene for phenylketonuria (PKU), an inherited diseased caused by a recessive
allele, which affects one out of 10,000 babies.
Instructions
- Divide the class into pairs. Give each student two sticks and the Handouts.
Explain that each pair of sticks represents the possible genetic material
that can be donated from one parent, and that each line on the stick represents
a different gene from that parent. Explain that for this activity they
will be investigating the inheritance patterns of PKU. Tell students which
color represents the gene for PKU.
- Have each pair of students decide which pair of sticks represents the
mother's genetic material and which represents the father's genetic material.
Ask them to locate the PKU alleles on their trait sticks. Then have students
determine the PKU genotypes for the parents and record the information
in the "Parent" position of the pedigree. Ask them to use the
colored pencils to indicate the phenotype of each parent on the form.
- Have students use the Punnett square at the bottom of the handout to
show all the possible genotypes for the children. Then have them record
both the genotype and phenotype for each child in the "Children"
positions on the pedigree.
- Next have each pair of students join with another pair. Ask them to
randomly select one child from each pair's pedigree to use as parents for
the next generation. Have them record the new parents' genotypes and phenotypes
in the "Parents" positions on the second handout.
- Using the new parents, have students once again show the possible crosses
for the second generation of children. Have them trace the transmission
of PKU throughout the families.
Discussion Questions
- Examine the Punnett square for the original parents. What was the probability
for each birth that these parents would have a child with PKU? What was
the probability for each birth that the child would be a carrier? How can
you tell?
- How many family members (including the parents, first generation of
children, and second generation of children) actually had PKU? How many
were carriers? Are these numbers the same as the probabilities? Why or
why not?
- Suppose you knew there was a history of PKU in your family. What are
some of the advantages of creating a pedigree? What are the limitations
of using a pedigree? Would you want to have the pedigree made? Why or why
not?
- If you knew you were a carrier for PKU, that is, you had one recessive
gene for the trait, how would you feel about marrying a person who is also
a carrier for the trait? Would you want to have children with the person?
Explain your answer.
- Would you want to know if the person you plan to marry is a carrier
for PKU? Should this person be required to tell you? Give reasons for your
answers.
Extensions
- Repeat the activity using the second set of colored lines to represent
genes for a dominant genetic trait. Point out that someone who is heterozygous (or has one copy of a dominant gene)
for a dominant disorder will have the disorder.
- Have students use the sticks to investigate the concept of independent
assortment. Choose two colors on the sticks to represent two genes for
the crosses. Then repeat the activity, having students create new pedigrees
and Punnett squares.
Links
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Resource Book Index: Promises and Perils
