Promises and Perils
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Activity 3: Predicting Genetic Combinations


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.

pedigree diagram
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.

punnett square A Punnett Square that shows the possible
crosses between two parents with the alleles Pp. Here, white indicates the recessive trait.


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:


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.


  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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

  1. 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?

  2. 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?

  3. 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?

  4. 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.

  5. 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.


  1. 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.

  2. 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.


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