Cats and Punnett squares

This exercise is a part of Educator Guide: How to Lick Cat Allergies / View Guide

Purpose: Scientists would like to breed cats that don’t trigger allergies in people. By constructing and analyzing a Punnett square for two low-allergen cats, students will review key concepts including patterns and probabilities of inheritance, genotype, phenotype, genes, alleles, chromosomes and mutations. Students will apply critical thinking skills to interpret the likely allergen levels of the resulting cats.

Procedural overview: Students will study data on the genotypes and phenotypes of two Siberian cats and answer associated questions. Then, student groups will construct a Punnett square of a cross between the cats and analyze the results. 

Approximate class time: 1 class period to complete the data questions, construct the Punnett square, analyze the results and debrief as a class.

Supplies: 
Grid paper for making the Punnett square
Classroom resources on genetic concepts
A projector for introducing the activity (optional)
Cats and Punnett Squares student activity guide 

Directions for teachers: 
Introduce students to various methods for fending off cat allergies by having them read “How to lick cat allergies.” The story discusses the genetics behind the main cat allergen and explains how people are trying to find a reliable way to breed low-allergen cats but have not yet been successful. 

Tell students that they will look at some data on two low-allergen Siberian cats. (Note: While this activity uses data from recent genetic research that can be found here https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5753643/, some assumptions have been made and so this scenario should be presented as a hypothetical one.)

To understand the genetic data, it is important that students know the basics of DNA, genes, gene transcription, translation, genotype and phenotype. Background questions No. 1 through 4 below can help facilitate a class discussion that assesses student knowledge of genetic concepts and reviews those concepts as needed. If students need further review, have them consult resources and/or present the information to the class.

Important points are:

  • DNA, or deoxyribonucleic acid, typically exists as a double-stranded helix, and each strand is made of a sequence of nucleotides. DNA contains information that gives your cells instructions and so has a major role in determining your traits.
  • DNA is packaged in chromosomes that are passed down from parents to offspring. Animals generally have pairs of chromosomes, typically one from each parent. 
  • Genes are small sections of DNA that have the instructions for making a protein or part of a protein. Proteins are used to digest food, build cells and move your muscles, among other roles. 
  • In gene transcription, an RNA copy of a DNA strand is made. The RNA is used to make proteins in a process called translation.
  • Animals with one chromosome from each parent generally have two versions of each gene. The two versions of a gene are called alleles. The two alleles can be the same or different. 
  • Mutations are changes in the DNA sequence that makes up a gene. Alleles with mutations have a sequence of nucleotides different from the typical sequence for that gene. Mutations can be inherited or can happen spontaneously, and they can be helpful, neutral or harmful. 
  • A genotype is all the information carried in the DNA (and by extension genes) of an organism. 
  • A phenotype is the observable characteristics of an organism.

Next, instruct students to read the section of the activity titled “Analysis of mutation data on Siberian cats,” which provides information on mutations identified in two genes, Ch1 and Ch2, in two cats. Break students into small groups and have them analyze the data by answering questions No. 5 through 11. Review the answers as a class. 

Review how Punnett squares are used to help predict the genotypes and phenotypes in breeding experiments. Have students work to create a Punnett square for a cross between the two cats (question No. 12). Be sure to explain that the top of the Punnett square lists all the possible combinations of alleles for the two genes in one parent and the left side does the same for the other parent.

By filling out the square, students will determine all the possible genotypes for offspring of the two cats. 

If students have previously worked with Punnett squares, challenge them to set up a square for the two genes. If students have not previously worked with Punnett squares, go through the process of determining the possible genotypes for the parents. Then explain how to set up the table and fill in each cell. Check with students as they work to make sure they are on the right track.

When students have completed the Punnett square, review the genotypes and phenotypes and have students answer the remaining questions. Review the answers as a class.

Student directions:

Background questions

Answer the following questions to assess your knowledge of some genetics terms and concepts. Use your notes or classroom resources to look up terms that are new to you. 

1. Compare gene transcription and translation. 

Transcription is the process by which a gene’s DNA sequence is copied to make RNA. Translation is the process by which RNA is used to make a protein. 

2. What is an allele? What can make alleles different?

An allele is a form of a gene. Some alleles have mutations that make them different from the typical form found throughout a population. 

3. What is a genotype? 

A genotype is the genetic information, or DNA, of an organism. 

4. What is a phenotype? 

A phenotype is the observable characteristics of an organism.

Analysis of mutation data on Siberian cats 

Some people have claimed that Siberian cats are naturally low in the common cat allergen Fel d1 and therefore are less likely to cause allergic reactions in people. A recent genetic study investigated common alleles and mutated forms in the Ch1 and Ch2 genes, which are known to control production of Fel d1 in Siberian and non-Siberian cats. 

The researchers identified the common alleles of the two genes and three mutated forms of the gene that may be of interest. Two of the mutated forms do not make functional Fel d1. This may result in lower amounts of the allergen in a cat with those alleles. The following table describes and gives temporary names for the alleles identified in the study. 

Table 1

GeneAlleleAllele description and distributionEffect of mutation
Ch1Ch1+ Most common form across all felines
Ch1H Mutated form found only in Siberian cats Error in Fel d1
Ch2 Ch2+ Most common form across all felines
Ch2S Mutated form found in all Siberian cats and some non-Siberian cats No error in Fel d1
Ch2H Mutated form found only in Siberian catsError in Fel d1

The researchers then gathered complete data on the genotype and phenotype of two Siberian cats. Note that a previous study of a variety of cats determined that the average salivary concentration of the allergen Fel d1 ranged from 0.4 to 35 µg/mL.

Table 2

CatCh1 alleles present in the genome Ch2 alleles present in the genomeSalivary concentration of Fel d1 (µg/mL)
1Ch1H Ch2S, Ch2H 0.48
2Ch1+ Ch2S 2.19

Answer the following questions about the data in the study.

5. Based on Table 1, what are the names of the most common alleles of the Ch1 and Ch2 genes that are found across all felines?   

Ch1+ and Ch2+

6. What are the names of the three mutated alleles that are found in Siberian cats? Which of these alleles affect the production of the allergen Fel d1? 

Ch1H, Ch2S and Ch2H are mutated alleles that are found in Siberian cats. Ch1H and Ch2H affect the production of Fel d1.

7. Based on Table 2, which cat has the mutated forms of the alleles that are thought to affect the production of Fel d1? Does the presence of the mutations seem to be related to the level of Fel d1? 

Cat 1 has the two mutated forms Ch1H and Ch2H and it has a lower level of Fel d1. The mutations seem to be related to the level.

8. Which column(s) of the table indicate genotype information? Explain your answer.

The columns about the alleles, because the genotype is the genetic information and alleles are genes with different DNA sequences. 

9. Which column(s) of the table indicate phenotype information? Explain your answer. 

The information about the concentration of the allergen, because the phenotype is an observable characteristic.

10. Many people have claimed that Siberian cats cause fewer allergies. What data in the second table might support this observation?

The values of the allergen level for the two cats is at the low end of the range for all cats, even for cat 2, which does not have any of the two mutations known to affect protein production.

11. One research goal is to identify mutations in the Ch1 and Ch2 genes that are common to Siberian cats but not found in non-Siberian cats. Why is this interesting? 

Forms of genes that are shared by Siberian cats and not non-Siberian cats may be the ones responsible for the lower allergen levels.

Construct and analyze a Punnett square

Parents contribute to the DNA of their offspring. Generally for animals, one of a pair of chromosomes and its alleles comes from one parent, and the other chromosome and its alleles comes from the other parent. Each pair of alleles is the genotype for a specific trait. 

The Punnett square can predict the possible genotypes of offspring from two parents. The top of the Punnett square lists all the possible combinations of alleles that could come from one parent, and the left side lists all the possible combinations from the other parent.

In this activity, you will build a Punnett square for the Ch1 and Ch2 genes in the two cats described above. Because two genes are involved, the Punnett square will have four columns for the genotypes of one parent and four rows for the other parent.

Two notes: In this exercise, we are assuming that the two genes are not linked. Also, you may have worked with Punnett squares in which alleles are identified as recessive or dominant. But the research on the alleles of Ch1 and Ch2 is still too recent to know inheritance patterns. The good news is you can still create a Punnett square by listing both alleles for each of the two genes in the cells.

12. In your small group, construct a Punnett square for a cross between cats 1 and 2. Show the possible genotypes for cat 1 on the top of the square and the possible genotypes for cat 2 on the left side of the square. Then fill in the squares for the possible genotypes of the cats’ offspring. 

Ch1HCh2S Ch1HCh2H Ch1HCh2S Ch1H Ch2H
Ch1+Ch2S Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H
Ch1+Ch2S Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H
Ch1+Ch2S Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H
Ch1+Ch2S Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H

Now analyze the results of your Punnett square.

13. Highlight the genotypes of offspring that contain BOTH of the mutated alleles that affect protein production. (Answers are bolded in the table below.)

Ch1HCh2S Ch1HCh2H Ch1HCh2S Ch1H Ch2H
Ch1+Ch2S Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H
Ch1+Ch2S Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H
Ch1+Ch2S Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H
Ch1+Ch2S Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H Ch1+Ch1HCh2SCh2S Ch1+Ch1HCh2SCh2H

14. What phenotype would you predict for cats with BOTH mutations?

The cats will probably have lower levels of the allergen. 

15. What is the probability that a cat would have both mutations?

There is a six out of sixteen chance, or about 37.5 percent, for a cat with both mutations. 

16. The article “How to lick cat allergies” says that only one out of 15 offspring of low-allergen parents is low-allergen. List as many factors as you can think of that could explain why this number is smaller than what your Punnett square describes? 

Some possibilities include: The cats in this study might not be representative of the population of low-allergen cats. There might be other genes involved in turning production of Fel d1 on and off. When both alleles of Ch1 are present, Ch1+ could compensate for Ch1H. When both alleles of Ch2 are present, Ch2S could make more protein than Ch2H. The two genes might be linked in a way that affects the pattern of inheritance. Environmental factors might play a role in how much Feld1 is produced. Some other allergen might be produced by cats.

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