Unit 5 FRQ (Punnett Squares) Answers

AP Bio Free Response Question Answers for Punnett Squares

👋 Welcome to the AP Bio Unit 5 FRQ (Punnett Squares) Answers. Have your responses handy as you go through the rubrics to see how you did!

⏱ Remember, the AP Biology exam has 6 free-response questions, and you will be given 90 minutes to complete the FRQ section. (This means you should give yourself ~15 minutes to go through each practice FRQ.)

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Setup

The Eastern gray squirrel, Sciurus carolinensis, thrives in across the southeastern United States. Including their tails, they can be almost twenty-two inches long. They can live over a decade if they are not eaten by birds of prey, house pets, or even humans. A suburban neighborhood in the southern delta has an average population of squirrels, but many of the residents have begun to notice several of the squirrels do not have the characteristic long, bushy tails. Several of the neighborhood squirrels have skinny, hairless tails that resemble rat’s tails. A local biologist seeks to understand the mutation and begins trapping neighborhood squirrels to collect data. Over several months, the hundreds of squirrels are trapped, catalogued, and released. The data is summarized in Table 1. 

After careful research, the local biologist determines that the rat-tail is the result of a homozygous recessive genotype, commonly denoted as tt. The biologist hypothesizes that most squirrels have a heterozygous genotype of Tt for their tails. 

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Table 1

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Questions with Answers & Rubric

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(a) Assuming that the average squirrel is heterozygous, draw a Punnett square of two parent squirrels for tail type. Calculate the phenotypic and genotypic frequencies of the offspring. 

🏆 1pt: Draw

🏆 2pts: Calculate

• TT = 25%, Tt = 50%, tt = 25%. Thus:
• Wild-type or normal squirrels - 75%
• Mutant or rat-tailed squirrels - 25%

📄 Additional Resources

• Study Guide: Punnett Squares

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(b) Your Punnett square from Part A will define a specific number of expected rat-tailed squirrels. Using that as your expected value and the observed values from Table 1, calculate the Chi-square value for wild-type versus mutant phenotypes.

🏆 1pt: Calculate

• Chi-square = 14.95

🏆 1pt: Calculate

• Expected = 209 wild-type, 70 mutant
• Observed - 237 wild-type, 42 mutant

📄 Additional Resources

• Replay: Chi Square

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(c) In reference to your own calculations, explain if the local biologist’s hypothesis (that the average squirrel is heterozygous) is correct. 

🏆 3pts: Explain

• 1pt = Incorrect
• 1pt = The Chi-square value is greater than the critical value; reject the null hypothesis.
• 1pt = There is a significant difference between observed and expected values. Therefore, the researcher is incorrect: there is no way the average squirrel is heterozygous.

📄 Additional Resources

• Replay: Mendelian and Non-Mendelian Genetics and Chi Square

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(d) Squirrels are a species of rodent, which are characterized by teeth that are always growing for chewing and gnawing. They are closely related to rabbits, who dwell on the ground and are larger in size. Some rabbits, such as snowshoe hares (Lepus americanus), exhibit changes in fur color based on seasonal changes. Identify the phenomenon that results in these changes.

🏆 1pt: Identify

• Phenotypic plasticity

📄 Additional Resources

• Study Guide: Environmental Effects on Phenotype

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Next Steps

• • 🧠 Want to continue reinforcing your knowledge of Unit 5? Check out Unit 5 Trivia, either as a document or as a game.
• • ⏭ Ready to move on to the next topic? Take a look at the collection of Unit 6 resources.
• • 📚 Want to review multiple units? Check out all of the AP Bio FRQs.