Unit 5 FRQ (Punnett Squares)

AP Bio Unit 5 FRQ: Punnett Squares

Explore the Inheritance of Traits in Squirrel Populations

👋 Welcome to the AP Bio Unit 5 FRQ (Punnett Squares).
These questions are a bit longer, so grab some paper and a pencil, or open up a blank document to work through them. Once you’re finished, you can check the rubric and answers to see how well you did!

⏱ Time Management Tip: The AP Biology exam contains 6 free-response questions, and you’ll have 90 minutes to complete the FRQ section. That means you should allocate around 15 minutes per FRQ to stay on track.

🤔 Need a refresher? Check out the Unit 5 Overview to review key concepts before starting.

😩 Feeling stuck? Review any Unit 5 resources that are available to refresh your understanding.

Scenario

The Eastern gray squirrel (Sciurus carolinensis) is a common species in the southeastern United States, typically recognized by their long, bushy tails. However, in a suburban neighborhood in the southern delta, many residents have noticed that some squirrels have skinny, hairless tails, resembling a rat’s tail.

A local biologist decided to investigate this unusual trait by capturing and cataloguing the neighborhood squirrels over a few months. The biologist determined that the rat-tail phenotype is a result of a homozygous recessive genotype (tt), while the wild-type (bushy-tailed) phenotype is either homozygous dominant (TT) or heterozygous (Tt).

The data collected by the biologist is summarized in Table 1.

Table 1: Squirrel Population Data

Questions

(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.

Response Guide:

• Punnett Square: Draw a Punnett square for two heterozygous parents (Tt x Tt).
• Genotypic Frequencies:
  • TT: 25%
  • Tt: 50%
  • tt: 25%
• Phenotypic Frequencies:
  • Wild-Type (TT or Tt): 75%
  • Rat-Tail (tt): 25%

(b) Using your Punnett square from Part A, calculate the Chi-square value for wild-type versus mutant phenotypes using the observed values from Table 1 as your expected value.

Response Guide:

• Expected Rat-Tailed Offspring: If we assume a phenotypic frequency of 25% rat-tailed squirrels, we would expect (237 + 42) x 0.25 = 69.75 rat-tailed squirrels in the population.
• Expected Wild-Type Offspring: (237 + 42) x 0.75 = 209.25 wild-type squirrels.
• Use the Chi-square formula to compare the expected vs. observed values:
  • Chi-square (χ²) = Σ [(Observed – Expected)² / Expected]
  • Observed Values: Wild-Type = 237, Mutant = 42
  • Expected Values: Wild-Type = 209.25, Mutant = 69.75

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

Response Guide:

• Analyze the Chi-square value and compare it to the critical value from a Chi-square distribution table.
• If the calculated Chi-square value is less than the critical value, the observed data is consistent with the hypothesis that the average squirrel is heterozygous.
• If the Chi-square value exceeds the critical value, the observed data rejects the hypothesis.

(d) Squirrels are a species of rodent, closely related to rabbits, which exhibit changes in fur color based on seasonal changes (e.g., snowshoe hares change fur color to adapt to winter and summer). Identify the phenomenon that results in these changes.

Response Guide:

• Phenomenon: Seasonal Molt and Phenotypic Plasticity
• Seasonal fur color changes in hares like snowshoe hares are an example of phenotypic plasticity, where organisms change their phenotype in response to environmental cues (e.g., changing fur color for camouflage during different seasons to avoid predation).

Answers & Rubric

💯 Check how you did!
Compare your responses to the guide below to see how many points you would have earned.

Scoring Guide

(a) Punnett Square, Phenotypic, and Genotypic Frequencies

• 3 Points Total
  • 1 pt for drawing a correct Punnett Square (Tt x Tt).
  • 1 pt for accurately determining the genotypic frequencies.
  • 1 pt for accurately determining the phenotypic frequencies.

(b) Chi-Square Calculation

• 3 Points Total
  • 1 pt for correctly calculating expected values based on phenotypic ratios.
  • 1 pt for using the Chi-square formula.
  • 1 pt for calculating and providing a conclusion regarding the hypothesis.

(c) Conclusion Based on Chi-Square

• 2 Points Total
  • 1 pt for correctly interpreting the Chi-square value.
  • 1 pt for providing a valid conclusion on whether the biologist’s hypothesis is supported.

(d) Identification of Phenomenon

• 2 Points Total
  • 1 pt for correctly identifying the phenomenon as phenotypic plasticity or seasonal molt.
  • 1 pt for accurately describing how this helps organisms adapt to environmental changes.

Bonus Concept: Genetic Drift

What role does genetic drift potentially play in the observed frequency of the rat-tail mutation in this isolated squirrel population?

Response Options:

1. Genetic drift may lead to random fluctuations in the frequency of the rat-tail allele, potentially leading to increased or decreased prevalence of the trait in the population, especially in a small, isolated population.
2. Incorrect Option: Genetic drift always decreases the frequency of rare alleles like the rat-tail mutation.
3. Incorrect Option: Genetic drift has no effect on small, isolated populations.
4. Incorrect Option: Genetic drift only affects neutral mutations.