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In one species of squirrel, Sciurus carolinensis, fur colour is controlled by one gene, with two codominant alleles - AQA - A-Level Biology - Question 1 - 2021 - Paper 1

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In one species of squirrel, Sciurus carolinensis, fur colour is controlled by one gene, with two codominant alleles. $C^G$ represents the allele for grey fur colour... show full transcript

Worked Solution & Example Answer:In one species of squirrel, Sciurus carolinensis, fur colour is controlled by one gene, with two codominant alleles - AQA - A-Level Biology - Question 1 - 2021 - Paper 1

Step 1

Use the Hardy-Weinberg equation to estimate how many squirrels had brown-black fur.

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Answer

Let the frequencies of alleles be represented as follows:

  • Let pp = frequency of CGC^G
  • Let qq = frequency of CBC^B Given that 2 out of 34 squirrels had black fur (genotype CBCBC^B C^B), we can set up the equation: q2=234q^2 = \frac{2}{34} Calculating qq:
    q=234≈0.2424q = \sqrt{\frac{2}{34}} \approx 0.2424 Thus, the frequency of the grey allele p=1−q≈0.7576p = 1 - q \approx 0.7576. The expected frequency of squirrels with brown-black fur (genotype CGCBC^G C^B) is given by: 2pq=2(p)(q)≈2(0.7576)(0.2424)≈0.36742pq = 2(p)(q)\approx 2(0.7576)(0.2424) \approx 0.3674 Multiplying by the total population size: 34×0.3674≈12.5extsquirrels.34 \times 0.3674 \approx 12.5 ext{ squirrels}. Therefore, approximately 12 squirrels had brown-black fur.

Step 2

Use all of the information to calculate the actual frequency of the $C^G$ allele.

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Answer

The actual number of squirrels with brown-black fur is given as 16. The total population is still 34 squirrels. To find the frequency of the CGC^G allele, we can use the following equation of allele frequency: CG=2imes1634+234÷2≈0.71.C^G = 2 imes \frac{16}{34} + \frac{2}{34} \div 2 \approx 0.71. Thus, the actual frequency of the CGC^G allele is approximately 0.71.

Step 3

Tick one box for the most likely conclusion.

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The appropriate choice is: B. The mutation that caused black fur happened in a common ancestor of S. carolinensis and other closely related species. This conclusion aligns with the observed genetic similarities between the populations in North America and the UK.

Step 4

Calculate the percentage reduction in size of the protein coded for by the $C^B$ allele compared with the protein coded for by the $C^G$ allele.

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Answer

Given:

  • Protein length for CGC^G allele: 306 amino acids.
  • Length of deletion: 24 base-pairs, which equals 8 amino acids (as 1 amino acid corresponds to 3 base pairs). Hence, the length of the CBC^B protein is: 306−8=298extaminoacids.306 - 8 = 298 ext{ amino acids}. Now, to find the percentage reduction: Percentage Reduction=(306−298306)×100≈2.61%.\text{Percentage Reduction} = \left( \frac{306 - 298}{306} \right) \times 100 \approx 2.61\%. This indicates a 2.61% reduction in protein size for the CBC^B allele.

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