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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
Question 1
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, a... 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 in this population had brown-black fur.
Answer
To find the expected frequency of the brown-black fur phenotype using the Hardy-Weinberg principle, we first define: Let the frequencies of the alleles be:
- = frequency of allele C^g
- = frequency of allele C^b
We know from the population that 2 out of 34 squirrels had black fur, indicating they are homozygous for C^b (C^bC^b). Thus, we can calculate:
From this, we can find q:
To find p, since , we have:
Now we can estimate the number of squirrels with brown-black fur (C^gC^b): Using the Hardy-Weinberg equation, the expected proportion is given by:
Thus, the number of brown-black fur squirrels is:
Step 2
Use all of the information to calculate the actual frequency of the C^g allele.
Answer
To calculate the actual frequency of the C^g allele in the population, we need to use the actual numbers:
- Total squirrels = 34
- Brown-black fur squirrels (C^gC^b) = 16
- Black fur squirrels (C^bC^b) = 2
To find the number of squirrels with grey fur (C^gC^g):
- Grey fur squirrels = 34 - (16 + 2) = 16.
Now, using these values:
- Total alleles = 34 x 2 = 68
- Alleles from black fur = 2 x 2 = 4 from C^bC^b.
- Alleles from brown-black fur = 16 x 1 = 16 from C^gC^b.
- Alleles from grey fur = 16 x 2 = 32 from C^gC^g.
Counting the alleles:
- Total C^b alleles = 4 + 16 = 20
- Total C^g alleles = 32 + 16 = 48
Now find the frequency of C^g:
Step 3
Use this information to deduce which one of the following conclusions is most likely true. Tick (✓) one box.
Answer
The correct conclusion is: B. The mutation that caused black fur happened in a common ancestor of S. carolinensis and other closely related species.
This is supported by the fact that the same mutation causing black fur has been observed in closely related species.
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.
Answer
Let:
- Length of C^g protein = L
- Length of C^b protein = 306 amino acids long.
- We need to find the length of C^g.
Assuming the length of C^g is 942 amino acids (as from the context), the percentage reduction is calculated as: