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

To find the actual frequency of the $C^G$ allele, we first calculate the total number of squirrels with brown-black fur, which is 16. We can use the formula for allele frequency:

For the $C^G$ allele:

$q^2 = 6/34$

From this, calculating:

$q = ext{sqrt}(6/34)$

This results in an actual frequency for the $C^G$ allele which can be approximated as:

$0.71$

The most likely true statement is:

B. The mutation that caused black fur happened in a common ancestor of S. carolinensis and other closely related species.

The length of the protein coded by the $C^G$ allele is assumed to be the full length, which is 306 amino acids. The reduction in size due to the deletion mutation is calculated using:

ext{Percentage reduction} = rac{ ext{Length of } C^G - ext{Length of } C^B}{ ext{Length of } C^G} imes 100\ ext{Assuming the new protein is of lesser length let’s assume it’s } 314

Carrying out this calculation gives the final answer.

The deletion mutation in the $C^B$ allele results in the production of a receptor protein that leads to permanent activation of the receptor. This occurs because the receptor does not require the binding or leaving of aMSH to activate, which leads to only the dark pigment being produced.