In a population where the ratio of the phenotypes of brown fur to white fur is 1:1, this means that for every rabbit with brown fur, there is one rabbit with white fur.
To calculate the frequency of the white fur trait, you can use the following formula:
\[ \text{Frequency of white fur} = \frac{\text{Number of rabbits with white fur}}{\text{Total number of rabbits}} \]
Since the ratio is 1:1, we can assume there are equal numbers of brown and white fur rabbits. Thus, if we represent the total number of rabbits as 2 (1 brown fur and 1 white fur), the frequency of white fur rabbits is:
\[ \text{Frequency of white fur} = \frac{1}{2} = 0.5 \text{ or } 50% \]
Therefore, the correct response is 50%.