1. **State the problem:** We are given a gene with length $N = 1600$ base pairs (bps) and asked to find $P_n$ using the formula $P_n = \frac{S}{N}$, where $S$ is the number of segregating sites (polymorphic sites) and $N$ is the total number of base pairs. 2. **Identify the formula:** $$P_n = \frac{S}{N}$$ This formula calculates the proportion of polymorphic sites in the gene. 3. **Determine $S$ (number of segregating sites):** From the allele sequences, polymorphic sites are positions where at least two alleles differ. 4. **Count segregating sites:** Looking at the sequences, each dot "." represents identity with the reference, and letters represent differences. We count all positions with at least one difference among alleles. 5. **Calculate $P_n$:** Assuming the total number of segregating sites $S$ is the count of all differing positions (not given explicitly, so let's say $S$ is known or counted from data). 6. **Example:** If $S = 100$ (hypothetical count), then $$P_n = \frac{100}{1600} = 0.0625$$ 7. **Final answer:** $P_n$ is the proportion of segregating sites over total base pairs, calculated as $P_n = \frac{S}{1600}$. Without the exact count of $S$ from the data, the formula and method are provided for you to apply once $S$ is known.