Two plants are heterozygous for n genes on different chromosomes, and they are crossed…

Step 1. (a) $81 = 3^4$ and $256 = 4^4$, so $\tfrac{81}{256} = \left(\tfrac{3}{4}\right)^4$. So $n = 4$.

Step 2. (b) Four heterozygous genes give $2^4 = 16$ kinds of gamete.

Step 3. (c) The share that is recessive for all four genes is $\left(\tfrac{1}{4}\right)^4 = \tfrac{1}{256}$, which is $0.39\%$.