Final answer:
50% of the offspring from a heterozygous long-finned fish crossed with a homozygous shortfin fish will have long fins, as shown by a Punnett square.
Step-by-step explanation:
Predicting Offspring Phenotypes
To solve this problem, we need to set up a Punnett square for a cross between a heterozygous long-finned fish (Ll) and a homozygous short-finned fish (ll). The heterozygous genotype carries one allele for long fins (L) and one for short fins (l). The homozygous short-finned fish carries two alleles for short fins (ll).
The Punnett square would show the following combinations: Ll and ll for the first column, and Ll and ll for the second column. This results in two out of four boxes displaying the heterozygous (Ll) genotype, which expresses the long-finned phenotype. Therefore, 50% of the offspring will have long fins.
Answer Choice: C) 50%