Final answer:
A two-strand double crossover between two linked genes produces 50% recombinant and 50% nonrecombinant gametes because of two crossover events on the same two chromatids, with half retaining recombinant DNA segments and half remaining unchanged.
Step-by-step explanation:
A two-strand double crossover between two linked genes results in recombinant gametes because the process involves two separate crossover events on the same two chromatids. Initially, the first crossover event swaps segments between chromatids, and a subsequent second crossover swaps them back, but between different segments.
Therefore, the end result is that the two original non-sister chromatids essentially undergo an exchange and retain recombinant DNA segments. Due to both crossovers occurring in the same two chromatids, the other two chromatids remain unchanged, resulting in a mix of both recombinant and nonrecombinant gametes.
However, since the double crossover effectively restores the original configuration for the segments outside the crossover region, the correct answer is 50% recombinant and 50% nonrecombinant (Option a).