Final answer:
A 50% recovery of single-crossover products is the maximum because only half of the gametes will exhibit crossover of homologous chromosomes during meiosis, resulting in a mix of recombinant and nonrecombinant gametes. Factors like the mechanism of meiosis, possible double crossovers, and gene distance affect this limit. Option number C is correct.
Step-by-step explanation:
The reason a 50% recovery of single-crossover products is the upper limit, even when crossing over always occurs between two linked genes, is due to the mechanism of meiosis and genetic recombination. During meiosis, only half of the gametes formed will exhibit the crossover of homologous chromosomes. This means that a maximum of 50% of the resulting gametes can have recombinant chromosomes that are a mix of parental alleles. If single crossovers occurred in all meiotic events, each would produce two recombinant and two nonrecombinant gametes, thus making the recombination frequency 50% at most.
Moreover, while crossing over can produce new combinations of alleles on chromosomes, it does not necessarily occur between any specific pair of genes every time. Double crossovers can also alter the outcomes and complicate the calculation of recombination frequencies. It's also crucial to consider that while genes on the same chromosome are linked, they can sometimes act independently if they are far enough apart, as crossover events can occur between them.